LP After Delayed Head CT for SAH – Is It Required?

Written by Nickolas Srica

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The sensitivity of a negative non-contrasted head CT performed to rule out atraumatic subarachnoid hemorrhage (SAH) decreases when done >6 hours after headache onset, but further workup – including LP – requires shared decision making.

Why does this matter?
We continue to see more and more data supporting the notion that a negative non-contrasted head CT within 6 hours of headache onset during evaluation for atraumatic SAH is a sufficient rule-out test, and no further workup is needed in these cases (hence the ACEP Clinical Policy on Headache also supporting this). But what about a negative CT >6 hours after onset? Is a lumbar puncture necessary to rule it out in all of these patients?

Is performing a lumbar puncture worth the headache?
This was an evidence review of 4 previous studies including a retrospective cohort study, two prospective cohort studies, and a case-control study in order to assess the importance of lumbar puncture in the setting of a negative head CT >6 hours from headache onset during SAH evaluation. Three of the studies showed the potential risk of missing SAH by forgoing LP, as CT <6 hours vs. > 6 hours of symptom onset showed a significant drop in sensitivity (first study: <6h, 98.5% vs >6h, 90%; and in the second study: <6h, 100% vs >6h, 85.7%). One of the studies showed combining CT and LP ruled out SAH with 100% sensitivity up to 14 days after symptom onset, but traumatic taps led to a significant drop in specificity to 67%, with many false positives. The authors calculated a 1.1% post-test probability of SAH with a negative head CT >6 hours from symptom onset, and their testing threshold calculation for which emergency clinicians should perform LP was 1.0% (which is lower than some previous studies). Given that the post-test probability of SAH with head CT >6 hours from time of onset and the calculated testing threshold are essentially equal, the authors concluded that emergency physicians might reasonably decide to either perform or forego the LP, but shared decision-making should be utilized to make that decision. 

Source
Is A Lumbar Puncture Required to Rule Out Atraumatic Subarachnoid Hemorrhage in Emergency Department Patients with Headache and Normal Brain Computed Tomography More Than Six Hours After Symptom Onset?

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