Allergic contact dermatitis from iodine preparations: a conundrum

Iodine preparations are widely used antiseptics, yet limited information exists on their irritant potential and threshold for diagnostic patch testing. We examine this issue by using iodine in different preparations and concentrations. A total of 24 fair-skinned, healthy volunteers without a history of iodine allergy, ranging in age from 18 to 65 years (mean age 49.5 + 10.7 SD), were recruited. Concentrations of 0.5%, 1%, 5% and 10% iodine in petrolatum (pet.), 0.5%, 0.75% and 1% iodine in 70% isopropyl alcohol (IPA) and 1%, 5%, 7.5% and 10% of povidone-iodine (PVP-I) were applied for 2 days to the intrascapular area on the back or to the volar forearm between cubital fossa and wrist using Finn Chambers on Scanpor. Test sites were read 2 days (D2) and 4 days (D4) after patch application. Skin reactions were graded according to the following scheme: 0 = no reaction, + = questionable erythema, 1 = definite erythema, 2 = erythema and induration and 3 = vesiculation. Mild-to-moderate reactions (+ to 2) were observed in 75% of the subjects patched with 5% iodine in pet. at 2-4 days after application. Almost all subjects reacted to 10% iodine at D2 and D4, with 65% exhibiting erythema and induration or vesiculation. A large number (33%) of the subjects developed some reactions to the low concentration (0.5%) of iodine in 70% IPA at D2. Vesicles were seen in 54% of the subjects patched with 1% iodine in 70% IPA at D4. Only 1 subject reacted to 7.5% and 10% PVP-I. Iodine can be irritant to normal skin in pet. and in 70% IPA. Pet. possibly enhances skin contact with iodine, thus increasing its irritant capacity. Alcohol removes sebum from the skin surface, and it might increase iodine penetration into the skin, causing a higher degree of irritation. PVP-I is relatively non-irritant, because its iodine is complexed in an iodophor. For diagnostic patch testing, we recommend using iodine at less than 1% in pet. and at less than 0.5% in 70% IPA. For PVP-I, 10% appears non-irritant. With the variation in patch-test irritant response, interpretation of the patch-test response in the light of clinical history is mandatory.