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NBME 21 Answers

nbme21/Block 3/Question#21

A screening program is instituted for detection of ...

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submitted by drdoom(209),

2,500 students ... but you find out during your initial screen that 500 already have the disease. So, strikeout those people. That leaves 2,000 students who don’t have the disease.

Over the course of 1 year, you discover 200 students developed the infection. Thus:

200 new cases / 2,000 people who didn’t have the disease when you started your study = 10 percent

Tricky, tricky NBME ...

sympathetikey  Ah, I see. Thank you! +  
niboonsh  Im mad at how simple this question actually is +2  
sahusema  Incidence is measured from those AT RISK. People with the disease are not considered to be at risk. So 2500 - 500 = 2000 people at-risk. Of those 2000, within one year 200 develop the disease. So 200/2000 of the at-risk population develop the disease. 20/2000 = 10% = incidence +1  




 +3  upvote downvote
submitted by drdoom(209),

Don’t forget that incidence is the number of new cases which emerge in an unaffected population. Incidence is trying to get at the question -> “In a given year, how many new people develop this disease?”

In other words, you cannot count people who already have the disease. You have to exclude those people from your calculation. You want to know, among all the people out there who DO NOT have the disease, how many times this year was someone (newly) diagnosed?

Said differently still, you don’t want to “double-count” people who developed the disease before your study. As an epidemiologist, that would screw up your sense of how infective or transmissible a disease is. You want to know, “from time1 to time2 how many new cases emerged?”

questioneverything  You would count the total risk pool. Chlamydia is not a chronic disease so you would treat those 500 people and they would return to the risk pool. +  
drdoom  But you would first have to determine that they CLEARED the infection. What if you gave them tx and then they come back and say, "doc i got the chlamydia" -- is this a new case or did the tx fail? You're assuming it cleared but maybe it didn't. That's why you want to EXCLUDE from the start anyone who might already have disease of interest. +1  




Ok I get that if 500 already have the disease then the risk pool is dropped to 2000 students but the question specifically says that the test is done a year later...if 500 people had chlamydia, you would treat them. You don't become immune to chlamydia after infection so they would go back into the risk pool, meaning the pool would return to 2500. The answer should be 8%, this was a bad question.

thepacksurvives  Yeah, this was my issue. I got it wrong because of this-- still don't understand the logic bc you can get chlamydia multiple times +1  
hungrybox  FUCK you're right. Damn I didn't even think about that. That's fucking dumb. I guess this is why nobody gets perfect scores on this exam lol. Once you get smart enough, the errors in the questions start tripping you up. Lucky for me I'm lightyears behind that stage lmao +3  
usmile1  to make it even more poorly written, it says they are doing a screening program for FIRST YEAR women college students. So one year later, are they following this same group of students, or would they be screening the incoming first years? +3  
dashou19  I think the same at first, but after a second read, the question stem said "additional" 200 students, which means the first 500 students don't count. +  




 +0  upvote downvote
submitted by drdoom(209),

Also consider this great description from the NIH’s MeSH database:

INCIDENCE: The number of new cases of a given disease during a given period in a specified population. It also is used for the rate at which new events occur in a defined population. It is differentiated from PREVALENCE, which refers to all cases, new or old, in the population at a given time.

https://meshb.nlm.nih.gov/record/ui?ui=D015994

questioneverything  The prevalence of chlamydia in this group would be 0. It is not a chronic disease. +  




 +0  upvote downvote
submitted by usmle11a(12),

guys!!

the question doesnt say that the 500 got cured, so it will be 2000 who are at risk.