Evaluation of the overall accuracy of the DeLaval cell counter for somatic cell counts in ovine milk.

The DeLaval cell counter (DCC) is a portable device designed for on-farm somatic cell count (SCC) analysis in bovine milk. This study evaluated the performance of the DCC when analyzing ovine milk. A total of 29 composite ovine milk samples, ranging between 20 x 10(3) and 2,200 x 10(3) cells/mL, were divided into 15 aliquots/milk sample corresponding to 5 SCC methods using 3 types of preservation (unpreserved, azidiol, and bronopol). The SCC methods were the Fossomatic (FSCC), the DCC in undiluted samples, and the DCC in samples diluted 1:1 in 3 different types of diluents (PBS + Triton X-100, PBS + ethidium bromide + Triton X-100, and PBS + propidium iodide + Triton X-100). All analyses were carried out in duplicate. In addition, each sample was analyzed in quadruplicate by the direct microscopic method (DMSCC) using Pyronin Y-methyl green as a stain. Comparison of methods was based on overall accuracy studies (means comparison, repeatability, and regression studies vs. DMSCC and FSCC as reference methods). The DCC methods used to analyze milk samples diluted in staining solution (with ethidium bromide or propidium iodide) showed large coefficients of regression (b = 0.91 to 1.01) and correlation (r > 0.99) when compared with the DMSCC and FSCC methods. In these samples the DCC gave repeatability values (s(r) = 33 to 48 x 10(3) cells/mL) similar to the DMSCC (s(r) = 36 x 10(3) cells/mL), and their log SCC means (5.52 to 5.54) did not differ from the reference value (5.54). However, undiluted samples analyzed by the DCC method showed large standard deviations of repeatability and SCC values lower than those by the DMSCC or FSCC methods, probably because of the high solids content in ovine milk. The type of preservation did not affect the outcomes. Consequently, the DCC was determined to be accurate when analyzing diluted ovine milk based on comparison with the SCC reference methods.