I am missing something with this question because I do not understand question 4g which says to Discuss how the answers to parts b and f differ.
I got the same answer for both b and f, so I must be missing a teaching point.
can anyone help explain the point of b and f?
thank you
If you got the same answer for b & f you most likely made a math error or got the same LR for parts A and E. Review the chapter and/or Michael's lecture to see how to avoid that mistake.
thanks, the point is that i reviewed the chapter and I am not understanding why the two questions are different.
in both cases the pre test risk is >5% so, I don't understand what is being asked
I'm not sure if this was the confusion, but the underlying premise of the question is that she should get a CTPA if her post-test probability after d-dimer is >= 5%.
The question makes you do the calculation to obtain LR associated with the d-dimer result in 2 different ways. Using 2 different LR results and the same pretest probability of 10% is supposed to give you 2 different post-test probabilities.
Thank you for clarifying. I think I understand your confusion now. I've changed the stem of the question to clarify, adding "(post-test)" in the sentence below:
For
this problem, we will say we should do
a CTPA if the (post-test) probability of PE is ≥ 5%.
Does that help?
Tom
This is not relevant to solve the question, but can you explain how the 32 corresponds to the 5% in the question stem as pasted below? I tried to do the calculations based on the chapter, but am missing something on how these two numbers relate.
"We will say we should do a CTPA if the probability of PE is ≥ 5%, i.e., we are willing to do up to 32 CTPAs to diagnose one PE".
Dear Jean,
You aren't missing anything; it's a typo. I'm guessing in a previous version of the problem the treatment threshold was 3%, in which case it should have said you are willing to do up to 32 CTPAs in patients without a PE to diagnose one PE.
In any case, assume the 5% threshold is right, in which case you are willing to do 20 CTPAs to diagnose one PE.
Tom
