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 Delivery of 0.01 ml of milk required that the loop be held in a vertical position and withdrawn quickly, but not with forced rapidity.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Standard procedure calls for the loop to be withdrawn vertically from the milk (1, 3).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 During the course of these studies, it also became evident that the speed with which the loop was withdrawn influenced the volume of milk delivered.
When the milk container is level, the loop is easily withdrawn vertically at an angle of about 90° to the surface of the milk, or it may be withdrawn at angles considerably smaller.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.
1Applied Research Institute, 2 E. 23rd St., New York 10, N. Y.