The California mastitis test: what is the value?

The California mastitis test is a widely used cow-side test for the detection of subclinical mastitis in dairy cows. It is easy and cheap to perform on farm, with its use therefore widespread around the world. The test is performed by adding a reagent to a milk sample, agitating the sample and then evaluating the reaction by using a recognised scoring system. Each quarter of an udder is tested by drawing milk into a paddle where the test can be performed. The test can also be performed in other ways, such as on a composite sample or through in-line testing in a milking parlour or automated milking system. The threshold of the test is approximately 400 000 cells/ml. This makes the test useful for determining the infected quarter following a high somatic cell count composite test result, diagnosing subclinical mastitis in a quarter at dry off or assisting with the diagnosis of clinical mastitis, eg in a toxic cow. The limitations of the test are that it is not quantitative, there is variability in operator interpretation and the exact threshold of detection is not known for each testing method. Overall, the test may be a valuable addition to cow-side mastitis diagnosis when used in the appropriate circumstances.