诊断学概念ppt课件

Predictive ValuesBob L. Larson, DVM, PhD University of MissouriCommercial Agriculture, Beef Focus TeamUse 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)1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic Tests Must be tested against a “gold standard” Although no perfect test exists, a gold standard is an accepted, reliable test Properties of diagnostic testsSensitivity = proportion of true positives that test positiveHow effective is the test at identifying animals with the condition?Specificity = proportion of true negatives that test negativeHow effective is the test at identifying animals without the condition?1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest PositiveTest Negative1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest Positive True PositiveTest Negative1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest PositiveTest Negative True Negative1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest Positive False PositiveTest Negative1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest PositiveTest Negative False Negative1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest Positive a b a+bTest Negative c d c+da+c b+d a+b+c+dSensitivity = a/(a+c)1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest Positive a b a+bTest Negative c d c+da+c b+d a+b+c+dSensitivity = a/(a+c)Specificity= d/(b+d)1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest Positive a b a+bTest Negative c d c+da+c b+d a+b+c+dSensitivity = a/(a+c)Specificity= d/(b+d)False negative = c/(a+c)1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic TestsDisease StatusPresent AbsentTest Positive a b a+bTest Negative c d c+da+c b+d a+b+c+dSensitivity = a/(a+c)Specificity= d/(b+d)False negative = c/(a+c)False positive = b/(b+d)1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsN u m ber of animal sMeasurementDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)Population APopulation B1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsN u m ber of animal sMeasurementDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)Population APopulation B1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsN u m ber of animal sMeasurementDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)Population APopulation B1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsN u m ber of animal sMeasurementDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)Population APopulation B1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsN u m ber of animal sMeasurementDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)Population APopulation B1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)N u m berMeasurementWhere cut-off is placed determines sensitivity and specificity1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)N u m berMeasurementWhere cut-off is placed determines sensitivity and specificityNormal AbnormalFew False NegativesHigh Sensitivity- result + result1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)N u m berMeasurementWhere cut-off is placed determines sensitivity and specificityNormal AbnormalFew False PositivesHigh Specificity- result + result1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)N u m berMeasurementWhere cut-off is placed determines sensitivity and specificityNormal AbnormalTrade off between False and False +- result + result1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)N u m berMeasurementTest with High SensitivityNormal AbnormalA negative result allows you to rule-out whatever you are testing - result + resultSnNout1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic tests use an arbitrary cut-off to separate two populations (normal vs. abnormal)N u m berMeasurementTest with High SpecificityNormal AbnormalA positive result allows you to rule-in whatever you are testing - result + resultSpPin1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsDiagnostic 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.1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsBecause N u m berMeasurementMy illustration is not consistent with true disease dynamics in a population (the real world)1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsN u m berMeasurementBecause My illustration is not consistent with true disease dynamics in a population (the real world)1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsPredictive 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 specificity1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsPPV and NPV Assume a test for bovine Johnes is 60% sensitive and 90% specific On examination of a 5 year old dairy cow, with typical history, clinical signs and positive herd-matesWe assume 90% of cows with this description are positive (prevalence = 90%)Dz + Dz -Test Positive 540 10 550Test Negative 360 90 450900 100 1,000Prevalence is 90% (900/1000 = 90%)1. Measurement of disease events2. Infectious disease epidemiology3. Interpreting diagnostic testsPPV and NPV Assume a test for bovine Johnes i