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Background: Isocyanates with the general formula R-(N=C=O) are theoretically contact sensitizers. However, allergic contact dermatitis (ACD) from isocyanates is seldom reported. In previous reports, patients reacted to their isocyanate-based work materials but not to commercial patch-test preparations of isocyanates. Therefore, we suspected that the low frequency of reported ACD from isocyanates was partly due to inadequate commercial preparations. A past study also showed the concentrations of diphenylmethane-4,4'-diisocyanate (4,4'-MDI) in petrolatum preparations to be much lower than declared. Objective: In this study, 2,4-toluene diisocyanate (2,4-TDI), 1,6-hexamethylene diisocyanate (1,6-HDI), and isophorone diisocyanate (IPDI) were investigated in a similar fashion. Methods: In preparations from 12 dermatology departments and two suppliers of patch-test allergens, we determined the isocyanate content as the isocyanate-dibutylamine derivative, using liquid chromatography and mass spectrometry. The preparations were considered stable if the ratio between the stated and found concentrations was within the range of 0.8 to 1.2. Results: Although 28 of 36 investigated preparations had ratios outside of the stable range, they were in its vicinity, which indicates that preparations of 2,4-TDI, 1,6-HDI, and IPDI are more stable than are preparations of 4,4'-MDI where previously reported results showed ratios far outside of stable range. Conclusion: As opposed to preparations of 4,4'-MDI, preparations of 2,4-TDI, 1,6-HDI, and IPDI can be considered to be stable. (Less)