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NBME 20 Answers

nbme20/Block 3/Question#20

A 67-year-old woman comes to the physician for a ...

Low sensitivity

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submitted by hayayah(525),

Sensitivity tests are used for screening. Specificity tests are used for confirmation after positive screenings.

Sensitivity tests are used for seeing how many people truly have the disease. Specificity tests are for those who do not have the disease.

A highly sensitive test, when negative, rules OUT disease. A highly specific test, when positive, rules IN disease. So, a test with with low sensitivity cannot rule out a disease. A test with low specificity can't rule in disease.

The doctor and patient want to screen for colon cancer and rule it out. The doctor would want a test with high sensitivity to be able to do that. He knows that testing her stool for blood will not rule out the possibility of colon CA.

sympathetikey  SeN Out (Snout) --> sensitive test; - test rules out SPec In (Specin) --> specific test; + test rules in +1  
usmlecrasher  can anyone pls explain why it is not << potential false- positive results >> ??? +  
almondbreeze  correct me if I'm wrong, but 'high FP (choice C)=low specificity (choice B)'. Whereas high specificity is required to rule in dz +1  
almondbreeze  picked positive predictive value myself. can anyone explain why not PPV? +  
williamfreakingosler  The principle @hayayah is talking about (a negative test being relied upon to reliably rule out) is negative predictive value ("NPV"). I don't see why "uncertain NPV" isn't the correct answer, particularly because NPV is predicated on the disease having the same base rate in the person(s) being tested as in the population that was characterized for the test statistic. Given that the patient has a strong family history of colon cancer, the NPV of FOBT is uncertain. Said another way, the sensitivity of a test does not change with the population, but the NPV does. The whole reason the doctor is denying FOBT is because of bayesian thinking (a priori information related to family history), and from my point of view bayesian logic is more relevant to PPV/NPV than to sensitivity, hence my confusion over why NPV isn't the right answer. +2  
ibestalkinyo  I thought negative predictive value for the same reasoning +  
raga7  AFTER THE RESULT OF TEST WE CAN USED PPV OR PPN, BUT FOR TEH FIRST TIME LOOKING ANY DESEASE USE SENSITIVITY OR SPECIFICITY. +