DiseaseNo disease 60 people with disease 40 people without disease Total population = 100

DiseaseNo disease True positive tests = 60 True negative tests = 40 IDEAL! + - True positive True negative

DiseaseNo disease True positive tests = 48 True negative tests = 40 All the positive tests show disease, even though some cases are missed + - True positive True negative

DiseaseNo disease True positive - False positive True positive tests = 48 True negative tests = 40 False positive tests = 12 +

DiseaseNo disease Positive tests = = 60 Negative tests = = 40 False positive True positive True negative False negative True positive tests = 48 True negative tests = 40 False positive tests = 12 False negative tests = 12

DiseaseNo disease + - True positive = 48 True negative = 28 False negative = 12 False positive = Positive tests = true positive + false positive Negative tests = true negative + false negative + +

Positive and negative predictive value Disease present Disease absent Test positive Test negative True positive True negative False positive False negative TP + FP FN + TN Positive predictive value = TP/(TP + FP) Negative predictive value = TN/(FN + TN)

Positive and negative predictive value Disease present Disease absent Test positive Test negative Positive predictive value = 48/60 = 80% Negative predictive value = 28/40 = 70%

Positive predictive value PPV is the proportion of positive tests that actually show disease PPV = 48/60 = 80% For every 100 positive test results, 80 will be true positive and 20 will be false positive

Negative predictive value NPV is the proportion of negative tests that show no disease NPV = 28/40 = 70% For every 100 negative test results, 70 will be true negative and 30 will be false positive

New situations What does a PPV of 85% mean? For every 100 positive tests in patients from a similar population how many would be true positives? Answer: N = 85

New situations A positive test result usually means that a patient will have treatment or a further test. In an asthma clinic a positive test is followed by corticosteroid treatment. If there are 20 positive tests and a PPV of 75%, how many patients would be given corticosteroids unnecessarily? Answer: N = 5

New situations The NPV for the same test in the same clinic is 90%. For every 20 patients who test negative, how many will be false negative? Answer: N = 2

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Positive and negative predictive value Disease present Disease absent Test positive Test negative Positive predictive value = 44/50 = 88% Negative predictive value = 27/30 = 90% How many patients would be given treatment unnecessarily? N = 6