Contact allergy to propylene glycol. Do we use the appropriate test concentration?

The authors describe a case in which contact allergy to propylene glycol would have been missed if a concentration higher than 10 per cent in water had not been used for patch testing. They suggest raising the patch-test concentration to at least 20 per cent and using 50 or even 100 per cent in doubtful cases, although cautioning that further experience with larger numbers of patients is necessary.