NBME biostat q

[MudPhud20XX]

A 67 yr old woman comes to the physician for a health maintenance examination. Her brother and mother have a history of colon cancer. The physician recommends colonoscopy, but the patient says that she would prefer only for her stool to be tested for blood. The physician explains that testing the stool for occult blood is not appropriate in this case. The physician is most likely concerned about which of the following regarding this test?

A. Low sensitivity
B. Low specificity
C. Potential for a false-positive result
D. Uncertain negative predictive value
E. Uncertain positive predictive value

I want to say A. Anyone has an idea? Why not D?

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[Missorleans]

I think the answer may have been B, low specificity, since occult blood doesn't necessarily mean cancer. This question was definitely discussed in the thread for that NBME, try a google search to get the page.

 

[Apoplexy__]

Yeah, I remember this question. Look in the appropriate NBME thread.

I'm pretty sure it's A though, because the goal here is to screen for + rule out colon cancer -- therefore, high sensitivity is necessary. A fecal occult blood can be negative and you can still have colon cancer. A colonoscopy is far more unlikely to be false negative.

D doesn't make sense because there's nothing uncertain about the predictive values of these tests. I haven't personally checked, but I'm sure there are plenty of studies identifying the reliability of the two tests in terms of predictive values and other metrics.