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NBME 20 Answers

nbme20/Block 3/Question#20 (44.5 difficulty score)
A 67-year-old woman comes to the physician ...
Low sensitivity🔍,📺

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submitted by hayayah(1101),
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tiitivenysS ettss rae seud orf snre.iencg iScyptifcei estst ear sude orf ncmtrnoiafio faetr psivotei s.cnsirngee

isiivyStnte tsest rae deus fro giseen how yamn eolpep lurty ahev teh yfctcpieiSi stest rea rof otesh ohw do nto have hte aees.ids

A gyihlh setniseiv ,estt hwne t,gaevine surle TUO .aesseid A lihhgy pecficis te,st nweh v,ioeipts ulers NI aeessd.i o,S a tset iwht htiw lwo stsvieinyit cantno rleu tou a A test hwit wol efiscptciyi 'acnt lure ni desisa.e

heT coodrt nad eitnpta nwat ot cneser fro ocnol ecranc adn erul ti uot. eTh drctoo dlwuo nawt a tets whit ihgh ienvstsitiy to be leab ot do .atth eH kwons hatt gttnsei ehr ootsl for olbod wlli not erul tuo het iysslitobpi of clnoo .CA

sympathetikey  SeN Out (Snout) --> sensitive test; - test rules out SPec In (Specin) --> specific test; + test rules in +25  
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(5),
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bcsauee ethy aveh kesda ubato THE T,ETS adn spivcittyysecis/itifine era het epprtories of teh e.tts wasereh PVP m&a;p PVN rae deandenpt noup het potnioupal bigen dttese, tsi' ton hte ntnrcsiii yrtropep fo teh tte.s

lovebug  @aazib05 simple and clear. Thank you! +  

submitted by bbr(27),

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).

trazobone  What a beautiful explanation THANK YOU +