Patch Testing and Photopatch Testing
Because sunscreen allergy may be photoinduced, the evaluation of suspected patients usually includes both standard and photopatch testing. A full discussion of photopatch-testing procedures is beyond the scope of this review. Briefly, prior to patch application, minimal erythema dose (MED) testing is often performed to assess the minimal erythemogenic dose for skin that is not exposed to sunlight. For most fair-skinned individuals, the MED in the UVA range is 20 to 30 J/cm2. Most photoallergens will be defined by smaller doses, however, and the dose of UVA used to irradiate photopatch tests for sunscreen allergy ranges from 1 to 10 J/cm2 in most studies.[4,20,22] MED testing with UVB is often done at the same time to assess general (nonallergenic) sensitivity at this wavelength. Allergen patches are placed on the back, as in standard patch testing, but are done so in duplicate. One set of photoallergens is irradiated with UVA, and the other set serves as a control. If patches on both sides show a positive reaction, contact allergy is confirmed. If only the irradiated side shows a positive reaction, photocontact allergy is present.[20]
Sunscreen allergen concentrations and vehicles used for patch testing and photopatch testing are not well characterized. Hasan and colleagues suggested that in eliciting a photoresponse, the allergen concentration is much more important than the actual dose of UVA; they suggested 5 J of UVA for standard testing.[9] Petrolatum is most often used as the diluent for sunscreen antigens; however, the bioavailability of the antigens tested may be limited with the use of this vehicle. For this reason, ethanol has been suggested for detecting PABA sensitivity.[21] Other problems inherent in photopatch testing include the "masked patch test" (so called when the patient is so photosensitive that the UVA dose alone will elicit a reaction), irritant reactions, inadvertent sensitization, and false-positive or false-negative reactions owing to the selection of an inappropriate type or amount of light.[4,22]
Dermatitis. 2006;17(1):3-11. © 2006 American Contact Dermatitis Society
Cite this: Sunscreen Allergy: A Review of Epidemiology, Clinical Characteristics, and Responsible Allergens - Medscape - Mar 01, 2006.
