The biocide polyhexamethylene biguanide remains an uncommon contact allergen.

Polyhexamethylene biguanide (PHMB) is used as a preservative in cosmetics and personal care products. Previous studies had shown a frequency of sensitization to PHMB of 0.5% and 0.4% in unselected dermatitis patients. The objective of this study was to determine the current frequency of sensitization to PHMB. From July 2005 until December 2005, a total of 1975 consecutive patients were patch tested with PHMB. 10 patients (0.5%) had positive patch test reactions to PHMB 2.5% aq. (9+ and 1++) and 16 patients (0.8%) to PHMB 5.0% aq. (12 +, and 4 ++). The test reaction pattern (reaction index and positivity ratio) of both preparations and a limited concordance between results from both concentrations (Cohen's kappa = 0.35) are probably indicative of a substantial number of false positive reactions. Potential causal exposures were assessed by a case by case analysis and by referring to surrogate markers of exposure in terms of concomitant reactions. Occupational exposures were identified as a probable cause of sensitization. Further risk factors included leg dermatitis and old age. The frequency of sensitization remains low. It is very unlikely that exposure to cosmetics or personal care products may have played a role in the few cases sensitized.