CD33 Be aware of the decrescendo patch test trend

Abstract

Patch testing is the gold standard for the diagnosis of allergic contact dermatitis. Some haptens are known to be more irritating than others, which leads to difficulty in interpreting irritant, doubtful and weak positive reactions. We aimed to explore whether trends in the strength of patch test readings correlated with the interpretation of relevance of patch test results. A retrospective chart review was conducted at our tertiary referral patch testing centre of patch test reactions to titanium oxalate hydrate (TO) from January 2019 to December 2023, and benzalkonium chloride (BAK) from January 2014 to December 2023. Data on patch test trends, strength of reactions and relevance of reactions were collected. In total 77 reactions were recorded for titanium oxalate hydrate A decrescendo pattern was observed in 30 of 77 (39%) reactions from day 2 to day 4. None of these reactions were deemed of relevance. Of the 27 reactions that had a crescendo pattern from day 2 to day 4, 7 of 27 (26%) were of relevance, whereas 2 of 15 (13%) of the positive reactions that remained unchanged from day 2 to day 4 were found to be relevant (P = 0.02). For BAK, a decrescendo pattern was observed in 109 of 396 (27.5%) reactions from day 2 to day 4 and only 10 of 109 (9.2%) were deemed of relevance, whereas 36 of 206 (17.5%) crescendo reactions were deemed to be relevant (P = 0.006). It can be difficult to interpret patch tests from haptens that are also known irritants. A decrescendo pattern is significantly more likely to be seen in irritant reactions and less likely to be associated with relevance for both titanium oxalate hydrate and BAK. It is important to be aware of the evolution of patch test reactions over time.