Clinically relevant contact allergy to formaldehyde may be missed by testing with formaldehyde 1.0%.

Background: It has been found that patch testing with 15μl formaldehyde 2.0% aq detects twice more allergies than testing with 1.0%. The clinical relevance of positive patch test reactions is often difficult to determine. Use tests are simple to do and help to evaluate the significance of patch test results. Objectives: To study the clinical relevance of contact allergy to formaldehyde detected by 2.0% (0.60 mg/cm²) but not by 1.0%. Patients/Methods: 18 patients positive to formaldehyde 2.0% but negative to 1.0% and a control group of 19 dermatitis patients without allergy to parabens, formaldehyde and formaldehyde releasers were included in the study. Formaldehyde 2000 ppm, the maximum concentration permitted in leave-on cosmetics according to the EU Cosmetics Directive, was added to a batch of a moisturiser preserved with parabens. The same batch without formaldehyde served as control. The study was double-blinded and randomised. The patients were provided with both moisturisers and instructed to apply one of them twice a day on a marked-out 5×5 cm area on the inside of one upper arm and the other moisturiser on the other arm. Reading of the test sites was done once a week for a maximum of 4 weeks. Results: In the control group there were no allergic reactions to any of the moisturisers. 9 of the 17 formaldehyde-allergic patients reacted with an allergic reaction to the moisturiser which contained formaldehyde (p<0.001). No positive reactions were observed to the moisturiser without formaldehyde. Conclusions: Our results demonstrate that contact allergy to formaldehyde 2.0% may be clinically relevant.