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NBME 24 Answers

nbme24/Block 1/Question#15

Results of a 5-year screening program for HIV ...

5%

Results of a 5-year screening program for HIV ...

5%

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seagull
good work. I found this question annoying and gave up doing those considering the amount of time we are given.
+3

_yeetmasterflex
How would we have known not to include the intake year? From average **annual** incidence?
+

lamhtu
Do not include intake year because the question stem is asking average annual incidence. The 4000 positives at intake could have acquired HIV whenever, not just in the last year.
+1

neels11
literally didn't think there was an actual way to figure this out.
but my thought process was: okay incidence means NEW cases. so the annual average at the end of 5 years would be:
(# of NEW people that tested positive at the end of year 5) /
(# of people at that were at risk at the beginning of year 5) <--- aka at the end of year 4
250/5050 = 4.95%
also if you look at year 5: you'll see that the at risk population is 4800 when 300 new cases were found the year before. 5050 at the end of year 4 MINUS the 300 new cases at the end of year 4 should give you 4750 as the new population at risk. but notice that end of year 5 we have 4800. idk if that means 50 people were false positives before or 50 people were added but in incidence births/death/etc don't matter
it's kind of like UWORLD ID 1270. assuming average annual incidence is the same as cumulative incidence
this was just a bunch of word vomit. sorry if it was unbearable to follow
+

So number of cases with positive serology during "intake" represents prevalence and should not be accounted for when calculating population at risk. Population at risk = Total population - prevalence. Doing that, sum up the annual incidences left = 400 + 250 + 250 + 300 + 300 = 1500.

Applying incidence formula = 1500/(10,000-4000) = 0.25 over a total of 5 years.

Annual = 0.25/5 = 0.05%

Exclude the intake year. Add all the positive results (400+250+300+300+250 = 1500). Divide 1500 by the total number of patients from the start of the incidence period, year 1, so 6000 ---> 1500/6000=0.25, this is the average across all 5 years. For annual, divide 0.25 by the number of years, 5 --> 0.25/5 = 0.05 --> 5%.

That's how I got it.

submitted by karljeon(23), 2019-06-06T19:46:29Z

I don't know if there is an equation for this, but I basically pumped out every division across the table to get ~5% on average.

Here they are: 400 / 6,000 = 0.067 250 / 5,600 = 0.045 300 / 5,350 = 0.056 300 / 5,050 = 0.059 250 / 4,800 = 0.052

The average of these %s for all the years = 5.58%. So that's close enough to 5%.