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Summary Variations in precision of the Breed method for cell counts in milk were investigated by utilizing different working factors (WF) for the same smear and by using the same WF over a major portion of the probable cell count range. A significant inverse relationship was found between precision and the WF. With a constant WF on the other hand, the precision of the count increased very significantly as the actual cell count increased. Formulas showing the relationship between the expected high and low for any given cell count were computed via the least squares method for a WF of 20,000. Evidence was presented that a WF of 5,000 or below would be necessary when a good estimate of cell content is important.

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.

Summary By means of direct microscopic observation of cells in stained smears and projection of a series of color transparencies, differences in criteria used by individuals for identification of cells to be counted in Prescott-Breed smears were discovered and described. Extensive studies revealed a unique pattern of cell distribution within smears which can materially affect the outcome of a cell count depending upon the area selected for cell counting. Among other factors, variation in size of the smear was found to have a marked affect upon resultant sample count.

The Direct Microscopic Somatic Cell Count can be used advantageously in control of abnormal milk to perform simultaneously screening and confirmation of milk samples. Confidence limits computed according to the individual microscope Strip Factor are used to interpret the count made on a single strip of one of the two milk films. The sample is assigned on the basis of the interpreted count to one of three categories: (a) less than the legal cell concentration maximum ( P

Summary Milk smears were prepared and the weights of milk delivered were determined following use of the syringe and use of the 0.01 ml loop. Large and significant increases in the volume of milk delivered resulted when the angle of the loop with the milk was 20° rather than 90° or when the flat surface of the loop was brought out of the milk in the manner of a dipper. Slow withdrawal of the loop resulted in loss of milk and delivery of small volumes. Delivery of 0.01 ml of milk required that the loop be held in a vertical position and withdrawn quickly, but not with forced rapidity. The platinum loop, calibrated to deliver 0.01 ml of milk, has been used extensively for counting leukocytes in milk (1, 3, 5). Some limitations have been encountered, particularly the tendency for the loop to pick up an excessive volume of cell-laden fat from the suface of poorly mixed milk (1, 6). With well-mixed milk, leukocyte counts from smears made with the loop using carefully standardized techniques were found not to differ significantly from those made with the syringe1 (6). Standard procedure calls for the loop to be withdrawn vertically from the milk (1, 3). It has been observed, however, that workers from different milk quality control laboratories may withdraw the loop in differing ways so that the volumes of milk removed may vary substantially (2). During the course of these studies, it also became evident that the speed with which the loop was withdrawn influenced the volume of milk delivered. When the milk container is level, the loop is easily withdrawn vertically at an angle of about 90° to the surface of the milk, or it may be withdrawn at angles considerably smaller. When the milk container is tipped, it becomes convenient to withdraw the loop at an angle of about 20°. With the container tipped, the loop may also be used as a “dipper” by bringing it out with the broad surface almost horizontal with that of the milk. Tendencies toward use of each of these procedures have been noted in general laboratory practice. Because leukocyte counts have become a vital part of many milk quality control programs, any causes of error inherent in use of the loop should be understood. The following observations were made in order to evaluate the nature and magnitude of differences in volume of milk delivered by the loop as a consequence of variations in the manner of withdrawal of the loop from the milk.