Evaluation of the electrical conductivity of milk as a mastitis indicator

The only cow-side test which can be (has been) applied on each quarter automatically (in-line measurement) is the measurement of electrical conductivity. This has been investigated in many studies worldwide, but with often contradictory results. The discussion of the usefulness of electrical conductivity as a parameter for mastitis detection within the IDF Group of Experts A2 (Mastitis) indicated the need to acquire and to analyse the current knowledge on this topic. However, it has not been the intention of this work to prepare guidelines for the standardization of the measurement of electrical conductivity in milk. The electrical conductivity values in milk of healthy quarters may show a high variability due to influences such as stage of lactation, breed, milking interval, or due to factors related to the general cow status (for example oestrus). Therefore, the distribution of electrical conductivity values in healthy quarters could overlap with the distribution of electrical conductivity values in milk of mastitic quarters. Even if some data suggest that electrical conductivity could be applied to the diagnosis of clinical mastitis, the application of electrical conductivity as a parameter to indicate mastitis by measurements of composite or foremilk samples may not be sufficient in comparison with other mastitis diagnostic parameters. This lack of precision has led to the use of in-line quarter conductivity measuring devices in connection with on-line computer programs. Depending on the thresholds used, it was found that for subclinical mastitis in the case of low thresholds the frequency of false-positives was high (low specificity), while with high thresholds the frequency of false-negatives was high (low sensitivity). To assess the validity of electrical conductivity measurements for the diagnosis of subclinical and clinical mastitis cases meta-analysis of available published data was performed. Meta-analysis suggests that electrical conductivity in milk does not identify mastitis quarters or cows with sufficient accuracy. In conclusion, more detailed information on milk electrical conductivity as a mastitis diagnostic parameter is needed to prepare guidelines for both a standardized measuring method and data interpretation.