Effect of delayed lumbar puncture on the diagnosis of acute bacterial meningitis in adults EMJ 2010:27:433-438

The basic overview for everyone

Meningitis is a bad thing. The bacterial one at least. Around the world it kills kiddies in droves. It’s a big deal. Thanks to vaccines and antibiotics it is not so much of a big deal (as in common) where I live. When it happens it’s a terrifying disease it just doesn’t happen that often. Even more so since the pneumococcal vaccine.

The classic (headache, photophobia, neck stiffness) presentation is no longer considered classic because we see it so rarely that we now only see the really hard cases – the kid with the fever and a sore throat who’s dead by morning. In the third world you’ll see the classic presentation all the time.

The test we do to make the diagnosis is the lumbar puncture – the one they do on House in every episode. And as tests go it’s not bad.

Very rarely, and mainly more than 40 years ago, you would put the needle in to take some fluid and the patients brain would squeeze out of his skull from the pressure change. This is as one would imagine a bad thing.

We are all terrified of this happening to us when we do the test (like most things in medicine we are more scared of doing harm than we are keen to do the right thing) so we often get a scan of the patients brain first to see if there’s anything obvious like blood or an abscess in there that might make the patients brain squirt out.

Rarely does this scan do anything other than make us feel better. It is rarely helpful.

As a result we delay doing the useful test so we can do a less useful one.

Most of the time these days we have enough common sense to start treating the patient before we do any tests. Treating bacterial meningitis quickly is one of the few diseases where treating it early makes a genuine difference.

Now I think these guys conclusions are mainly right but I’m not sure the study they’ve published gives them much evidence to say it.

The more complex critical appraisal bit

They tried to look at people who had meningitis and see how long they waited for a lumbar puncture and why they waited and what impact this had on how they did. But there are lots of problems.

– it’s a chart review – they looked at notes and decided what was wrong with the patient from there. Which can be useful but often you can read the chart whichever way you wish to prove your point. What do you do with missing data? What if someone made a decision on data that wasn’t written down? If you don’t tell how you decided what the chart said then everything that follows is dubious

– the patients they choose to look at are those pulled by discharge coding – so you only get in the study if someone thought you had meningitis and wrote it down. This misses those who had meningitis but no one made the diagnosis and thought they had something else. The famous paper on how to do a chart review is here.

– they excluded people who didn’t get a lumbar puncture – this was 10% of their patients. This is a big problem as there was probably a reason why they didn’t get an LP and so you can’t make statements about all meningitis patients, only the ones who got an LP. Though one would question the validity of a disgnosis of meningitis that doesn’t involve an LP.

– they questioned and reviewed the diagnosis of meningitis – they may well be right that some of the people who got coded as meningitis weren’t meningitis but you can’t do this with a chart review with their methods

– the gold standard seemed to be the British Infection Society guidelines – as with most guidelines these are often evidence light (there’s usually not much evidence in existence). I am aware of no evidence that shows that following the guidelines saves lives even though I agree with the guidelines in most respects.

– they do statistical analysis on symptoms of small numbers of patients from a chart review – this bit is completely pointless – when I think someone has meningitis my aim isn’t to ensure whether I’ve written down whether they vomited before coming in, I ask, but that doesn’t mean I write it down so you can’t find it out retrospectively.

– they try to make firm statements about whether or not a patient should have had a CT based on what was written on the chart when in reality there are lots of reasons, many logistical, why this happens or not.

– they note that none of the patients (they selectively chose) had LP prior to antibiotics even though “antibiotics are immediately started after the LP is performed, or sooner if there is a delay of more than 30 mins”. This is kind of weird as the reason they didn’t get the antibiotics before LP may have been that the docs thought there may have been a >30 min delay (for whatever reason) in which case they were following guidelines.

We do lots of needless CT scans – the most useful point I found was that of the 62 patients (two thirds of all) who got scanned none had anything to worry about on CT.

I agree with these guys that we miss valuable information by sending people for silly scans that take too long to move the patients for and get a report and make a decision(not the scan itself which is seconds). I agree that the 4 hour target has caused problems here [though in recent news we appear to be scrapping it]. I agree that we’re all too scared to do the right thing. Unfortunately this isn’t great proof of that.

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