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Excellence In Physical Exam Series | Strep Throat

Matthieu DeClerck, MD and Mike Weinstock, MD
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Strep throat has many physical exam findings, some of which are nonspecific such as erythema and others which are very specific such as palatine petechiae.

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Vince D. -

Regarding palatine petechiae: Don't forget that we can never examine on sensitivity or specificity on their own. Even with a very strong specificity, if the sensitivity is in the single digits, the finding probably isn't clinically useful. When I calculate the likelihood ratio for a sensitivity of 7% and specificity of 95%, I get an unimpressive LR+ of 1.4.

I'm sure it was just a slip up from Dr. DeClerck and Dr. Weinstock (thanks for the excellent series btw!), but I'll expand further for anyone else who happens upon this comment.

Imagine it this way: You're seeing 100 patients with sore throats, and, being generous, 30% of them have Strep as the culprit. Using a sensitivity of 7%, you'll see palatine petichiae in 2.1 of the 30 people with Strep throat. Not very high yield, but maybe useful still? But wait! With a specificity of 95% and 70 patients presenting with non-Strep throat, you'll also identify palatine petichiae in 3.5 patients who *don't* have Strep at all.

The numbers get worse if your prevalence of Strep is lower than 30%.

Mike W., MD -

Thx Vince and you are right, we do need to factor in our pretest probability as well as many other factors. For example an IV drug user with fever and no cough would have a centor criteria of 2, obviously very different LR of having strep throat then the 16 year old with a sore throat, exudate and ant cerv LAN (also w a centor criteria of 2). Thx for the comments!
M

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Off The Cuff Full episode audio for MD edition 169:22 min - 79 MB - M4AHippo Urgent Care RAP September 2019 Written Summary 681 KB - PDF

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