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Anti-Tissue Transglutaminase Antibodies and Their Role in the Investigation of Coeliac Disease

Posted on: Friday, 31 March 2006, 06:00 CST

By Hill, P G; McMillan, S A

Abstract

Coeliac disease (CD), caused by an inappropriate T-cell-mediated immune response to the ingestion of cereal proteins in genetically susceptible individuals, is a common disorder with a prevalence of about 1% in Caucasian populations. It has a strong association with other autoimmune disorders, particularly type 1 diabetes and autoimmune thyroid disease. Although primarily affecting the small bowel, CD is a multisystem disorder and the adult or child patient may initially present to a wide range of clinical specialities. The concept of the 'coeliac iceberg' has been used to emphasize that many cases currently remain undiagnosed. The identification of tissue transglutaminase (TGA)-2 as the antigen against which the autoantibodies are directed has led to a greater understanding of the pathogenesis of CD and to the development of improved serological tests. Enzyme-linked immunoassays using human tissue TGA as antigen have high diagnostic sensitivity and specificity for the detection of CD. This review examines the evidence for adopting IgA anti-tissue TGA as the first-line diagnostic test for CD. It recommends a laboratory algorithm for the use and interpretation of TGA to enable the clinical laboratory to play a full part in detecting and monitoring a disorder that is eminently treatable once the diagnosis has been considered and confirmed.

Ann Clin Biochem 2006; 43: 105-117

Clinical background to coeliac disease

Coeliac disease (CD) is an inflammatory condition of the small intestine caused by an inappropriate immune response to the ingestion of certain dietary cereal proteins in genetically susceptible individuals. The pathogenesis of the disease involves interactions between environmental, genetic and immunological factors. The only treatment at present is lifelong strict adherence to a gluten-free diet (GFD), which permits recovery of the intestinal mucosa.1

Prevalence

CD is one of the most common immune-mediated disorders. Its prevalence has been documented in Europe, North and South America, North Africa, South and West Asia and Australia; it is considered to be rare in Chinese, Japanese and African-Caribbean people. Large studies in the USA and Europe in children and adults have shown the prevalence to be 0.5-1% among populations of European ancestry, particularly in countries where wheat forms part of the staple diet.2-6 Earlier estimates suggested a much lower prevalence (0.05- 0.1%), and the dramatic increase is probably due to better recognition of active CD in patients who do not present with the classical gastrointestinal symptoms and to the use of sensitive serological assays based on the detection of anti-transglutaminase (anti-TGA) antibodies to screen for the disease.

Clinical presentation

Previously regarded as a childhood disease, CD is now recognized to affect mostly adults, with about 25% of patients being diagnosed over 60 years of age.7 CD may present at any age after the introduction of gluten into the diet and the clinical presentation is variable, ranging from subclinical to severe. Although primarily a disorder affecting the small bowel, symptoms can range from classical gastrointestinal symptoms to non-specific gastrointestinal symptoms and extraintestinal manifestations. At presentation, gastrointestinal symptoms (diarrhoea, abdominal distension and failure to thrive) are common in infants and young children, whereas a more insidious onset with non-specific symptoms is typical of older age groups. Presentation in adult life may initially be to a wide range of clinical specialties; for example, to Primary Care with the complaint of 'tired all the time', to Clinical Haematology with persistent anaemia, to Gynaecology because of infertility and recurrent miscarriages, or to an Orthopaedic department because of fractures. Although the classical symptoms of steatorrhoea and marked weight loss are now uncommon presentations in adult life, diarrhoea remains a common symptom at diagnosis. It is, however, frequently not the presenting complaint and only becomes apparent on direct questioning.7 The absence of diarrhoea in a patient with other symptoms does not exclude CD. Common signs and symptoms include abdominal pain, frequency of bowel movements, weight loss, bone disease, anaemia, fatigue and biochemical evidence of malabsorption (e.g. low serum folate, ferritin or calcium). Extra- intestinal presenting symptoms include osteoporosis, short stature, dental enamel defects, miscarriage, infertility, arthritis, arthralgias and neurological problems.8,9 Neurological features include peripheral neuropathy, cerebellar ataxia, epilepsy and migraine.10,11

Other diseases can be associated with CD, including dermatitis herpetiformis, immune-mediated endocrine disorders (most commonly type 1 diabetes and thyroid disease8,9), Down's syndrome, Turner's syndrome and Williams syndrome. About 10% of first degree relatives of CD patients will also have CD. IgA deficiency is more common in patients with CD and the diagnosis may therefore be missed if IgA- based serological tests are used for screening. The incidence of certain types of cancers are also increased in patients with CD; these include non-Hodgkin's lymphoma, enteropathy-associated T-cell lymphoma and small intestinal adenocarcinoma.4

Serological, genetic and histological markers have been used as screening techniques to identify patients with CD; however, some of these patients are asymptomatic. The iceberg concept has been used to draw attention to the fact that many cases are asymptomatic and therefore remain undiagnosed if screening tests are restricted to patients presenting with the classical signs and symptoms of the disease. Clinical CD is represented by the exposed tip of the iceberg, with silent and latent disease hidden below the waterline.12,13

Diagnosis of coeliac disease

Diagnostic criteria for CD in both children and adults are still based on the guidelines proposed in 1990 by the European Society for Paediatric Gastroenterology and Nutrition.14 These require the demonstration of villous atrophy in biopsies taken from the proximal small bowel while the patient is still consuming a gluten- containing diet, followed by a response, usually assessed clinically, to a GFD. However, more recent guidelines from the USA8,15,16 have suggested an algorithm for diagnosis, as indicated in Figure 1.

