Some skin allergy tests give late results

Some patients may have delayed allergic patch tests reactions that are not identified during the customary of 5 days reading. The patch tests involve keeping a sample of a suspected allergy-causing substance, or allergen, in place next to the skin, and seeing if a reaction develops. Researchers from America studied to identify which allergens showed positive reactions after day 5. They studied the medical record of patients who had underwent patch testing for suspected allergy contact and returned for delayed readings between 7 to 10 days or beyond. A total of 843 patients reaction were interpreted on day 5 and at least once during 7 days interval. Researchers found that 170 allergens that had been tested in at least 50 patients, 117 had reactions that differed on day 5 when compared with days 7 to 21. Only 14 allergens had significantly higher late positive reaction rates. The allergens with higher delayed positive reactions included some metals, antibiotics, and a few preservatives. On the other hand, some positive reactions on day 5 were found negative when checked later. It has been concluded that late patch test readings were useful when interpreting reactions to metals and topical antibiotics but it was not useful in the diagnosis of reactions to other allergens. Because reactions to certain fragrance and preservative allergens may dissipate after 5 days, patch test reactions therefore are optimally read at days 3 and 5, but an additional reading on day 7 or beyond is useful if patch tests to metals and topical antibiotics are performed.

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