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NAVLE® Question of the day

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September 5, 2024

NAVLE

A practice is using an FeLV test with a sensitivity of 90% and a specificity of 95%.

Assuming the prevalence of feline leukemia in the area is 5%, what is the predictive value negative (PVN) of the test?

A - 55%

B - 45%

C - 88%

D - 75%

E - 99%

Sorry. You chose "B - 45%" but the correct answer is "E - 99%"

PVN is 99%.

The trick with this kind of question is to pick an imaginary number of animals that you test, like 1000, and fill out your 2x2 table from there.

Follow the links to see diagrams step by step.

If prev is 5% then there must be 50/1000 cats with FeLV and 950 cats that are disease-free.

A 90% sensitive test will correctly call 45/50 positive (box "a"), and IN-correctly call 5/50 negative, (box "c": these are the false negs).

If 50/1000 animals are infected, then 950/1000 are disease-free.

Your 95% specific test will correctly call 902/950 disease-free (box "d": 0.95 X 950=902) and IN-correctly call 48/950 positive, (box "b": these are the false pos).

Now your a,b,c,d boxes are all filled, it is easy to calculate PVN=d/(c+d)=902/(902+5)=99%.