Accuracy of the detection of intramammary infection using quarter somatic cell count when taking parity and stage of lactation of the dairy cow into account

The aim of this study was to determine if taking parity and stage of lactation of the dairy cow into account could improve the sensitivity (Se) and specificity (Sp) of a SCC-test to detect quarter intramammary infection (IMI). A total of 27 315 quarter milk samples were collected from 277 Holstein cows from one experimental herd from 1980 to 1994. Two analyses were carried out. In the first analysis (MIN+MAJ analysis), quarter samples showing at least a single isolation of a minor or a major pathogen were considered as infected. In the second analysis (MAJ analysis), only quarter samples showing at least a single isolation of a major pathogen were considered as infected. The SCC threshold value above which a quarter was defined positive was the one which maximised the Youden index (YI). Se, Sp and YI were calculated on (1) the whole study sample (herd level), and (2) 6 subsamples defined according to parity (primiparous versus multiparous) and stage of lactation (0 to 60 days, 61 to 120 days and 120 to 400 days). Se, Sp and YI were also recalculated at the herd level on the basis of Se and Sp obtained for each subsample. The SCC threshold selected on the whole study sample was 265 000 cells.mL(-1) in the MIN+MAJ analysis and 420 000 cells.mL(-1) in the MAJ analysis. At these thresholds, the Se and Sp values were 71.2% and 75.8% respectively (MIN+MAJ analysis) and 82.9% and 83.6% respectively (MAJ analysis). For both analyses, the SCC thresholds, Se and Sp varied according to parity and stage of lactation. Finally, the results showed that taking the cow characteristics into account had a very limited impact on the accuracy of IMI detection.