Interestingly, a recent article has suggested that forehead size can help to identify patients with CD. The craniofacial morphology of CD patients reveals an altered pattern of craniofacial growth when compared with the general population.17

When a patient has symptoms suggestive of CD, a range of laboratory tests can be performed to support the diagnosis. These include haematological tests to reveal anaemia or abnormal red cell morphology; biochemical tests for identification of thyroid disease or diabetes mellitus; and tests which may be indicative of malabsorption, such as serum folate, ferritin, calcium or alkaline phosphatase. Testing for IgA antibodies to tissue transglutaminase and/or IgA antiendomysial antibodies has shown excellent sensitivity and specificity, as discussed later in this review.18 It has been shown that the increase in diagnosis rates for CD can be attributed to greater awareness in Primary Care, where practitioners are ideally placed to assess patients for the multisystem manifestations of the disease.19

A small bowel biopsy is still the gold standard for the diagnosis of CD. The interpretation of the histological lesions is based on criteria proposed by Marsh20 and modified by Oberhuber.21 This classification is used both for diagnosis and for assessment after the introduction of a GFD. Type 1 shows infiltrative lesions, characterized by normal mucosal architecture but with an increase in intraepithelial lymphocytes (IELs). Type 2 is characterized by hyperplastic lesions and increase in crypt depth without villous flattening. Type 3 a shows mild villous flattening and crypt hypertrophy, type 3b marked villous flattening with crypt hypertrophy and in type 3c the mucosa is completely flat with crypt hypertrophy. Type 4 shows hypoplastic lesions characterized by villous atrophy but with normal crypt height and IEL count. Recently, a simplified grading system (which excludes Marsh type 4) has been suggested to minimize disagreement between pathologists and facilitate comparison between serial biopsies in the follow-up of treated patients.22 The authors suggest that the lesions characterizing CD should be divided into non-atrophic lesions, Grade A (Marsh/Oberhuber type 1 and 2), and atrophic lesions, Grade B, with type 3 a and 3b classified as Grade Bl and type 3c as Grade B2.

The pathogenesis of coeliac disease

As previously stated, the development of CD involves interactions between environmental, genetic and immunological factors.

Figure 1 General algorithm for evaluation of patients with suspected coeliac disease. Detailed recommendations for serological tests (highlighted here in grey) are shown in Figure 2 (TGA = tissue transglutaminase antibody, EMA = endomysial antibody)

Environmental factors: dietary cereal proteins

Cereals are an important source of nutrients for humans; however, in patients with CD they represent the external trigger for the disease. The storage protein fractions of the endosperm of cereal gra\ins can be classified by their solubility properties. The alcohol soluble fractions are collectively designated as prolamins due to their high content of the amino acids, proline and glutamine. Depending on the cereal, they have been termed gliadin (wheat), secalin (rye), hordein (barley) and avenin (oats). It is generally accepted that prolamins are the major triggering factors in CD.23 Wheat gluten contains two major protein fractions, gliadins and glutenins, both of which are active in causing the disease.24,25

Wheat, barley and rye have a common ancestral origin in the grass family. Oats are a more distant relative of wheat, barley and rye and are now considered to be toxic in only a minority of patients with CD. Wheat, barley and rye are derived from the Triticeae tribe of grasses, whereas oats come from a different tribe (Aveneae) but from the same subfamily (Pooldeae.) In contrast, rice, maize, sorghum and millet are more distantly related and do not activate CD.25 The very high glutamine and proline content in gliadin, secalin and hordein may be one of the factors in the pathogenesis of CD, rendering these proteins relatively resistant to proteolytic digestion by intestinal enzymes26 resulting in large peptides of high glutamine and proline content.

Genetic factors

Inheritance plays a major role in the pathogenesis of CD. Concordance in monozygotic twin pairs is between 70% and 75%; there is also an increased prevalence of CD within affected families (5- 15%). There is a strong genetic predisposition, with the major risk attributed to genes of the major histocompatibility complex class II antigens human leukocyte antigen (HLA) DQAl *O501-DQB1 "02 (DQ2) and HLA-DQA1 "0301-DQB1 "0302 (DQ8). These genes, which are located on chromosome 6p21.3, are present in nearly all patients with CD. The HLA-DQ2 heterodimer that confers CD susceptibility is formed by a beta chain encoded by the allele DQB1 "02 (either DQBl "0201 or *0202) and an alpha chain encoded by the allele DQA151 *05. DQ2 is present in 90% or more of CD patients. The HLA-DQ8-associated heterodimer is present in the remaining 5-10% of patients and is formed by the beta and alpha chain encoded by DQB1H*0302 and DQA1 *3. The DQ2 susceptibility alleles can be inherited in the cis form (both alleles coming from the one chromosome) or the trans form (one DQ allele coming from a chromosome of each parent).

