Patterns of concomitant allergic reactions in patients suggest cross-sensitization between octylisothiazolinone and...

Recent animal tests have indicated cross-reactivity between methylisothiazolinone (MI), octylisothiazolinone (OIT), and benzisothiazolinone (BIT).

To retrospectively analyse concomitant reactions to different isothiazolinones in patients in an occupational dermatology clinic during the ongoing MI contact allergy epidemic.

We searched patch test files (January 2012 to February 2017) for patients with allergic reactions to OIT, BIT, and MI, and analysed their patch test results, occupations, exposure, and diagnoses.

We found 61 (9.4%) allergic reactions to MI, 19 (2.9%) reactions to OIT and 9 (1.4%) reactions to BIT in a total of 647 consecutively tested patients. Seventeen (89%) of the OIT-positive patients had concomitant reactions to MI, whereas only 3 (33%) BIT-positive patients had allergic reactions to MI. The MI reactions in OIT-positive patients were almost exclusively extreme reactions (+++). Exposure to OIT was identified in 26% of the OIT-positive patients, and 56% of the BIT-positive patients had been exposed to BIT.

Allergic reactions to OIT were strongly associated with extreme reactions to MI, which suggests cross-sensitization. In contrast, BIT reactions were mostly independent.