Question of the Day

Don’t overthink this: As prevalence of a disease goes DOWN, PPV of your serologic test also goes DOWN.

That is, as your disease becomes more and more rare, the predictive value of your same old test gets WORSE.

That is all you need to know. Read more only if you want to see the math (but you don’t need it).

Let’s say prevalence of pedunculated giblet disease is 30% in 1000 turtles (that’s 300 infected, 700 disease-free, then). A test with 90% sensitivity would correctly diagnose 270 (cell “a”) with the disease (true pos) and correctly say 30 were negative (cell “c”). A test with 90% specificity would correctly diagnose 630 (cell “d”) as disease-free (true negs) and correctly say 70 were positive (cell “b”): PPV=a/(a+b)=270/(270+70)= 79%.

Follow this link to See a PPV Diagram.

Now, do the same math on 1000 turtles, but assume prevalence is now only ~1%  after your vaccination program. You will see PPV goes down to ~1%.