January 3, 2012
Be Cautious with Allergy Tests
(Ivanhoe Newswire) — Two leading allergists urge clinicians to use caution when ordering allergy tests and to avoid making a diagnosis solely based on the results.
Robert Wood of the Johns Hopkins Children's Center and Scott Sicherer of Mt. Sinai Hospital in New York warn that blood tests and skin-pricking tests should be used only to confirm suspicion and never to look for allergies in an asymptomatic patient.
Test results, they add, should be interpreted in the context of a patient's symptoms and medical history. If a food allergy is suspected, Sicherer and Wood advise, the patient should undergo a food challenge – the gold standard for diagnosis – which involves consuming small doses of the suspected allergen under medical supervision.
Skin tests and blood tests are proxies that detect the presence of IgE antibodies, immune-system chemicals released in response to allergens. Skin testing involves pricking the skin with small amounts of an allergen and observing if and how the skin reacts. A large hive-like wheal at the injection site signals that the patient's immune system has created antibodies to the allergen. Blood tests, on the other hand, measure the levels of specific IgE antibodies circulating in the blood.
These tests can tell whether someone is sensitive to a particular substance but cannot reliably predict if a patient will have an actual allergic reaction, nor can they foretell how severe the reaction might be, the scientists say. Many people who have positive skin tests or measurably elevated IgE antibodies do not have allergies, they caution
"Allergy tests can help a clinician in making a diagnosis but tests by themselves are not diagnostic magic bullets or foolproof predictors of clinical disease," Wood was quoted as saying. "Many children with positive tests results do not have allergic symptoms and some children with negative test results have allergies."
Undiagnosed allergies can be dangerous, even fatal, but over-reliance on blood and skin tests can lead to a misdiagnosis, ill-advised food restrictions or unnecessary avoidance of environmental exposures, such as pets.
In addition, the researchers caution, physicians should be careful when comparing results from different tests and laboratories because commercial tests vary in sensitivity.
In their report, the scientists say, skin and blood tests can and should be used to confirm a suspected allergic trigger after observing clinical reactions suggestive of an allergy. Also, monitor the course of established food allergies via periodic testing. Levels of antibodies can help determine whether someone is still allergic, and progressively decreasing levels of antibodies can signify allergy resolution or outgrowing the allergy. Skin and blood tests can confirm an allergy to insect venom following a sting that causes anaphylaxis, a life-threatening allergic reaction marked by difficulty breathing, lightheadedness, dizziness and hives. Lastly, skin and blood tests can determine vaccine allergies (skin tests only).
SOURCE: Pediatrics, published online December, 2011