So I'm having dinner tonight with this group of happy medical students and they are bummin out about their EBM course and they ask me if I really use "EBM" and I say yes and then I do this little strep throat thing like this:

I see an adult in the office with a sore throat.  No kids.  No contact with kids

So I assume pre-test probability of about 5% Yeh – I made it up.  I'm probably close.

And the sensitivity of the test I'm doing is 85% (possibly better — but we'll be safe)

Specificty is very good – 99%

So let's do the poor man's version that I use to explain to the patient:

You have a 5% chance of having this BEFORE I do the test – so after the test you have a 15% chance of the 5% chance of having strep throat.

.15 x .05 = .0075 … let's round it up to 1%.

"Mrs Jones – you have a 1% chance that you have strep throat after this negative rapid-strep. Go home and drink warm tea with honey and you will feel better soon." (ok .. go find the evidence for THAT!)