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NBME 20 Answers

nbme20/Block 3/Question#20 (39.8 difficulty score)
A 67-year-old woman comes to the physician ...
Low sensitivityπŸ”

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submitted by hayayah(1056),
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yeStisinvti ttsse are sdeu orf ensrncig.e icSitcpyfie estts rae seud rof rnitiamcnofo raeft spieiovt isrgecn.nes

siiSiyetvnt ttses rea dsue orf genies woh nmya lepepo urlyt hvea het isedae.s pyciiifctSe ttsse aer orf oehts how od ton vaeh hte ae.eidss

A lhyghi nvisetesi ,sett newh gin,eetva ulesr TUO deeassi. A hgyhil iicepfsc test, hewn s,ioeitpv rluse IN ess.daie oS, a tste ihwt with owl iyvesnitsti tncona uerl uto a eseisd.a A test htwi lwo seyifictpic tn'ac uler ni eaisse.d

Teh otorcd adn aetptni nwat to necesr orf nocol rcncea dna rule it ou.t hTe rtdooc ouldw twan a ttse itwh ighh ynvestiitis to eb abel ot do ath.t eH kwnso atht sneitgt rhe ooslt ofr obold iwll tno erlu tou eht lisyitisbpo fo lcono .AC

sympathetikey  SeN Out (Snout) --> sensitive test; - test rules out SPec In (Specin) --> specific test; + test rules in +21  
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 +2  
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 +  

submitted by aazib05(4),

because they have asked about THE TEST, and sensitivity/specificity are the properties of the test. whereas PPV & NPV are dependant upon the population being tested, it's not the intrinsic property of the test.

lovebug  @aazib05 simple and clear. Thank you! +  

submitted by bbr(22),

Another way to think of this: She has a strong family history, so we are thinking she probably does indeed have this mutation (probably a True Positive). Our fear, would be we do the wrong test and aberrantly tell her that she is in the clear (False Negative). Having a high False Negative would be deleterious to this patient, and plugging this into a 2x2 table gives a low sensitivity (TP/ TP +FN).