April 26, 2011

Blast from the "past"

Reuben posted this on his site in September...


September 21st, 2010

Traumatic LP for Meningitis

by seth in .fevermeningitis/encephalitis
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Special thanks to Reuben for inviting me to post this review from my teaching resident rotation.
Question:
Is a “corrected” CSF WBC count accurate for diagnosing meningitis for a traumatic LP?

Background:
Traumatic lumbar punctures may obscure accurate diagnoses. Many authors suggest correcting the WBC count by various methods — the most popular seem to be either 700 RBC = 1 WBC, or by using the actual patient’s RBC:WBC ratio in the blood. While this seems intuitive, does it work?
Answer:
Probably not.
Basically, no; the calculations are not helpful. But if the WBC count is MUCH higher than expected, it’s probably a positive tap.
Key points:
  • The sources I could find simply assert that correction is a viable method; I could not find any actual evidence that these corrections are valid.
  • Multiple small studies show that corrections are generally not accurate (including ref. 1), with ROC curves equivalent regardless of how — or if! — correction is applied
  • However, a few small studies also show that bacterial meningitis may be obvious despite a traumatic tap (refs 2 & 3):
If the “observed:predicted” ratio of CSF WBCs is >10, then some authors conclude that it indicates bacterial meningitis. Sensitivity & specificity are both around 80-90% with this method.
I think a higher threshold is probably better (ratio >100) — see images below.
Example:
CBC:
5 RBC (Hgb 15; Hct 45)
5 WBC
This is a predicted ratio of 1000:1 (RBCs are reported as 10^6/mcL and WBCs are 10^3/mcL)
A purely traumatic tap in this patient would be expected to look like this:
CSF
2000 RBC
2 WBC
If the CSF looked like this:
2000 RBC
20 WBC
than it is “likely” to be bacterial meningitis (Observed:Predicted = 10)
Looking at the data, I think we can all agree that this CSF is infected:
2000 RBC
200 WBC
(Observed:Predicted=100)
Here are the results from the Bonadio paper:

Bonadio data
Looking at their raw data, the ratio of 100 looks like a much better diagnostic cutoff, although it is probably best to still treat (i.e. antibiose & admit) pending more accurate tests (i.e. culture) if the picture is less clear.
Here is a ROC curve for their data, which looks pretty good altogether:
References:
  1. Greenberg RG, Smith PB, Cotten CM, Moody MA, Clark RH, Benjamin DK Jr. Traumatic lumbar punctures in neonates: test performance of the cerebrospinal fluid white blood cell count. Pediatr Infect Dis J. 2008 Dec;27(12):1047-51.
    There a number of similar small studies that all agree that adjustments are not useful.
  2. Bonadio WA, Smith DS, Goddard S, Burroughs J and G Khaja. Distinguishing cerebrospinal fluid abnormalities in children with bacterial meningitis and traumatic lumbar puncture. The Journal of Infectious Diseases. July 1990: 162(1): 251-254.
  3. Mayefsky, JH. Determination of leukocytosis in traumatic spinal tap specimens. The American Journal of Medicine. June 1987: 82(6): 1175.
NB I didn’t put references for any of the textbooks or papers (most of which refer to the same 2-3 textbooks) that simply assert that calculations are helpful.

April 23, 2011

Patients aren't customers

Paul Krugman's excellent April 21, 2011 column criticizes those who argue that increasing market forces in the healthcare economy will decrease costs, especially since it hasn't:

“Consumer-based” medicine has been a bust everywhere it has been tried. To take the most directly relevant example, Medicare Advantage, which was originally called Medicare + Choice, was supposed to save money; it ended up costing substantially more than traditional Medicare. (PK 4/21/2011)

Similarly, my wife -- who does a substantial amount of healthcare billing at her job -- loves to point out that secondary insurance plans that many Medicare patients have decide what is covered or not covered based on what Medicare covers (at least in the nutrition counseling she bills).* Which raises the question: what is the point of having extra coverage if it doesn't cover anything extra?

(This is slightly different than Medicare Advantage; my point here is that purchasing insurance on the free market sounds nice but doesn't work.)

Further, there is clearly something different about "purchasing" healthcare than, say, a new 3D flatscreen HDTV. The choice between spending $12,000 on a new TV or an appendectomy isn't a choice if one of those options involves a pretty good chance of getting septic and dying. I'm not a trained ethicist or anything, but that seems about the moral equivalent of a mugger threatening to stab you in the right lower quadrant if you don't give them $12,000.
Medical care, after all, is an area in which crucial decisions — life and death decisions — must be made. Yet making such decisions intelligently requires a vast amount of specialized knowledge. Furthermore, those decisions often must be made under conditions in which the patient is incapacitated, under severe stress, or needs action immediately, with no time for discussion, let alone comparison shopping. (PK 4/21/2011)
Krugman also fails to mention the middleman costs of privatization of health insurance. Overhead for private insurers is close to 30% -- about equal parts profit, paperwork, and effort spent denying claims. Note that most of that overhead is expressly about NOT providing patients with healthcare. Medicare has an overhead of <3%.

-nst



*Few insurance plans cover nutrition counseling in any substantive way. It's very difficult to get appropriate counseling for a myriad of medical problems -- such as gestational diabetes. But most insurance companies will cover foot amputations.




Update:

Shadowfax's outstanding blog about this topic (referred by BK at emedcentral)