Nearly all patients with CD carry these HLA genes; however, about 30% of the population carry these alleles while only 1 % develop CD, suggesting that other secondary genetic effects are also involved. Other susceptibility loci include genes involved in the immune response. These include the cytotoxic T lymphocyteassociated protein- 4 (CTLA-4) gene (although studies of this gene in CD have led to inconsistent results), the CD28 gene, inducible T-cell co- stimulator (ICOS) gene and other immune-associated genes found at the 5q31-q33 region. So far, extensive searches for additional genes have identified other candidate genes, but these have much weaker associations than the genes of the DQ loci.1,27

Immunological factors

Despite advances in the understanding of the pathogenesis of CD within the last decade, our knowledge of the immune recognition of cereal peptides and their effect on the immune response is still incomplete. In CD, it would appear that two pathways may be involved: the first is due to the direct toxic effect of cereal peptides on the gut mucosal epithelium involving cells of the innate immune system. Toxic peptide fragments induce mucosal damage when added in culture to duodenal mucosal biopsies and also lead to mucosal damage to the proximal and distal intestine when given in vivo.28 IELs are also known to be involved in the pathogenesis of CD. It has been suggested that a combination of IELs, IL-15 and up- regulated epithelial cells may lead to epithelial cell damage.29

The other pathway involves immunogenic peptide fragments which are capable of specifically stimulating HLA-DQ2/DQ8-restricted T- cell lines and T-cell clones derived from the jejunal mucosa or peripheral blood of patients with CD.30 Some of these immunogenic peptides are immunodominant (i.e. they are able to elicit a strong T- cell response). Although peptides are usually toxic or immunogenic, some peptide fragments can elicit both responses.

The identification in 1997 of the enzyme tissue transglutaminase- 2 (tTG) as the antigen within the endomysium (the connective tissue around smooth muscle) against which the endomysial antibody is directed has led to a greater understanding of the pathogenesis of this disorder.31 tTG is found in almost all cell types where it is usually retained intracellularly in an inactive form. When cells are exposed to mechanical or inflammatory stress, tTG is released into the extracellular space and associates with the fibronectin- containing extracellular matrix. tTG belongs to a family of at least eight calcium-dependent transamidating enzymes that catalyze the covalent and irreversible cross-linking of a protein containing a glutamine residue to a second protein with a lysine residue. In certain conditions (i.e. when no primary lysines are available), tTG merely deamidates a target glutamine in a substrate protein to a negatively charged glutamic acid. Gliadin peptides are substrates for tTG. However, the peptides that bind DQ2 or DQ8 in the case of CD are presumed to be glutamine/proline-rich peptides that remain following the intestinal digestion of dietary gluten. These gluten peptides lack the negatively charged amino acids that are preferred for binding to the disease-associated DQ2/DQ8 heterodimers. Therefore the absence of binding is unlikely to activate disease- relevant CD4 + T cells. tTG can deamidate glutamine, converting it to negatively charged glutamic acid. The enzyme acts on selected glutamines within the glutamine/proline-rich gluten peptides and some gluten peptides become better binders to the disease relevant DQ2/DQ8 molecules after deamidation. Once bound to DQ2/DQ8, these DQ- gluten peptide complexes have been shown to activate DQ2/DQ8 restrictive T cells, which can be isolated from the small intestine of patients with CD.

Crystallization studies of the DQ2 molecule containing a deamidated gluten peptide in the peptide groove revealed that the DQ2 molecule contains several pockets that favour the binding of negatively charged residues, such as those found in gluten peptides when glutamine is deamidated to glutamic acid. Such DQ-gluten complexes can efficiently activate T cells in the lamina propria of the intestinal mucosa. It is clear that gluten contains a myriad of peptides that after modification can bind to HLA DQ2/D08 and trigger T-cell responses. As mentioned previously, immunodominant epitopes of gliadin have been recognized. A 33 amino acid peptide derived from α-gliadin has been reported to have immunodominant characteristics; this 33-mer is extremely resistant to digestion by gastric, pancreatic or brush border enzymes and is readily available for T-cell recognition and activation.32

The interaction between gliadin fragments and tTG does not, however, explain why the disease presents at a specific point in time in the genetically susceptible individual who has been consuming the potentially toxic proteins and peptides for most of their life. It may be that partially digested gluten peptides cross the epithelial barrier to gain access to the subepithelial region. This uptake may be facilitated by a proinflammatory trigger such as infection or increased permeability of the mucosa. Infection may cause the release of tTG during tissue repair; therefore rendering the glutamine- and proline-rich peptides crossing the epithelial barrier suitable for binding to DQ2/DQ8 molecules on antigen presenting cells. Activation of CD4 + T cells would release y- interferon and activate the release of enzymes, such as matrix metalloproteinases, that damage the intestinal mucosa with the resulting loss of villous structure and crypt epithelial hyperplasia seen in CD. In CD, native or tTG-deamidated gluten peptides are presented by mature dendritic cells via HLA-DQ2 or HLA-DQ8 toT-cell receptors, causing activation of CD4 + T cells, resulting in immune activation. The stimulated CD4 + T cells are also able to induce differentiation of B cells into plasma cells producing specific anti- gliadin and anti-tissue transglutaminase (TGA) antibodies.30

