Positive patch-test reactions to para-phenylenediamine, their clinical relevance and the concept of clinical tolerance.

406 patients were patch tested over an 18-month period (January 1998 to June 1999) at a dermatology referral centre. 33 patients (8.1%) had a positive reaction to para-phenylenediamine (PPD). 20 patients were experiencing PPD hair dye dermatitis (present relevance, 61%), 2 were using PPD hair dyes without developing dermatitis (clinical tolerance, 6%), 4 had previously used PPD hair dyes with dermatitis in the past (past relevance, 12%), 3 had previously used PPD hair dyes without dermatitis (past exposure, 9%) and 4 had no known exposure to PPD (unknown relevance, 12%). Of the 20 patients whose test results were of present relevance, 17 avoided PPD hair dyes, resulting in total clearance of the dermatitis in all except 3, who had concurrent endogenous eczema. 3 continued using PPD hair dyes: 2 had recurrent contact dermatitis and 1 avoided dermatitis with meticulous technique. The 2 patients with clinical tolerance continued using PPD hair dyes with no dermatitis.