By Will Boggs, MD

NEW YORK (Reuters Health) - Reading patch tests for allergies beyond the customary 5 days may identify certain allergies missed by an earlier reading, according to a new report.

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.

"Late patch test readings were useful when interpreting reactions to metals and topical antibiotics," Dr. Mark D. P. Davis told Reuters Health.

Davis, from the Mayo Clinic, Rochester, Minnesota, and colleagues reviewed the records in their database to determine which allergens showed positive reactions after day 5.

Of the 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, the investigators report in the Journal of the American Academy of Dermatology.

Only 14 allergens showed 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 negative when checked later. "Substituting a later reading for the day 5 reading might exclude reactions to some allergens such as fragrances," the researchers note.

"I would add rather than substitute (a later reading)," Davis said. "If substituted instead of added, certain patch test reactions may 'dissipate.'

SOURCE: Journal of the American Academy of Dermatology, August 2008.

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