Dermatitis herpetiformis (DH) is a skin disease characterized by granular IgA deposits in the papillary dermis. The disease affects 10-20% of CD patients, usually occurring in patients with latent or mild disease.33 The small bowel histology is characterized by a more patchy distribution of villous atrophy with usually milder pathology. The antigenic specificity of the skin-bound IgA deposits has recently been found to be against epidermal transglutaminase (TGe or TG 3). Antibodies to tTG can also be detected in DH patients. It has been suggested that DH should be considered as the skin manifestation of gluten sensitivity in those patients with mild coeliac disease, who produce IgA antibodies to TGe.34

Background to serological tests

Serological tests for CD fall into two groups: those which detect antibodies to α-gliadin, the anti-gliadin antibodies (AGA); and those which detect autoantibodies (i.e. antibodies directed against endogenous antigens) - anti-reticulin (ARA), anti-endomysial (EMA) and TGA antibodies. Historically, the IgA antibodies giving a specific immunofluorescent staining pattern (Rl) on reticulin in rodent tissue were the first to be identified as associated with CD.35 Antibodies to several wheat gliadin fractions have been described and detection systems with α-gliadin as the antigen were widely used after 1980; these were principally quantitative enzyme-linked immunosorbent assays (ELISA). Performance was variable due tolack of standardization of the antigen used to coat the plates and the use of different solvents for the antigen selected.36 Additionally, some assays employed bovine serum albumin (BSA) as a blocking agent, which should be avoided in assays intended for the detection of CD as any disorders leading to increased gastrointestinal permeability may be associated with the development of serum antibodies to BSA (and other food proteins).37

Table 1 Approximate sensitivities and specificities of IgA-class antibody tests for detecting coeliac disease

In 1984 the detection of an IgA class antibody by indirect immunofluorescence to antigens present in monkey oesophagus was shown to have high sensitivity for CD;38 coupled with remarkable specificity, it proved to be an almost ideal diagnostic test.39 The antibody stained the structure around the muscle fibre bundles and was therefore named endomysial antibody (EMA). The recognition, in 1997, of tTG as the antigen for EMA led to the development of TGA as a further diagnostic test. Table 1 compares the approximate sensitivity and specificity for detecting CD for these four IgA antibodies.

Immunofluorescent staining on rodent tissue is still used in some laboratories as part of a general autoimmune screening profile; a positive ARA is therefore often an opportunistic finding and its detection should lead to the initiation by the laboratory of more specific tests for CD. In this context, the significant Rl-ARA are predominantly IgA class and will be missed if only antiIgG conjugates are used in screening and IgG class Rl-ARA are absent. The wider availability and use initially of AGA and then of EMA and TGA has contributed significantly to the increased awareness of the high prevalence of CD and to the concept of the 'coeliac iceberg'as previously described.12 Population screening has been advocated by some as a way of detecting a larger proportion of cases,40 but there is wider support for the screening of high risk groups, such as children with type 1 diabetes mellitus in whom the prevalence of CD is about 5%.41 There may also be a role for evaluating the HLA DQ2/ DQ8 alleles in such children;16 those without the appropriate alleles can be reassured with this single test of their very low risk of ever developing CD. Those with the appropriate alleles should be tested for CD on a regular basis, for example every five years. However, the value of this approach has recently been questioned because of the high prevalence of these HLA alleles in children with type 1 diabetes without CD.42

In addition to their role in diagnosis, quantitative antibody assays have a role in monitoring compliance with a GFD;43,44 with good compliance, AGA and TGA antibody concentrations fall to within the reference range, indicating mucosal recovery and avoiding the need for follow-up small bowel biopsy. Evidence is accumulating that antibody concentrations are directly correlated with the extent of mucosal injury45 and that in the diagnostic setting, levels of 6-10 times the upper limit of the reference range are diagnostic of CD (positive predictive value [PPV] - 1.0), suggesting that diagnostic guidelines could be revised so that small bowel biopsy is not mandatory when TGA concentrations are above an appropriate cut- off.46,47

Anti-actin antibodies have also been proposed both for diagnosis and for assessing mucosal recovery, although the sensitivity of these antibodies is variable.48,49 Currently, it is unclear if the development of these antibodies is involved in the pathogenesis of the disease or is an epiphenomenon. The presence of antiactin antibodies can lead to false-negative EMA results by preventing the recognition of the EMA pattern on oesophageal sections. This is discussed further in the section on laboratory strategy for detecting CD.

