In a test with more than two possible results, you should not calculate the likelihood ratio of a positive result, LR (+), or the likelihood ratio of a negative result, LR (-); you should calculate likelihood ratios LR(r) for each possible result r of the test. in the context of the problem on BNP, "r" refers to a BNP result that falls into a particular interval, like 80 - 150.
The general formula is:
LR (r) = P(r|D+)/P(r|D-)
This is the probability of result "r" in a patient With the disease Over the probability of result "r" in a patient WithOut the disease. (WOWO)
Instructions on how to calculate an interval likelihood ratio using data like those in Problem 4.3 were given in the lecture on 10/2 and are also in the answer to 4.3 B) at the back of the textbook.