Statistical Tests: Sensitivity, Specificity, PPV and NPV Page 1/3
The table below shows the results from looking at the diagnostic accuracy of a new rapid test for HIV in 100,000 subjects, compared to the Reference standard ELISA test. The rows of the table represent the test result and the columns the true disease status (as confirmed by ELISA).
|
|
HIV+ |
HIV- |
Total |
|
Test + |
378 |
397 |
775 |
|
Test - |
2 |
98,823 |
98,825 |
|
|
380 |
99,220 |
100,000 |
Welcome to the "Sensitivity, Specificity, PPV and NPV" quiz. There are 4 questions to answer.
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Q1. What is the Sensitivity of the new rapid test for HIV? Report the answer to 3 decimal places.
The answer is 0.995. The sensitivity of the test focuses on the HIV+ column of the table. It is the number of truly HIV+ patients who tested positive with the new rapid test divided by the total number of HIV+ patients. Therefore it is 378/380=0.995 to 3 decimal places.
Q2. What is the Specificity of the new rapid test for HIV? Report the answer to 3 decimal places.
The answer is 0.996. The specificity of the test focuses on the HIV- column of the table. It is the number of truly HIV- patients who tested negative with the new rapid test divided by the total number of HIV- patients. Therefore it is 98823/99220=0.996 to 3 decimal places.
Q3. What is the Positive predictive value (PPV) for the new rapid test for HIV in this cohort? Report the answer to 3 decimal places.
The answer is 0.488. The PPV of the test focuses on the Test+ row of the table. It is the number of truly HIV+ patients who tested positive with the new rapid test divided by the total number of positive tests using the new rapid test. Therefore it is 378/775=0.488 to 3 decimal places.
Q4. What is the Negative predictive value (NPV) for the new rapid test for HIV in this cohort? Report the answer to 5 decimal places.
The answer is 0.99998. The NPV of the test focuses on the Test- row of the table. It is the number of truly HIV- patients who tested negative with the new rapid test divided by the total number of negative tests using the new rapid test. Therefore it is 98823/98825=0.99998 to 5 decimal places.
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