Formaldehyde concentration in diagnostic patch testing: comparison of 1% with 2%

Exposure to formaldehyde is common from both consumer products and industry. The reliability of the patch test is essential for the diagnosis of formaldehyde allergy as it is difficult to suspect from the patient's history. The recommended formaldehyde patch test concentration has been reduced over the last decades from 4-5% to 2% and is currently 1%. The changes have not been based upon formal studies, but driven by an intention to reduce irritancy and false-positive results. The aim of the present study was prospectively to compare the outcome of simultaneous testing with formaldehyde 1% and 2% in consecutively patch-tested patients, with respect to frequency of positive patch test reactions, strength of patch test reactions to different formaldehyde test concentrations, irritancy and relevance. The study included 3734 consecutively patch tested patients. 121 gave a positive reaction to 1% and/or 2% formaldehyde in water. There was no statistically significant difference between 1 and 2% with respect to allergic reactions, but 2% gave significantly more irritant reactions. Doubtful, positive and follicular reactions at day (D) 2 frequently developed into true positive reactions at D 3 to 4. Reading at D 7 gave few additional positive cases compared to D 3/4. Problems related to relevance are discussed. Based on present knowledge, a 1% patch test concentration for formaldehyde is recommended.