Antigen specificity of serological tests

In CD serum antibodies are produced against a wide range of gliadin proteins,50 and both crude gliadin and purified ot-gliadin have been used as antigens in AGA methods. However, evidence suggests that although gliadin is the external trigger for CD, it is not the pathogenetic antigen.51 The 33-mer peptide described previously has high specificity for deamidation by tTG leading to the hypothesis that the resulting modified peptide, or perhaps an enzyme-peptide complex, is then able to bind to HLA-DQ2 molecules initiating the events which culminate in the flat mucosa typical ofCD.52

The identification of tTG as the antigen for EMA31 had a major impact on our understanding of the antigenie specificity of serological tests for CD, as well as stimulating further research into the pathogenesis of the disease and the development of better diagnostic tests. Further work has confirmed that ARA and EMA are identical antibodies normally measured by substrate- and species- dependent methods33 and distinct from AGA activity.54 It is now evident that both assays detect antibodies to tTG. Dieterich et al.31 showed that adsorption of serum from patients with untreated CD with tTG abolished the characteristic staining pattern of EMA. They also demonstrated that IgA antibodies to tTG could be detected in such serum samples in an ELISA using guinea pig liver tTG as antigen. The subsequent study by Korponay-Szabo53 confirmed tTG as the autoantigen for these antibodies. Using tTG knockout (TG2-/-) mice, these authors confirmed that the EMA and Rl-ARA binding patterns in serum samples from a large group of patients with CD (n = 61) or DH (n = 40) are entirely dependent on tTG and that the tissue distribution of the enzyme (as shown by monoclonal antibodies specific to tTG) co-localized with the IgA-EMA and IgA-ARA binding on wild-type mouse tissue. They were also able to show that the insertion of human recombinant tTG into TG 2-/- mouse tissue provided a substrate for detecting the antibodies that was as good as monkey oesophagus. By demonstrating that the immunofluorescent antibody staining patterns diagnostic of CD were exclusively tTG- dependent, the nature of the autoantigen was confirmed and the basis for a diagnostic test based on a specific antigen was established.

Tissue transglutaminase antibody

The first generation assays for TGA used guinea pig liver tTG as the antigen. Initial studies using selected CD patients and control subjects showed comparable sensitivity to EMA, but subsequent evaluations based on routine clinical use generally showed lower sensitivity and inadequate specificity with consequently low PPV for detecting CD.55,56

Second generation kits use either purified human tTG or human recombinant tTG. Several studies compared human and guinea pig antigens and confirmed that sensitivity and specificity were significantly improved with the human antigen for in-house and commercial assays.57-59 Recombinant tTG may be expressed in a variety of cell systems (e.g. human cell lines, Escherichia coli, baculovirus/eukaryotic cell system) and lack of standardization in antigen preparation might be expected to lead to variability in the antibody epitopes detected.

Although antibody assays based on crude gliadin or α- gliadin have now been superseded, it has been suggested that the use of tTG-gliadin peptide complexes as antigen ('third generation kits') may lead to assays with higher sensitivity and specificity than those based only on tTG as antigen. Experience with these kits is limited, but in a comparative study they showed no advantage over human recombinant antigen kits.60 The personal experience of one of us (SMcM) is of a small improvement in sensitivity but with no increase in specificity. A recent comparative study noted that these kits gave the largest numbers of false-positives in patients with cirrhosis.46

Analytically, TGA has advantages over EMA: it can be easily automated, does not require the use of primate tissue and standardization of individual components of the test is achievable. The question to be answered here is whether TGA using human antigen can replace EMA as a first-line test for diagnosis and for monitoring dietary compliance.

Use of IgA-tTG antibody for diagnosing coeliac disease

The Standards for Reporting Diagnostic Accuracy (STARD) criteria were established as standards for assessing diagnostic accuracy,61 but currently few comparative studies of EMA and TGA fulfil these standards. Too often, estimates of sensitivity are biased because patients have been selected for the gold standard test (duodenal biopsy) on the basis of either EMA or TGA (verification bias), or the selection criteria may omit mild cases of disease (spectrum bias). Assessment of TGA in EMA-positive samples is helpful in comparing the performance of different kits but gives little information on diagnostic accuracy in routine use.

Human-antigen-based TGA kits have shown high sensitivity and specificity in retrospective studies based on selected samples,62 in comparative studies of different kits,45,60,63 and in evaluations of diagnostic use in routine practice.64,65 In their retrospective study, Burgin-Wolff et al.62 assessed diagnostic sensitivity in serum samples from a group of 208 patients with untreated CD; the control group consisted of 157 patients, all of whom underwent small bowel biopsy. Based on the optimal receiver operating characteristic (ROC) curve cut-off, sensitivity was 96% and specificity was 99%. No significant differences in sensitivity or specificity were found when the data were analysed in three age groups ( <2 years, 2-16 years, ≥16 years).

The recent extensive study by Van Meensel45 compared analytical and diagnostic performance for 10 commercial kits which use human tTG as antigen (Table 2). They noted that in general, based on performance on an automated ELISA instrument, the kits showed good analytical performance. One kit showed poor linearity (R^sup 2^ = 0.603); seven of the remaining nine kits showed good linearity (R^sup 2^ = ≥0.95).Within-run coefficients of variation (CV) were also generally acceptable with only two kits having CV>10%. Diagnostic sensitivity was assessed in 70 consecutive patients with biopsy-confirmed CD (without IgA deficiency); selection of samples was based on the final clinical diagnosis rather than the results of another laboratory test (e.g. EMA: 90% of the samples were positive for EMA). Specificity was assessed on results of 50 consecutive disease controls in whom CD had been considered in the differential diagnosis and 20 patients with Crohn's disease; CD was excluded in all 70 by small bowel biopsy. On the basis of optimized cut-offs from ROC curves, sensitivity for the 10 kits varied from 91% to 99% and specificity was 99-100%. The areas under the ROC curves did not differ significantly between kits. They noted considerable discrepancy for some kits between the manufacturers' and ROC- optimized cut-offs, emphasizing the importance of thorough validation of cut-off points. They also recommended that attempts should be made by international professional bodies and diagnostic companies to harmonize TGA assays.

