Clinically relevant contact allergy to formaldehyde may be missed by testing with formaldehyde 1·0

Summary Background It has been found that patch testing with 15 μL formaldehyde 2·0% aq. detects twice as many allergies as by testing with 1·0%. The clinical relevance of positive patch test reactions is often difficult to determine. Repeated open application 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% formaldehyde (0·60 mg cm−2) but not by 1·0%. Methods Eighteen patients positive to formaldehyde 2·0% but negative to 1·0%, and a control group of 19 patients with dermatitis but without allergy to parabens, formaldehyde and formaldehyde releasers were included in the study. Formaldehyde 2000 p.p.m., the maximum concentration permitted in leave‐on cosmetics according to the EU Cosmetics Directive, was added to a batch of moisturizer preserved with parabens. The same batch without formaldehyde served as a control. The study was double‐blinded and randomized. The patients were provided with both moisturizers 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 moisturizer 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 either of the moisturizers. Nine of 17 formaldehyde‐allergic patients reacted with an allergic reaction to the moisturizer which contained formaldehyde (P