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A Complicated PPV Question!

5K views 30 replies 11 participants last post by  nsesereso 
#1 ·
A test yields 80% infected and 10% non-infected.....prevalence is 10%.
What is positive predictive value?

A. 47%
B. 54%
C. 75%
D. 25%
 
#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.:rolleyes:
 
#7 ·
Question:
A test yields 80% infected and 10% noninfected.....prevalence is 10%
...what is positive predicted value ??

a.47%
b.54%
c.75%
d.25%
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.
 
#14 ·
Answer is:
Imagine that population consist of 100 patients .Since the disease prevalence is 10% that means 10 patients have disease and 90 do not ..Performing a test with with 80% sensiitivity on 10 diseased patients yields 8 TF . performing a test with 90% specificity on 90patients without disease yield 9 FP ..
PPV = 8/(8+9) = 47%
BUT same question how specificity is 90% ?
 
#15 ·
Answer is:

BUT same question how specificity is 90% ?
Got it

As question asks 10% prevelance

sensitivity = 80 %

FP as question mentioned = 10% = and we know that 1-specificity=FP r specificity = 1-FP 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
 
#20 ·
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!
 
#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...:)
 
#26 ·
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...:)
Thank you soo much!! :) Great explanation
 
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