Objective: For the early detection of udder health disorders in praxi, it is necessary to use a method with high sensitivity. The aim of the pre - sent study was to investigate the sensitivity of a commercial Califor - nia Mastitis Test and a new reagent which was optimised for the de - tection of udder health disorders with somatic cell counts > 100,000 cells/ml. Material and methods: The viscosity of mixtures of test milks with standardised somatic cell counts (50,000-800,000 cells/ml) and test reagents were analysed in vitro. A total of 50 quarter fore - milk samples from cows without signs of clinical mastitis were in - vestigated using both California Mastitis Test reagents. Sensitivity and specificity were calculated. Results: The viscosity of mixtures increased with a higher somatic cell count. At a somatic cell count of 50,000 cells/ml, the mean viscosities were 3.20 mPa*s (commer - cial product) and 2.69 mPa*s (optimised reagent). At a somatic cell count of 100,000 cells/ml the viscosities increased to 3.43 mPa*s and 4.58 mPa*s, respectively. At a somatic cell count threshold of 100,000 cells/ml, marking the transition of the physiological status into a secretion disorder, the sensitivity of the commercial test was 76.7% and that of the optimised test was 97.7%. Conclusion and clinical relevance: The optimised California Mastitis Test allows - compared to the commercial product - the early detection of quarters or animals with high somatic cell counts. The results of the study indicate that the test result has to be evaluated immediately after the California Mastitis Test reaction.