Photopatch testing in photosensitive patients

S ir, The British Photodermatology Group (BPG) held a workshop on photopatch testing in November 1995, the findings and recommendations of which were published in the British Journal of Dermatology. 1 A standard series of six potential photoallergens was devised, incorporating agents which were widely used at the time and reported to be a relatively common cause of photocontact allergy. Agents of historical interest such as the antibacterial tetrachlorosalicylanilide, which were no longer clinically relevant, were excluded. From September 1996, we have performed photopatch tests on all patients with suspected photosensitivity who have been referred to the Liverpool Photobiology Unit for diagnostic photoinvestigation; the agents tested and procedure were as recommended by the BPG. 1 We have conducted a retrospective review of the positive results obtained in order to establish which allergens are most often identified as relevant, and in which photosensitive disorders these reactions are occurring. On day 0, the standard series of photocontact allergens ( Table 1) is applied to the patient’s back in duplicate, one set of patches for irradiation and one for control. The minimal erythema responses to ultraviolet (UV) A are also assessed. The patches are removed on the following day, and one set of allergens is irradiated with 5 J/cm2 of broad‐band UVA. Lower doses of radiation are given in those with low erythema thresholds to UVA. The readings are taken 48 h later. Reactions are graded as follows: 0, no response; ±, doubtful response; 1, mild erythema; 2, moderate erythema; 3, erythema and oedema; 4, erythema, oedema and vesicles. Grades 1 and above are regarded as positive reactions.