Table 2 Comparison of IgA-tissue transglutaminase antibody kits*

Tesai et al.64 reviewed results in stored serum samples from 426 consecutive individuals undergoing small bowel biopsy over a four- year period to the end of 2000. In all, 250 were from patients with untreated CD, the remaining 176 patients' samples (with no biopsy evidence of CD) were used as controls. Sensitivity and specificity at the manufacturer's cut-off were 91% and 96%, respectively, with a PPV of 87%. The sample population, with the high prevalence of CD of 59%, was not representative of samples normally received by a diagnostic laboratory and could favourably bias the PPV However, a prospective study65 that reviewed results from consecutive samples routinely received from 1554 subjects to exclude a diagnosis of CD found a similarly high PPV (85%). The prevalence of CD was 2.8% in these patients in whom the diagnosis was being considered. Sensitivity based on 75 consecutive new adult diagnoses with both small bowel biopsy and TGA results was 92% (specificity was 99%); the sensitivity of EMA in this series was also 92%.

A recent systematic review66 (to the end of 2003) reported a pooled specificity for EMA (monkey oesophagus or human umbilical cord as substrate) of close to 100% in adults and children, compared with 95-99% for human recombinant TGA. The pooled sensitivities in children and adults for EMA (monkey oesophagus) were 96% and 97% and for human recombinant TGA, 96% and 98%, respectively. The sensitivity in adults for EMA (umbilical cord) was lower (90%). A further literature review67 also concluded that EMA and TGA were both highly sensitive and specific for identifying individuals with CD; they also observed no differences in diagnostic performance for children and adults.

The weight of evidence therefore shows that human recombinant TGA kits have adequate sensitivity and specificity for the detection of CD in children and adults and can replace EMA as a first-line test. Not all kits perform to the same high analytical or diagnostic standards, and kits that have been shown in comparative studies to under-perform should not be used.

Use of IgA-tTG antibody for monitoring dietary compliance

As EMA and TGA are identical antibodies, it is not surprising that concentrations of both change in parallel on commencement of a GFD or following a gluten challenge.62 TGA has also been assessed as an early marker of response to and/or compliance with a GFD. In 18 subjects judged by dietetic assessment to be compliant with their GFD, TGA declined from diagnostic levels over the first 5-17 weeks with an apparent half-life of about four weeks.68 The time taken for TGA levels to return to normal will therefore depend on the level at diagnosis as well as on dietary compliance.

From the comparative study of 10 kits,45 there is evidence that kits may vary in their ability to detect an early response to withdrawal of dietary gluten. This report compared the TGA concentrations for the diagnostic sample from a single patient with samples taken at three months and one year after the introduction of a GFD. All kits showed a similar and large decrease after 12 months on a GFD, but large differences were noted in the percentage decrease at three months (10-90% decrease), suggesting that kits may differ in their value for monitoring the early response/compliance with a GFD. Further work is required to confirm this observation.

TGA can be used to monitor longer-term (12 months or more) response to a GFD, but further work is required to better understand the reasons for the apparent discrepancy between kits when assessing the response after three months on a GFD.

False-positive and false-negative IgA-ITG antibody results

False-positive results

Vecchi et al.69 noted a high prevalence of false-positive TGA results in a group of patients with chronic liver disease. EMA was negative in these patients, all of whom had raised serum immunoglobulin concentrations. Hill et al.65 commented on three patients with raised TGA concentrations who were EMA-negative. Two of them had raised concentrations of polyclonal IgA (8 and 17 g/L) as a consequence of chronic liver disease. Duodenal biopsy in one of them with alcoholic liver disease showed no histological evidence of CD. CD may present as a'transaminitis'and in many centres testing for CD will be included in the profile of tests carried out on patients attending clinics because of persistent abnormalities in liver function tests. We have seen further similar cases with slightly raised TGA values and negative EMA and all with total IgA > 4 g/L. Wide variability in the performance of TGA kits for samples from 54 patients with cirrhosis was noted with positivity varying from 0% to 33% in the 11 kits.46 For nine of the 11 kits, the false- positives fell in the range of up to four times the CD cut-off; for the other two kits, the false-positives were up to six and >10 times the CD cut-off (Table 2). We have also seen samples from two patients in whom TGA was requested during investigation for anaemia with slightly raised TGA concentrations and negative EMA; both were subsequently found to have myeloma and high monoclonal IgA concentrations. False-positives for other specific IgA antibodies in patients with IgA monoclonal gammopathy have previously been reported.70

It is likely that the false-positives in these two groups of patients are due to different mechanisms. In chronic liver disease, the high polyclonal IgA will reflect increased concentrations of many specific IgA antibodies, including TGA. For the myeloma patients, the extent of the interference is variable and probably related to the extent of non-specific binding of the monoclonal IgA to the ELISA wells.

