








USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam 

Thread Tools  Search this Thread  Display Modes 
#1




A Complicated PPV Question!
A test yields 80% infected and 10% noninfected.....prevalence is 10%.
What is positive predictive value? A. 47% B. 54% C. 75% D. 25% 
#2




A. 47%
Assuming total population of 100. 10% prevalence would mean total diseased number to be 10 out of which 80%(8) are ruled + by test. And so on. 8+9/17=0.47 Could you give the answer with possible explanation if I'm headed in the wrong direction. 
#3




sure i will let you know answer with explanation..waiting for some more answers...
Can you please explain how you calculated specificity?? 


#4




Okay. 1. I think I calculated something else in previous reply so ignore that. 2. Could you please write the whole question because either you wrote the question wrong or the answer I'm getting from your given values is not any one of the options.
Thank you. 
#5




Same here just as insomniac did. I also got a different answer somewhere around 80%

#6




New stool test for shigella infection yields postive results in 80% of infected patients and in 10% of uninfected patients ...prevalence of shigela in population is 10%..What is the probability that a patient who tests positive with the new test is infected with shigella ..
P.s: insomiac u were right BUT 8+9/17 = 0.47 .plz explain this equation. 
#7




Quote:
very confusing question! if i suppose 80% infected as sensitivity and 10% noninfected as specificity then with a prevalnce of 10% , the positive predictive value is 8%. now,if the question is correctly asked then i must have misinterpreted it. 'test yields 80% infected' means all the positive results ( TP and FP included ), then 80% is not the sensitivity n our whole calculation collapses n i can't find the answer then without sensitivity n specificity. 
#8




Quote:
with 80% sensitivity n 10% specificity, using 2 by 2 table, TP=80 , FP=90 FN=20, TN=10 PPV = 80/80+90 = 80/170 = 47% 
#9




Quote:
i am using this formula only using sensitivity and specificity,not putting 10% prevalence in consideration. if using 10% prevalence in the formula, i get a diferent answer! plz xplain medicalbiology! 
#10




Quote:
as q says test is positive in. 10 % who dnt hav disease consequentlt equation become 80/80+10= 88% If i m wrong correct me plz?? 
#11




organ14 i am also confused in specificty thing..... According to statement it should be 10% specificity .. I willl let u know uworld explanation soon but explanation is also not very helpful...

#12




Exactly hardtime FP should be 10 NOT 90..

#13




the question is wrongly put,
the specificity = 90% sensitivity = 80% prevalence of infection = 10% suppose we have 1000 pts, then infected noninfected test +ve 80 90 = 170 test ve 20 890 = 910 100 900 = 1000 so PPV = 80/170 = 47% 
#14




Answer is:
Quote:

#15




Got it
As question asks 10% prevelance sensitivity = 80 % FP as question mentioned = 10% = and we know that 1specificity=FP r specificity = 1FP rates So putting values = 1  .1 = .9 r 90/100 We make square Disease present disease absent test positive 8 (TP) 9 (FP) test negitive 2 (FN) 81 (FN) 10 90 100 By putting values we get the ans 
#16




Here's the explanation.
Seems like I guessed correctlywhat medicalbiology was trying to say in the first post.
Here's how I had gotten around it. 
The above post was thanked by:  
#17




but how do we know that 10 percent were labeled as false positive...

#18




Quote:
It states that right in the question. "New stool test for shigella infection yields postive results....."in 10% of uninfected patients"" 
#19




whoops my bad, i didnt read carefully . It all makes sense now, just gotta read carefully. I get tripped up cause i forget to just assume the amount of patients.

#20




Quote:
It would be really helpful if you could tell us where this question is from, because if it is from NBME I won't look at it, same goes for uworld Secondly, please just post the whole question next time. The way you put it at first is so confusing, there is no way I could have known what this question is asking... Please everyone who posts questions, lets try to do it the right way so that it helps us all!
__________________
Step1 [ ], Step 2CK [ ], Step 2 CS [Pass first attempt], Step 3 [ ] 
#21




I have a question...
what if the specificity and sensitivity is the same... ex. 45% and prevalence is 30%. Then asked to calculate prevalence.. what would the prevalence be in this example? 
#22




val7 itz from uworld....Actually it is not allowed to write whole question of copyright material..

#23




PPV = TP/(TP+FP). It was impossible to work out the TP or FP rate from the way the original question was worded.
Sky_blue2000, your question is confusing, If the prevalence is 30%, then the prevalence is 30% 
#24




Oh sorry!!
I meant what would the PPV be if the prevalence is 30% and specificity and sensitivity are both 45% 
#25




Lets say sample size is 1000 patients
Prevalance = 30% Therefore diseased = 300, non diseased = 700 We now need to work out our TP and FP numbers Sensitivity = 45% In other words this test will give a positive result in 45% of the people with the disease (ie its rubbish!) So the True Positive rate = 300 x 45% = 135. This means that the False Negative rate, which is all the patients our test missed, is 300  135 = 165 Now, specificity = 45% This means that, out of all the 700 patients without disease, our test will give a truly negative result only 45% of the time So our True Negative rate = 700 x 45% = 315 This means that the False Positive rate, which is all the patients without disease that our test gave a positive result for, is 700  315 = 385 We now have all the numbers needed to make a 4x4 table and work out anything else. The PPV = TP/(TP+FP) = 135/(135+385) = 26% So the test is worse than simply tossing a coin... 
The above post was thanked by:  
#26




Quote:
Thank you soo much!! Great explanation 
#27




Waoh!, that was a pain.:p
Quote:
Applying Bayes Theorem: PPV = Sensitivity x Prevalence/ [ (Sensitivity x Prevalence) + (1Specificity)(1Prevalence)] Plug in those values and you get ~26% PPV 
#28




Ha, i remember seeing that on uworld and clearly totally didnt take it in...

#29




confusion..
Quote:
i knw my ques is lame but jst want to b sure.. weak in maths :/ thanks..
__________________
Never let your fear be stronger than your Faith

#30




yep, multiplying something by 45% (percent) just means multiplying by "45 per 100" ie 45/100, or 0.45

#31




This same question was discussed in the beh science vedio of Kaplan. The answer is 47%. Insomnia's photograph explanation is correct!
__________________
In USMLE, “Any fool can know. The point is to understand.” 
Tags 
BiostatisticsEpidemiology, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Complicated CF Genetics Pedigree  ashanair70  USMLE Step 1 Forum  21  02202013 02:51 PM 
Grief versus Complicated Grief versus Major Depression  winner21  USMLE Step 2 CK Forum  2  05052012 09:51 AM 
What is the diagnosis of this complicated patient?  m82_ghasemi  USMLE Step 1 Forum  9  09272011 11:48 AM 
A complicated case of a smoker, carpenter, and alcoholic man!  aghammoud85  USMLE Step 2 CK Forum  4  05312011 03:00 PM 
Complicated unlicensed abortion  RRMadukha  USMLE Step 2 CK Forum  1  05262010 02:49 PM 
