Late reactions in patch tests: a 4-year review from a clinic of occupational dermatology

Late patch test reactions on day (D) 10-14, which on subsequent retesting appear in the normal time schedule on D2-D4, indicate active sensitization due to the patch test. The objective of this study was to report our series of late patch test reactions. We analysed the clinical records of all the patients who had reported late reactions for the date of the late reaction, the results of the first patch tests and retesting. Between January 2002 and February 2006, we had a total of 16 patients with late reactions (>or=D10). 12 of them were retested. Retesting confirmed active sensitization to 1 or more allergens in 11 patients (1.3% of 884 patients patch tested). In these 11 cases, none of the late reactions had clear relevance to the symptoms before its appearance. The commonest allergens were paraphenylenediamine (PPD) in 6 patients (0.72% of 826 PPD tests), elecampane (Inula helenium) and black-eyed Susan (Rudbeckia sp. hybr.) extracts in 3 patients, and 4-aminoazobenzene and Disperse Orange 3 in 2 patients. The late reaction did not generally affect the clinical course. Only 1 patient developed new dermatitis possibly due to active sensitization. Late reactions meeting the classic criteria of active sensitization were relatively common in our series from a special clinic of occupational dermatology.