Some patients may have delayed allergic patch test reactions that are not identified with a typical 5-day reading protocol.

To identify allergens with delayed-positive reactions and to determine whether a late reading (day 7-9) can be substituted for the day-5 reading.

We retrospectively reviewed medical records of patients who underwent patch testing for suspected allergic contact dermatitis from October 1997 through December 2006 and returned for delayed readings between days 7 and 10 or beyond. (This cohort was predominantly patients with suspected allergies to metals and corticosteroids).

A total of 36,064 individual reactions (471 allergens, 372 patients) were interpreted on day 5 and at least once during days 7 through 21. We analyzed the 170 allergens that were each tested in at least 50 patients. The 4 allergens with the highest percentage of delayed-positive reactions were gold sodium thiosulfate 0.5% (delayed-positive reactions in 22/353 patients), dodecyl gallate 0.25% (6/105), palladium chloride 2% (8/194), and neomycin sulfate 20% (10/253). We observed a low number of delayed-positive readings for p-phenylenediamine 1% (1/251) and for corticosteroids. Reactions to certain preservative and fragrance allergens dissipated after the day-5 reading. Most reactions that dissipated after day 5 were mild, whereas the reactions that became apparent after day 5 were strong.

This was a retrospective study. Most patients had suspected allergies to metals and corticosteroids.

Late patch test readings (day 7 or beyond) were useful when interpreting reactions to metals and topical antibiotics, but it was not useful in the diagnosis of reactions to other allergens, including topical corticosteroids. 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.