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June 20, 2013

A Graphic Reanalysis of the Leukocyte Count in Acute Appendicitis

The leukocyte count (aka white blood cell count; WBC in North America; WCC in Oz) has fallen out of favor with many, particularly in the diagnosis of acute appendicitis. Most EM docs I know love regaling each other with stories of the CT showing acute appy and the surgeon asking "what's the white count?" Always struck me like standing in the rain asking about the weather report.

(In all seriousness, I have a lot of respect for surgeons -- most that I know are very smart and incredibly hard-working; please don't cut me with your surgery knifey-thing).

A fun little paper that Scott Weingart has posted on his webtext shows likelihood ratios* (LR+) for appendicitis for different WBC levels.* Their sample is not huge but it's still nice.

The main conclusion is that WBC is only really useful in diagnosing appendicitis if it's below 7 or greater than 17, which is pretty much nobody.

Here is a graphic representation I put together of their results:

And here is their table with all the actual values:

This isn't to say that every patient with a possible appy should forego a CBC or that they all need CTs. See, for example, Choosing Wisely.

The lesson is that we should know the value -- or lack thereof -- of the tests that we do.

What's the deal with the title?

*Explanation of LRs that I wrote for The NNT a while back:

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

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