K E Y W O R D S : benzisothiazolinone, contact allergy, methylchloroisothiazolinone, methylisothiazolinone, spot test Contact allergy to isothiazolinones is frequent, especially to methylisothiazolinone (MI) alone and combined with methylchloroisothiazolinone (MCI/MI). While the incidence of MI allergy is decreasing, 1 it remains clinically relevant, in part due to occupational exposures.A spot test kit for detection of isothiazolinones in industrial water systems is commercially available (Tintometer Group) (http://www. lovibondwater.com/product/isothiazoline.aspx). This spot test was recently used to identify a clinically relevant but undeclared exposure to MI in the raw material of a facial gel mask, 2 suggesting the clinical value of the test.The manufacturer states that the test can detect isothiazolinones down to the level of 0.75 ppm; however, it is unclear whether this applies to the most relevant isothiazolinones responsible for contact allergy. We examined the sensitivity of this spot test against 3 common isothiazolinones, and report two clinical cases where application of the test helped us identify MI and MCI/MI sources in allergic patients. SPOT TESTThe spot test kit for isothiazolinones (Tintometer Group) was tested against 3 common isothiazolinones/mixtures: MCI/MI (CAS no. 55965-84-9); MI (CAS no. 2682-20-4); and benzisothiazolinone (BIT) (CAS no. 2634-33-5). The spot test is based on a colorimetric method, where the presence of isothiazolinone derivatives results in a blue color change. The exact basis of the test method is company proprietary information and the analytes used come in a coded form. A reading scale is provided (Figure 1). The detection limit of the test as declared by the manufacturer is 0.75 ppm, where the solution gets a blue undertone, and ends at 7.5 ppm, where it becomes dark blue. The test is negative if no color change occurs, that is, if the solution (in water) remains clear (Figure 1). A range of test solutions corresponding to the reading scale of the spot test (0-7.5 ppm) and added higher concentrations of 10-200 ppm (Table 1) were prepared by using sterile-filtered water (Fresenius Kabi, Bad Homburg vor der Höhe, Germany) and the patch test materials: MCI/MI (allergEAZE, SmartPractice, Phoenix, Arizona) 200 ppm, MI (allergEAZE) 2000 ppm, and BIT (97% Sigma-Aldrich, Copenhagen, Denmark) in w/w% as shown in Table 1. The spot tests were performed with the 5 coded reagents provided in the test kit in accordance with instructions from the manufacturer. The mixing of test solution and reagents takes approximately 5 minutes. The developed color was compared to the test kit standards. Unblinded readings were conducted by one investigator (LA or UFF), approximately 10 seconds after the mixing according to instructions.