Somatic cell count (SCC) represents the total number of somatic cells in milk and is used as an indicator of udder health and milk quality worldwide. The parameter is used for regulatory purposes as well as for the management of animal health on farms. The current microscopic-based reference method for SCC has been described as tedious, cumbersome and challenging to work with. Hence, the objective of this work is to define the criteria for reference methods, map all available methods for determination of SCC and to recommend one or two candidates that could potentially be used as a new reference method. The criteria for reference methods defined in this work were that the methods need to be robust, highly reproducible, well-established in the field of cell counting, open and adjustable, and to measure the same as the current method but with better performance. Numerous methods for the determination of SCC are available. They were described in detail and categorized as follows: 1) Microscopic counting (manual), 2) Microscopic counting (automated), 3) Flow cytometry (dedicated, open systems), 4) Flow cytometry (adjustable, open systems), 5) Image cytometry (fluorescence microscopy, dedicated, open systems), 6) Image cytometry (fluorescence microscopy, adjustable, open systems), 7) Impedance flow cytometry, and 8) Other, indirect methods. To be able to compare the performance of the different methods, a feasibility study was performed. The results revealed that the current reference method is not fit for purpose. The results further indicated that automated microscopy, open and adjustable variants of flow and image cytometry, respectively, are potential candidates for the reference method. However, detailed protocols on how such methods could be used routinely for reference testing purposes do not yet exist. In a next step, such protocols need to be developed and subsequently proficiency and ring trial data are to be generated so that the actual performance of the methods can be determined.
