Welcome back to MedSmarter’s USMLE Style Question of the Week. Here’s the breakdown of another high-yield Biostatistics question for those preparing for your USMLE Step 1 or just to gain some knowledge. As always you want to begin by reading the last sentence of the vignette first to get an understanding of what the question is asking for.
Question Break Down of the Week:
A new nasal swab test was recently developed to detect antibodies to SARS-CoV-2 in order to diagnose the infection. One thousand patients received the test, and while 100 people had the infection, only 80 of them tested positive. Of the 900 people who did not have the infection, 800 tested negative and 100 tested positive. Which of the following percentages indicates the specificity of this new test?
(A) 10%
(B) 44%
(C) 80%
(D) 89%
(E) 98%
The correct answer choice is D. Specificity refers to the ability of a test to correctly identify those without the disease or infection, which is represented by the true negative rate. In this case, there were 800 true negatives and 100 false positives, giving a total of 900 in the denominator. Thus, the specificity of the new nasal swab test is 800/900 or approximately 89%. This means that 89% of patients without the infection would test negative on this test.
Did you think the answer was different?
Did you think that the correct answer choice was other than D? You can view this video for a deeper discussion of why A, B, C, and E are not the correct answer choices.
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