QBDOTW: Patient Suspected to Have Myasthenia Gravis
Welcome to MedSmarter’s USMLE Style Question Break Down of the Week. For those preparing for the USMLE Step 1, this week we break down a high-yield Biostatistics question. As always you want to begin with 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:
You have a patient in your office, and you suspect they have myasthenia gravis. Individuals with myasthenia gravis classically present with complaints of muscle weakness and fatigue secondary to the formation of autoantibodies directed against the acetylcholine receptors at neuromuscular junctions. The most accurate method of diagnosis involves the detection of these autoantibodies. On average, this test is approximately 90% sensitive and 80% specific. If an individual has a positive test for autoantibodies against the acetylcholine receptor, what is the approximate probability of having this disease?
The correct answer choice is B: The positive predictive value (PPV) of the test can be calculated with the following formula, where TP is true-positive results and FP is false-positive results: TP / (TP + FP). We need to set up a hypothetical 2 × 2 table in which the number of subjects with the disease is equal to the number not having the disease (or to be said differently, the pretest probability becomes the prevalence). If we set the number of those with the disease as 10, then TP = 9 and FP = 2, given the sensitivity of 90% and specificity of 80%. Therefore, the PPV would be calculated as 9 / (9 + 2) = 81.8%, or about 82%.
Did you think the answer was different?
Did you think that the correct answer choice was other than B? You can view this video for a deeper discussion of why A, C, D, and E were not the correct answer choices.
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