Photoscratch testing in systemic drug-induced photosensitivity
Because of numerous false-negative results, photopatch testing is seldom relevant in systemic drug-induced photosensitivity. These false-negative photopatch test results can be attributed to the inability of the drug to penetrate into the epidermis. In order to enhance the penetration of the tested drug into the epidermis, some authors proposed to breach the cutaneous barrier. We performed a prospective study comparing photopatch and photoscratch testing. Fifteen patients presenting with a systemic drug-induced photosensitivity, proved by a favourable outcome after discontinuing the drug, were tested. For each drug, photopatch and photoscratch tests were performed. Two-thirds of the patients had negative photopatch and photoscratch tests with the suspected drugs. Photopatch and photoscratch tests were positive and relevant, respectively, in 3 and 4 patients. Photoscratch tests induced more false-positive results due to irritation confirmed on control subjects. Our study proves that photoscratch tests do not change the sensitivity of phototesting.