In routine practice, these are interferences that need to be recognized and appropriate reflex tests (e.g. total IgA concentrations and EMA) initiated by the laboratory when indicated (see 'laboratory strategy' below). The possibility of false- negative EMA results due, for example, to the presence of anti- actin antibodies, should also be considered when reviewing discordant TGA and EMA results. It may also be helpful to check such samples for Rl-ARA; their presence in this situation will help to confirm that the TGA result is a truepositive. For most diagnostic laboratories, such samples will form a very small proportion of all samples tested and do not detract from the high diagnostic accuracy of TGA. However, when unrecognized, for the individual patient these false-positives may lead to diagnostic delay and to unnecessary duodenal biopsy.

False-negative results

Neither EMA nor TGA has 100% sensitivity for the detection of CD. As shown previously, both have equivalent sensitivity in adults and children, and at diagnosis, some patients have the characteristic histological features but are antibody-negative by current tests. Some of these patients will be AGA-positive while remaining EMA- and TGA-negative. However, the low specificity (high false-positive rate) of AGA negates the value of the systematic use of both AGA and TGA to detect such patients. It has been calculated that for a sample population with a prevalence of CD of 5%, the PPV for CD of a raised AGA with a negative TGA is less than 2%, which is too low to justify small bowel biopsy in all such patients.45

We need to acknowledge that there will be a small percentage of patients with CD, who will not be detected with the current tests. When there are strong clinical grounds for suspecting CD, then duodenal biopsy should be performed in serologically negative subjects. It should be noted that biopsies may also occasionally give false-negative results, probably due to the patchy nature of the small bowel lesions. Patients with positive EMA and normal duodenal histology may progress to having symptoms of CD and will respond to a GFD.71 When EMA and/or TGA results are unequivocally positive, then in appropriate patients, the clinical response to a GFD may assist in the diagnosis.

Detection of selective IgA deficiency and diagnosis of CD

In an Italian multicentre study,72 2.6% of subjects with CD were found to have selective IgA deficiency (IgAD) compared with a prevalence of IgAD in the general population of about 0.2%. There is an increased incidence of HLA-DQ2 in IgAD and therefore an increased risk of CD.73 The testing strategy for CD must therefore ensure that such patients are identified and that the absence of IgA antibodies is not interpreted as a normal finding. In a quantitative ELISA procedure for IgA class AGA with adequate analytical sensitivity, patients with IgAD can be detected by abnormally low antibody concentrations.36 A similar approach can be used with some quantitative TGA procedures,74 although the variable analytical sensitivity of differe\nt manufacturers kits leads to variable overlap between TGA concentrations in IgA-deficient and IgA- sufficient subjects.37 Using the Celikey kit (Pharmacia Diagnostics, Freiburg, Germany), Fernandez et al?5 showed that an absorbance cut- off of 0.022 successfully identified all subjects with IgAD with a specificity of 94% (i.e. total IgA measurement was required in only 6% of samples). This is an aspect of kit design that should be improved; using a kit that discriminates between the abnormally low concentrations of TGA seen in IgAD and the great majority of IgA- sufficient individuals avoids the need to measure total IgA in all samples received for coeliac serology. For laboratories continuing to use EMA as their first-line test, or when using a TGA kit with inadequate analytical sensitivity, measurement of total IgA on all samples is required in order to detect those patients with IgAD.

Once IgAD is identified, appropriate IgG antibodies must be sought. Detection of IgG-EMA may be difficult because of higher background staining, particularly when human umbilical cord tissue is used as substrate, due to non-specific binding masking the recognition of the specific EMA staining pattern. The detection of IgGi antibodies to EMA avoids these problems and has high sensitivity (100%) and specificity (100%) for detecting CD in IgAD.76 These authors also found high diagnostic accuracy for IgG- TGA (sensitivity = 100%, specificity = 96%). A more recent study using human recombinant tTG also reported high sensitivity (98.7%) with no false-positives in a control group of IgAD subjects without evidence of CD by biopsy.77 These authors concluded that 'IgG TGA measurements should be integrated into diagnostic and screening strategies and CD should be considered in all IgAD individuals'. However, in a comparative study of nine IgG-TGA kits,45 using samples from five patients with untreated CD and IgAD, sensitivity was variable between the kits, indicating that further comparative studies are required. The two kits that were positive for all five samples showed the lowest specificities. Specificity was assessed in IgA-sufficient subjects without CD rather than in IgAD subjects without CD.

A laboratory strategy for detecting coeliac disease in children and adults

As there is no evidence that a panel of tests has greater diagnostic accuracy than either IgA-EMA or IgA-TGA, the use of a panel of tests should be abandoned. The strategy we propose uses TGA as the first-line test as it has many analytical advantages over EMA. TGA kits should be selected based on evidence of good analytical and diagnostic performance, supported by clinical studies. The criteria used in kit selection should include linearity, precision, diagnostic sensitivity and specificity (see Table 2) and the ability of the kit to rule out selective IgAD.

