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May 17, 1999

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Study Questions Usefulness of Common Allergy Test

A new study from Johns Hopkins Children's Center may encourage physicians to spare people the discomfort of a skin test to confirm a fairly common diagnosis allergy to cats.

In a study published in the May issue of the Journal of Allergy and Clinical Immunology, Children's Center researchers examined the predictive value of three typical tests to diagnose respiratory allergy to cats: skin prick, intradermal and radioallergosorbent tests. The most common first step in allergy diagnosis is a simple and relatively painless skin prick test (SPT). If the results are negative, the SPT is usually followed by a more uncomfortable intradermal skin test (IDST), in which a needle is inserted under the skin. The third type, radioallergosorbent (RAST), is an in vitro test that exposes a sample of the patient's blood to concentrated allergens.

Researchers studied 120 participants between the ages of 18 and 65, most of whom had other allergies, asthma or both. People who believed they were allergic to cats were recruited by advertisement; allergy clinic patients with asthma and a negative SPT reaction to cat allergen made up the balance of the study group. The participants were evaluated by patient history, SPTs, IDSTs if the SPT came back negative, and RASTs. They were then placed in an allergy "challenge" room with two cats, a bed and a small carpet, and monitored for allergic symptoms.

Results showed positive reactions to the "challenge" in 38 of 41 subjects with a positive SPT, 10 of 39 with a negative SPT, six of 26 with a positive IDST, four of 13 with a negative IDST, 27 of 27 with a positive RAST, and 12 of 44 with a negative RAST.

A positive result from an SPT and a RAST most often indicated allergy to cats, and negative results most often ruled it out. The IDSTs almost invariably confirmed the SPT and RAST indications, leading the team to conclude that IDSTs are the least diagnostically useful or necessary of the three tests.

"These findings lead me to suggest that the routine use of IDSTs in the evaluation of respiratory allergy should be an exception rather than the rule," said Robert A. Wood, M.D., associate professor of pediatrics and the study's lead author.

Although the Children's Center study focused on adults, the implications for children are positive, as the study authors recommend limiting the use of a test that can cause fear, stress and discomfort in young patients. There is also some evidence from an earlier study that supports the same conclusions for diagnosing sensitivity to other allergens, such as grass pollen.

The study was supported by the Eudowood Foundation for the Consumptives of Maryland and a National Institutes of Health institutional training grant.

Co-authors include Robert A. Wood, M.D., Wanda Phipatanakul, M.D., Robert G. Hamilton, Ph.D., and Peyton Eggleston, M.D.

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