COMPARISON OF SOMATIC-CELL COUNTING WITH THE FOSSOMATIC-360 AND THE SOMASCOPE

Duplicate samples of mixed milk (n = 13 927) were collected to compare somatic cell counting with the Somascope and the Fossomatic 360. To e stimate sampling and assay variability, somatic cell count (C) was mea sured with the Fossomatic 360 on 295 duplicate milk samples. The total variance was 1042 x 10(6). The contribution of the measuring instrume nt (Fossomatic 360) and the sampling procedure were 12.7 and 87.3%, re spectively. The overall variance of the paired differences of the loga rithmically transformed numerical value of C (expressed in mu L(-1)) ( ln{C}) of the 295 duplicate milk samples all measured with the Fossoma tic 360, was 0.08. Additionally, accuracy of the Somascope was evaluat ed on 9558 duplicate milk samples. In one of the duplicate milk sample s, C was measured with the Fossomatic 360; in the other, with the Soma scope. The variance of the paired differences of ln{C} between the Som ascope and the Fossomatic 360 was 0.14. Comparison of the In{C} per cl ass (class width = 1 on the log, scale) showed 77.4, 28.1 and 11.3% ov erestimation of In{C}, measured with the Somascope, in the classes les s than or equal to 2,2-3 and 3-4, respectively, and 1.4, 2.6 and 4.6% underestimation in the classes 6-7, 7-8 and >8, respectively, indicati ng a lower accuracy of the Somascope in these classes. The Pearson cor relation of ln{C} between the Fossomatic 360 and the Somascope was 0.9 6. The Pearson correlation of In{C} of the duplicate milk samples meas ured twice with the Fossomatic 360 was 0.97. Simple linear regression (Somascope against Fossomatic) showed a significant intercept, b(0) (/-s.e.) = 1.13 (+/-0.011) and the slope, b(1) (+/-s.e.) = 0.803 (+/-0. 0022), differed from the ideal value of 1 (P < 0.001). The overall res idual variance of the linear regression analysis was 0.08. With a weig hted non-linear regression analysis, a relationship X(som) = P-1 + X(f os)/(1 + P(2)X(fos)) was calculated, where P-1, is a constant that des cribes the overestimate of C (greatest at lowest values) with the Soma scope, and P(2)X(fos) is a term that reflects the underestimate with t he Somascope at higher values. The overall prevalence of cows with C h igher than 250 x 10(3) mL(-1), based on the results of the Fossomatic 360 and on the Somascope, was 20.2 and 21.9%, respectively. Test agree ment (K), sensitivity and specificity for detecting cows with C > 250 x 10(3) mL(-1) using the Somascope as compared to the Fossomatic 360, was 0.89, 0.95 and 0.97, respectively. The Somascope, when correctly c alibrated, is an acceptable alternative for measuring C for field tria l purposes and for dairy herd improvement programme.