COLLABORATIVE STUDY OF SOME SCREENING TESTS FOR DETECTION OF ABNORMAL MILK1

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Abstract
The California Mastitis Test, Modified Whiteside Test, Wisconsin Mastitis Test, tube Catalase Test, Milk Gel Index, and Direct Microscopic Somatic Cell Count (NMC Method) were done according to a detailed protocol in five laboratories. Each laboratory tested about 250 bulk tank milk samples in blind duplicate. Screening tests were compared at various critical scores with respect to their identification of milk samples with cell concentration above (positive) and below (negative) 1.0 and 1.5 million/ml. The percent of positive samples correctly identified is the Utility of the screening test; the percent of negative samples mis-identified is the Cost. At the 1.5 million cells per milliliter limit, and using U.S.P.H.S. recommended critical scores, the mean Cost/Utility estimates were: CMT 68/98, MWT 64/97, WMT 13/84, and CAT 38/94. Lowering the critical score for WMT to > 20 increased its Utility to an acceptable 89% at 17% Cost. At the 1.0 million/ml cell concentration limit the ranking of tests did not change materially. Laboratories varied widely in Cost of screening with all tests, and, particularly for subjectively-scored tests, in the critical score required for equivalent Utility. In all comparisons, the MGI was the test of choice, with the WMT ranking next.

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