诊断学概念幻灯片.pptVIP

Predictive Values Bob L. Larson, DVM, PhD University of Missouri Commercial Agriculture, Beef Focus Team Use and Interpretation of Diagnostic Tests Help to make a diagnosis does not stand alone Judge severity of disease Predict subsequent clinical course of disease Estimate likely responsiveness to therapy Determine response to therapy post therapy Diagnostic Tests Must be tested against a “gold standard” Although no perfect test exists, a gold standard is an accepted, reliable test Properties of diagnostic tests Sensitivity proportion of true positives that test positive How effective is the test at identifying animals with the condition? Specificity proportion of true negatives that test negative How effective is the test at identifying animals without the condition? Diagnostic Tests Diagnostic Tests Diagnostic Tests Diagnostic Tests Diagnostic Tests Diagnostic Tests Diagnostic Tests Diagnostic Tests Diagnostic Tests Diagnostic Tests Sensitivity and specificity are inversely related Sensitivity and specificity are properties of the diagnostic test and tell us if the test is valid Test sensitivity and specificity have little effect on the confidence a veterinarian has in an individual result. Predictive ValueWhat we are really interested in Predictive values tell us if a test is useful Positive Predictive Value PPV The probability that an animal with a positive test result is truly positive Negative Predictive Value NPV The probability that an animal with a negative test result is truly negative Dependent on disease prevalence and test sensitivity and specificity PPV and NPV Assume a test for bovine Johne’s is 60% sensitive and 90% specific On examination of a 5 year old dairy cow, with typical history, clinical signs and positive herd-mates We assume 90% of cows with this description are positive prevalence 90% PPV and NPV Assume a test for bovine Johne’s is 60% sensitive and 90% specific On examination of a 5 year old dairy cow, with typical