Family Medicine Notes

Today's unrelated tidbits:

• A great tool for helping men with a decision for/against PSA testing.  This thing is buried deep in the guts AAFP website - so deep that I couldn't find it.  Here it is. 
• A very good review article on dx/work-up of fatigue.
• Excellent guidelines on Otitis, pharyngitis and bronchitis
• Baysean Calculator.

The last one is the best of many calculators out on the web.  Here's why - it combines the calculations of likelihood ratios with the calculation of PPV and NPV.  Huh?  Terms for normal humans: 

a) How good is the test that I'm doing for this disease? (sensitivity) let's take one that is VERY common:  the "rapid strep" test.  It's pretty good, but not perfect.  Let's say it's 85% sensitive.  This means that 15% of the time it will miss strep throat.  So ... does a patient in my office with a sore throat and a negative "rapid strep" have a 15% chance of having strep throat?  Nope.  Read on.

b) How accurate is the test? (specificity).  If I get a positive result - how sure can I be that the patient really has it?  In the case of the rapid strep - darn sure.  Let's say 99%.

c) How likely is THIS patient to have this disease given their symptoms?  Long story - Kids are more likely to have it than adults.  Let's say an adult has a 10% chance of having a sore throat caused by strep - and a kids have a 25% chance. 

So now we can use the calculator.  Enter a prevalence of 25% (it's a kid with a  sore throat) and a sensitivty of 80% and a specificty of 99%. 

PTL- .. The post-test likelihood of a negative test = .048  So a kid with a negative rapid strep has only a 5% chance of having strep throat - not 15% like we thought above.

But we can play with this plot even more.  We know from the long story above (click it - it's a very good article) that the kid with a cough and no adenopathy is much less likely to have strep than the kid with no cough and positive anterior adenopathy.  So let's say our patient has a cough and runny nose and not much of a fever.  Hmm .. doesn't sound much like strep.  But he does have a sore throat.  Let's move his "prevalence" score to 10%.

Yikes.  Now the PTL- is .01 -- not very likely that this kid has strep.

So now you see why I'm not compelled to send a follow-up culture (with sensitivity of 90% rather than 85%) if I get a negative rapid strep.