Allergy to phosphorus sesquisulfide: 0.5% pet. is a more appropriate concentration for patch testing.

Patch testing was performed with phosphorus sesquisulfide P4S3 in 2 groups containing equal numbers of patients using different concentrations (0.5% P4S3 in pet. and 1% P4S3 in pet., the usual suggested test concentration as recommended by the International Contact Dermatitis Research Group). We found that there was a statistically significant increase in the number of clinically irrelevant irritant reactions in the group tested with the concentration (chi 2 = 16, p < 0.0004). We recommend that patch testing with phosphorous sesquisulfide should be at a reduced concentration of 0.5% pet.