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Diagnostic Test Characteristics

Diagnostic Test Characteristics. Professor Mobeen Iqbal Shifa College of Medicine. Case.

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Diagnostic Test Characteristics

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  1. Diagnostic Test Characteristics Professor Mobeen Iqbal Shifa College of Medicine

  2. Case • At the local outpatient clinic of the department of gastroenterology, 100 consecutive patients are included in a study for a new non-invasive diagnostic test for detection of carcinoma of the colon. The new test was positive in 3 out of 4 patients with biopsy proven diagnosis of carcinoma. The test was also positive in 7 patients without biopsy proven carcinoma.

  3. Sensitivity= TP/TP+FN • =3/3+1=3/4=0.75=75% SENSITIVITY The proportion of people with the diagnosis (N=4) who are correctly identified (N=3)

  4. Specificity=TN/TN+FP • =89/89+7=89/96=0.93=93% The proportion of people without the diagnosis (N=96) who are correctly identified (N=89)

  5. PPV=TP/TP+FP • =3/10=30%

  6. NPV=TN/TN+FN • =89/90=99%

  7. 4 people with the disease, 3 are picked up by the test

  8. test is positive for a further 7 people without disease

  9. The remainder of the sample are negative on the test

  10. SENSITIVITY • SENSITIVITY is the proportion of people with the disease correctly identified by the test • It measures the proportion of false NEGATIVES

  11. SENSITIVITY In this case, sensitivity is ¾ or 75%

  12. SPECIFICITY • SPECIFICITY is the proportion of people without the disease correctly identified by the test • It measures the proportion of false POSITIVES

  13. SPECIFICITY In this case, specificity is (96-7)/96 or 93%

  14. If someone is positive on the test, what are the chances that he/she has the disease? • Probability = 3/10 = 30% • This is the POSITIVE PREDICTIVE VALUE (the value of the test in predicting a positive result)

  15. If someone is negative on the test, what are the chances that he/she does not have the disease? • Probability = 89/90 = 99% • This is the NEGATIVE PREDICTIVE VALUE (the value of the test in predicting a negative result)

  16. SENSITIVITY, SPECIFICITY AND PREDICTIVE VALUES • For sensitivity and specificity, the reference variable (‘denominator) is the DISEASE • For predictive value, the reference variable (‘denominator’) is the TEST

  17. acknowledgment • Tom Sensky, Teaching EBM/EBMH St Hugh’s College Oxford for some of the slides in the presentation

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