The effects of three changes in the optical equipment used for Direct Microscopic Somatic Cell Count in milk (National Mastitis Council method) on performance of the method have been measured and subjected to statistical analysis of probable significance. Replicate slides of six milk samples were counted by eight technicians among the authors' laboratories. Three levels of cell concentration were represented among the samples. No consistent differences in estimated cell concentration were demonstrated in the comparisons of narrow versus wide reticle band, wide field versus Huyghenian eyepieces, or low (high dry) versus high (oil immersion) magnification. In a few instances within each comparison, individual counters produced markedly different results when the optical equipment was modified. These individual biases were large and consistent with regard to change in objective magnification. No evidence was found to preclude use of wide field eyepieces in performing the DMSCC.

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Author notes

1A contribution from the Subcommittee on Screening Tests, National Mastitis Council, Inc.

2United States Department of Agriculture, ARS, Animal Science Research Division, Beltsville, Maryland 20705.

3Department of Clinical Pathology, School of Veterinary Medicine, University of California, Davis, California 95616.

4Statens Veterinaere Serumlaboratorium, 4, Odinsvej, DK 4100 Ringsted, Denmark.

5Department of Veterinary Microbiology and Immunology, Ontario Veterinary College, University of Guelph, Ontario, Canada.

6Department of Large Animal Medicine, Obstetrics and Surgery, New York State Veterinary College, Cornell University, Ithaca, New York 14850

7Laboratory Division, State Department of Health, P. O. Box 1689, Hartford, Connecticut 06101.