LEC3_TESTING_2011

LEC3_TESTING_2011 - HIVTesting November8,2011 HIVTest...

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HIV Testing November 8, 2011
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HIV Test What is an HIV test? Antibody test  Does not directly measure the virus Measures whether the body has produced  antibodies to the virus
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Validity of HIV Tests How do we assess this? Two important concepts: Sensitivity  – ability to identify true positives;  confirm presence of disease   TP/(TP + FN) Specificity  – ability to identify true negatives;  confirm absence of disease    TN/(TN + FP)
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Gold-standard for diagnosis In order to evaluate the validity of a clinical test,     a gold-standard  for diagnosis is required. Gold-standards are definitive and often highly  invasive. What would the gold-standard be for an HIV test ?
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Group (a) True Positive Group (b) False Positive Group (c) False Negative Group (d) True Negative The 2x2 Table describes screening test outcomes: Disease present Disease absent Positive result Negative result Application of Screening to  Populations
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Group (a) True Positive Group (b) False Positive Group (c) False Negative Group (d) True Negative Characteristics of Screening  Tests 1) Sensitivity: proportion of those with disease who test positive in the screened group = a/(a+c) or TP/(TP+FN) Positive result Negative result Disease present Disease absent
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Group (a) True Positive Group (b) False Positive Group (c) False Negative Group (d) True Negative Characteristics of Screening  Tests  2) Specificity: proportion of those without disease who test negative in screened group =(d)/(d+b) or TN/(TN+FP) Positive result Negative result Disease present Disease absent
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2000 True positive 0 False positive 0 False negative 8000 True negative Disease present Disease absent Positive result Negative result n = 2000 n = 8000 The Ideal Situation--100%  Agreement
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1996 True Positive 8 False Positive 4 False Negative 7994 True Negative Disease present Disease absent Positive result Negative result n = 2000 n = 8000 A More Likely Outcome
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Trade-offs  High sensitivity may come at the expense of  many false positives Sensitivity = TP/(TP+FN) Similarly, high specificity may come at the  expense of many false negatives Specificity = TN/(TN+FP)
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LEC3_TESTING_2011 - HIVTesting November8,2011 HIVTest...

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