Determination of threshold UV-A elicitation dose in photopatch testing

Photopatch testing, although widely used in the diagnosis of photosensitivity disorders, is not Standardized. We performed this study to determine the threshold ultraviolet tight A (UV-A) dose required to elicit photopatch test responses. 4 patients with previously positive tests were reexposed to the offending allergen, using an incremental dosage regime. Isopropyl dibenzoylmethane (Eusolex 8020), mexenone (benzophenone-10) and oxybeozone (benzophenone-3) produced positive responses at 1.0, 1.0 and 0.7 J/cm², respectively. Responses to phenothiazines were deemed phototoxic. These results demonstrate that high doses of UV-A (e.g., 10–15 J/cm²) are unnecessary, and that 5 J/cm² should become the current Standard.