Figure 2 Algorithm for use of serum tissue transglutaminase antibody (TGA) for detecting coeliac disease. Please refer to main body of text for Interpretations 1-8 (TGA = tissue transglutaminase antibody, EMA = endomysial antibody)

Figure 2 shows an algorithm for the use of IgA-TGA for detecting CD. IgA-EMA has been retained in this scheme as a second-line test in order to improve the specificity and therefore increase the PPV as previously described.78 This cascade approach will identify those samples with false-positive TGA due to raised serum IgA concentrations. These samples are EMA-negative and usually with TGA results up to 3-4 times the CD cut-off.46 However, the qualitative nature of the EMA procedure inevitably leads to a narrow range of TGA results where TGA is positive (i.e. above the CD cut-off) but where EMA may be positive or negative. In our experience this is from the TGA cut-off level up to about twice that level.65 EMA results in this 'low positive' area vary considerably between laboratories and those laboratories measuring EMA should be confident of their ability to detect EMA in approximately 50% of these samples when TGA is in this low-positive area.

When TGA is positive but EMA negative, serum immunoglobulins should be measured. Significant increases in polyclonal or monoclonal IgA can lead to false increases in TGA. When TGA is more than 10 times the CD cut-off and EMA negative, then either analytical error or a false-negative EMA should be suspected. If a repeat sample (or repeat analysis on the same sample) shows similar results, then duodenal biopsy should be advised as with most kits46 such TGA concentrations are highly predictive of CD. Low concentrations of TGA positivity and negative EMA with a normal serum IgA concentration do not exclude CD and the decision whether to proceed to biopsy or to wait and repeat serology in 3-6 months must be made by the clinician, depending upon symptoms and family history.

If the TGA value is below the cut-off to exclude IgAD, then measurement of serum IgA must be carried out. The proportion of samples requiring IgA will depend on the proportion of samples from young children. The majority of those with low TGA concentrations will be young children with appropriate IgA concentrations for age. In adults, low IgA concentrations due to secondary IgA deficiency (e.g. IgG myeloma) should be followed up appropriately. Selective IgAD is indicated by serum IgA of < 0.05 g/L with normal IgG and IgM concentrations. We suggest that all samples in which selective IgAD is identified should be referred to the National Blood Service for follow-up tests including checking for antibodies to IgA. IgG-TGA (by an acceptable method) should be carried out on samples with selective IgAD. IgG^sub 1^-EMA is an acceptable alternative and may be more suitable for laboratories processing smaller numbers of samples.

Interpreting the results

The subheadings in this section refer to Figure 2.

Interpretation 1: The quantitative result should be reported with the reference range. If the clinical details on the request form indicate that the patient may have reduced their wheat intake (e.g.'symptoms after eating bread'), then a comment such as the following (based on the recommendations of the Primary Care Society for Gastroenterology79) should be added:

Subjects must have been on a diet containing adequate gluten (4 slices of bread daily) for 6 weeks prior to the test. Negative results do not exclude CD if the patient has significantly reduced their wheat intake.

Interpretation 2: These results should be reported as strongly suggestive of CD. If the request is not from a gastroenterologist/ paediatrician, then a comment should be added that referral to a gastroenterologist/ paediatrician for further assessment is appropriate and stating that dietary gluten should not be reduced prior to such a referral.

Interpretation 3: The report should indicate that while not excluding CD, this pattern of results is more likely due to a falsely increased TGA due to the high serum IgA concentration.

Interpretation 4: If the high TGA and negative EMA are confirmed, the report should indicate that these are unusual results but that this TGA level has a high predictive value for CD, referral for further assessment is appropriate.

Interpretation 5: Small increases in TGA and negative EMA, without an increase in serum IgA, do not exclude CD. Patients with this pattern of results should be reviewed by the attending clinician and tests repeated in 3-6 months or referred to a gastroenterologist/paediatrician if there is a family history or other clinical indications of CD.

Interpretation 6: These results should be reported as for Interpretation 1. If further results are to follow, this should be indicated on the report.

Interpretation 7: The requesting doctor should be made aware that the results are consistent with selective IgAD, with a note that IgA class antibodies cannot be used to exclude CD in subjects with this condition. The report should also state that while these results are negative for CD, patients with selective IgAD and significant gastrointestinal symptoms should be referred to a gastroenterologist for further assessment. The report should also state that the sample has been referred to the National Blood Service for further tests in view of the finding of selective IgAD.

Interpretation 8: These results should be reported as for Interpretation 2, with the addition of comments that the results are consistent with selective IgAD and that the sample has been referred to the National Blood Service for further tests in few of this finding.

Conclusions

CD is an eminently treatable condition once the diagnosis has been considered and confirmed. The use of TGA antibodies with the adoption of the testing strategy as outlined here will enable the laboratory to play a full part in the diagnosis and monitoring of this disorder.

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Accepted for publication 10 January 2006

PG Hill1 and SA McMillan2

Addresses

1 Department of Chemical Pathology, Haematology and Immunology, Derbyshire Royal Infirmary, Derby Hospitals NHS Foundation Trust, Derby DE1 2QY;

2 Regional Immunology Service, Kelvin Laboratories, Royal Group of Hospitals, Belfast BT12 6BN, UK

Correspondence

Dr SA McMillan

E-mail: stan.mcmillan@bll.n-i.nhs.uk

This article has been prepared at the invitation of the Clinical Sciences Reviews Committee of the Association for Clinical Biochemistry.

Copyright Royal Society of Medicine Press Ltd. Mar 2006

Source: Annals of Clinical Biochemistry

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