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"