Overestimation of the predictive value of positives by the usual calculations of the specificity of diagnostic tests.

The specificity of a diagnostic test for a given disease, i.e. the percentage of true negatives, can be calculated from either the results from a group of healthy animals or possibly from a group which also contains diseased animals which are free of the particular disease for which the test has been performed. The specificity may be much lower in the latter case and the predictive value of positives thus greatly reduced. In the example of creatine kinase being used for the diagnosis of muscle diseases in dogs, the specificity at the thresholds of 105 and 150 U/L (upper limits of the 95% and 99.7% interquantiles) decreased from 0.98 and 1.0 to 0.66 and 0.78, respectively.