Question of the Day

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 2×2 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%.