Friday, July 26, 2013

Most Positive LPs Aren't SAH

There's still active debate regarding whether CTs are now sensitive enough to pick up all subarachnoid hemorrhage, and whether lumbar puncture is about to go the way of the dodo.  Their argument is based on the premise that, despite the imperfect gold standard of this practice changing study, very few negative CTs result in positive lumbar punctures.

However, as this chart review study shows, in real-world practice, most positive LPs may in fact be false positives.  This is a 10 year review at Barnes Jewish Hospital, where they were able to identify 57 patients with negative CT and positive LP, who subsequently underwent angiography for diagnostic confirmation.  Of these, three patients had positive findings – only two of which were ultimately determined to be true positives.

This is a little different than previous reviews, in which 53% of negative CT/positive LP represented true positives.  These authors suggest the previous higher-yield results are likely the result of dependence on xanthrochromia as part of the diagnostic evaluation, and a cohort with a longer duration of symptoms prior to LP.

The truth is likely somewhere in the middle, splitting the difference between useless and half-wrong.  This probably aids the "CT, no LP" camp – of which I tend towards – by demonstrating the preponderance of false positives inherent to LP for SAH.

"Yield of Cather Angiography After Computed Tomography Negative, Lumbar Puncture Positive Subarachnoid Hemorrhage"
www.ncbi.nlm.nih.gov/pubmed/23619131‎

2 comments:

  1. Ryan, It doesn't sig. alter your conclusion, but most of these are not false + LPs for SAH. They are false positive LPs for ANEURYSMAL SAH. Many of these results may represent Perimesencephalic nonaneurysmal subarachnoid hemorrhage. No if you want to argue that these pts do well even if we miss them, especially if CT negative, I would not disagree.

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    1. I have never heard of perimesencephalic nonaneurysmal subarachnoid hemorrhage – you have enlightened where doleful darkness pervades. From what I'm reading, it seems these folks have excellent outcomes and return to normal level of functioning – so, indeed, that might support a statement that some of these +LP are not false positives, but rather true positives for a subclinical form of SAH that doesn't need to be detected.

      One of my former residents pointed out some aneurysmal SAH will have negative cerebral angiograms in the acute phase, if the aneurysm is filled with clot. I am not certain the prevalence of this phenomenon in the CT-negative LP-positive version of SAH.

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