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Published byRachel Duncan Modified over 3 years ago

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

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

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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

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

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DiseaseNo disease + - - + Positive tests = 48 + 12 = 60 Negative tests = 28 + 12 = 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

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

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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)

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Positive and negative predictive value Disease present Disease absent Test positive Test negative 48 28 12 60 40 Positive predictive value = 48/60 = 80% Negative predictive value = 28/40 = 70%

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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

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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

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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

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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

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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 44 27 6 3 50 30 Positive predictive value = 44/50 = 88% Negative predictive value = 27/30 = 90% How many patients would be given treatment unnecessarily? N = 6

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