8. A test for the presence of Zika virus is developed. The test is administered

Question

8. A test for the presence of Zika virus is developed. The test is administered to a group of 160 subjects known to have Zika. Of this group, 156 test positive. The test is also administered to a group of 400 subjects known to not have Zika. Of this group, 8 test positive. These results are similarly presented in the table below: Have Zika Do Not Have Zika Total Test Positive Negative Total 164 396 560 156 392 400 160 a. Calculate the sensitivity and specificity of this test directly from the data. b. Assume that the prevalence of the disease is 3%. Calculate the NPV and PPV with this prevalence. c. What conclusions can be drawn regarding the effectiveness of this test?

Explanation / Answer

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a. Sensitivity = TP/TP+FN = 156/(156+4) = .975

Specificity = TN/TN+FP = (392)/(392+8) = .9800

b. Prevalence = 3%

PPV
= Sensitivity*Prevalence / Sensitity*Prevalence+(1-Specificity)*(1-Prevalence)
= .975*.03/(.975*.03+(1-.98)*.97)
=0.03

NPV = (specificity x (1 – prevalence)) / [ (specificity x (1 – prevalence)) + ((1 – sensitivity) x prevalence) ]
= (.98*.97)/(.98*.97+(1-.975)*.03)
= 0.9992

c. The test does a good job at correctly identifying True Negatives cases.

But a truer measure are NPV and PPV. The PPV and NPV describe the performance of a diagnostic test or other statistical measure against the prevalence rate. The NPV of .6012 suggests the identification of True positives out of all positive assigned test results is not that strong as compared to identification of correct negative cases from all negatively predicted cases

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