Evaluation of changes in somatic cell counts as ...

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Title Evaluation of changes in somatic cell counts as indicators of new intramammary infections
Author(s) Ian R. Dohoo, K. E. Leslie
Journal Preventive Veterinary Medicine
Date 1991
Volume 10
Issue 3
Start page 225
End page 237
Abstract Data from 1565 milk samples collected weekly from the University of Guelph mastitis research herd were used to evaluate somatic cell counts (SCC) as a screening test for prevalent intramammary infections. Changes in SCC were evaluated as a screening test for incident infections. The sensitivity and specificity of SCC for prevalent infections depended on whether cows with only minor pathogen infections were classified as infected or not. Including them in the infected cow category resulted in estimates of sensitivity and specificity of 0.726 and 0.855, respectively, at a threshold of 200 000 cells/ml. Several methods of measuring the change in a cow's cell count over a 28-day interval were evaluated. Observing a change from under to over a threshold of 200 000 cells/ml was optimum for predicting new infections. While the sensitivity of this procedure was low (0.388),the specificity was high (0.919) and relatively few cows (0.126) which were already infected at the start of the interval met the criterion. This last value was referred to as the TPNAR (test positive, not at risk) and was required, along with the sensitivity and specificity, to completely define the operating characteristics of the test and to predict the expected performance of the test. Changes in the apparent 28-day incidence risk in a herd would be expected to be much smaller than changes in the true risk. The correlation between the observed incidence risk and the true risk in the study herd over a number of test periods ranged from 0.57 to 0.83..

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