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Rationale 4,4′‐Methylene diphenyl diisocyanate (MDI) is a highly reactive isocyanate used in the production of polyurethanes. Workers exposed to these products may develop sensitization to the diisocyanate compounds, leading to occupational asthma. Quantifying MDI levels is necessary to ensure workplace safety. MDI is metabolized by acetylation and/or conjugation to macromolecules for excretion into urine. All metabolites can be chemically hydrolyzed to form the free diamine 4,4′‐methylenedianiline (MDA) as a urinary biomarker of MDI exposure. Current methods involve long sample preparation, or have been designed using costly automation. There is therefore a need to develop a new practical method for assessing exposure to MDI. Methods Urine samples were acidified and heated to form MDA, followed by neutralization and liquid–liquid extraction. Extracts were separated by reversed‐phase chromatography on a HSS T3 column followed by analysis on a triple quadrupole mass spectrometer in multiple reaction monitoring (MRM) mode. Results 13C15N‐MDA was selected as the internal standard (IS) of choice following an investigation of internal standard stability. The hydrolysis efficiency, forming free MDA from conjugated metabolites in vivo, was evaluated using 4,4′‐methylenebis(acetanilide) spiked into urine and complete hydrolysis occurred after 1 h. A dynamic range of 5 to 500 nM was achieved, and was useful for monitoring MDI exposure considering the biological guidance value (BGV) of 10 μg/L (~50 nM) proposed by the German Research Foundation (DFG). The limit of detection (LOD) and limit of quantification (LOQ) of the method were 0.8 and 2.7 nM, respectively. The intra‐day and inter‐day precisions were 4.33% and 4.27%, respectively. Finally, the method was tested with inter‐laboratory samples from the German External Quality Assessment Scheme (G‐EQUAS) program and the results submitted were all within the allowable tolerance range. Conclusions A practical and validated method for the analysis of small‐ to medium‐sized batches of samples has been developed for the biological monitoring of MDI exposure in human urine.

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