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Sensitivity & Specificity

Sensitivity & Specificity. Sam Thomson 8/12/10. Sensitivity. Proportion of people with the condition who have a positive test result = True Positives/(True Positives +False Negatives) = TP/(TP+FN). Contingency Table. Question.

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Sensitivity & Specificity

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  1. Sensitivity & Specificity Sam Thomson 8/12/10

  2. Sensitivity • Proportion of people with the condition who have a positive test result = True Positives/(True Positives +False Negatives) = TP/(TP+FN)

  3. Contingency Table

  4. Question • A new test to screen for pulmonary embolism is used in 100 patients who present to the ED. The test is positive in 30 of the 40 patients who are proven to have a PE. Of the remaining 60 patients, only 5 have a positive test. What is the sensitivity of the new test?   • A.8.33%  • B.30% • C.40%   • D.66.66% • E.75%

  5. Answer • A contingency table can be constructed from the above data: • Sensitivity = TP/(TP+FN) • Sensitivity = 30/(30+10) • E. 75%

  6. Specificity • Proportion of patients without the condition who have a negative test result = True Negatives/(True Negatives +False Positives) = TN/(TN+FP) • High Specificity = Few False Positives

  7. Question • A rapid finger-prick blood test to help diagnose DVT is developed. Comparing the test to current standard techniques a study is done on 1,000 patients:What is the specificity of the new test?  • A.680/880   • B.200/220 • C.680/780   • D.680/700   • E.200/300

  8. Answer • Specificity = TN/(TN+FP) • = 680/(680+100) • C. 680/780

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