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Background: Methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) and methylisothiazolinone I) are tested to detect contact allergy to these isothiazolinones.

Objectives: To study if an aqueous patch test preparation with MCI and MI in a mix of 0.015% and 0.2%, spectively, detects more contact allergies than the commonly used preparations of MCI/MI in 0.02% . and MI in 0.2% aq.

Methods: A total of 1555 patients with dermatitis in five Swedish dermatology departments were sted consecutively with MCI/MI 0.215% aq., MCI/MI 0.02% aq., and MI 0.2% aq.

Results: The share of contact allergy to MCI/MI 0.215% aq., MCI/MI 0.02% aq., and MI 0.2% aq. varied the test centers between 7.9% and 25.9%, 3.2% and 10.3%, and 5.8% and 12.3%, respectively. MCI/MI 215% aq. detected significantly more patch-test positive individuals than both MCI/MI 0.02% aq. (P < 01) and MI 0.2% aq. (P < .001), as well as either one of MCI/MI and MI (P < .001). In the patients only acting to MCI/MI 0.215% aq., 57.7% were recorded as having a dermatitis that was explained or gravated by exposure to either MCI/MI or MI.

Conclusion: The results speak in favor of replacing the preparations MCI/MI 0.02% aq. and MI 0.2% aq. th MCI/MI 0.215% aq. as the screening substance in the Swedish baseline series, which has been plemented in 2020.