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Is it worth 700 LPs to find one aneurysmal subarachnoid hemorrhage?

Are you in the 700 club? Most emergency physicians are.

If you trained in emergency medicine you learned that the work-up of a potentially dangerous headache isn’t finished until the CSF is back. In this month’s SMART EM (www.smartem.org) we flesh out our educational session from ACEP 2012, a talk which asked how many LPs it takes to find one aneurysmal subarachnoid hemorrhage—and what the fallout is.

The literature on this topic is both fresh and remarkably good. The studies are recent, large and prospective, with excellent follow-up. And the results are straightforward. It turns out that following classic dogma means performing roughly 700 lumbar punctures for every one aneurysmal subarachnoid hemorrhage detected. Thus, if you trained in emergency medicine, you’re probably in the 700 club.

And why are we in the club? Faith. For years we have been true believers that performing an LP after a negative CT increases a patient’s safety. But the numbers raise an important question: if we’re poking hundreds of backs for just one to benefit, how much safety is lost? LPs, after all, cause infections, pain, headaches, and on rare occasion serious morbidity. They also lead to false positives, which typically mean a second LP, or perhaps advanced imaging with radiation, and contrast exposure. Sometimes patients are admitted after a false positive LP and have cerebral angiography, a procedure with definite risks. And once in a blue moon someone undergoes risky neurosurgery due to a false positive, because imaging can’t discern which aneurysms were causing a headache and which are incidental.

There’s more. The 700 calculation presumes that patients undergoing LP have acute onset, first time, mostly worst-of-life headaches. In relevant studies roughly 8-10% of such patients have a subarachnoid hemorrhage, making this a high risk group. But that’s not most headache patients. In patients with half the risk, the ratio of LPs to aneurysmal SAH jumps from 700 to 1400. And for those with lower risk headaches the number is in the thousands, which helps to explain why finding an aneurysmal SAH by LP is a Ghostbusters moment.

Now, I love doing LPs. It is a deft hand that swiftly and smoothly threads the needle, coaxing fluid. But 700 is a lot. And thousands of LPs is pretty much LP hell—at both ends of the needle. After all, in addition to being painful and having side effects, up to a third of LPs are falsely positive.

After seeing these numbers we decided to calculate a ‘Test Threshold’ – a formula that crunches the life-saving benefits of finding a true positive LP, and the life-threatening harms of LP for everyone else (we left out minor harms like headache, pain, and infection). The answer surprised us: the threshold is roughly 1%. In other words, performing LPs on more than about 100 people for each true positive LP leads to more life-threatening harm than life-saving good.

This is a revelation. According to our best calculations, unless a patient has high risk headache features (syncope, stiff neck, etc.) the most beneficial approach to diagnosing SAH is a CT scan—with no LP—because the scan typically gets you well under the 1% mark.

It’s a tough pill to swallow, so don’t take our word for it. Listen to the full audio at www.smartem.org, then contemplate, and consider. Then tell us what you think. To us it seems like the 700 club, and the dogma of classic teaching, are more religion than science.

David H. Newman, MD is Author of Hippocrates’ Shadow: Secrets From The House Of Medicine

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