Sensitivity & Specificity Sam Thomson 8/12/10. Sensitivity Proportion of people with the condition who have a positive test result Proportion of people.

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Presentation transcript:

Sensitivity & Specificity Sam Thomson 8/12/10

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

Contingency Table Disease Present Disease Absent Test Positive True Positive (TP) False Positive (FP) Test Negative False Negative (FN) True Negative (TN)

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 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% A.8.33% B.30% B.30% C.40% C.40% D.66.66% D.66.66% E.75% E.75%

Answer A contingency table can be constructed from the above data: A contingency table can be constructed from the above data: Sensitivity = TP/(TP+FN) Sensitivity = TP/(TP+FN) Sensitivity = 30/(30+10) Sensitivity = 30/(30+10) E. 75% E. 75% PE Diagnosed No PE Test positive 305 Test negative 1055

Specificity Proportion of patients without the condition who have a negative test result 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 High Specificity = Few False Positives

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 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 A.680/880 B.200/220 B.200/220 C.680/780 C.680/780 D.680/700 D.680/700 E.200/300 E.200/300 DVT present DVT absent New test positive New test negative 20680

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