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!)