Just Added!

New Videos with Amal Mattu, MD

Watch NowGo

LP after CT for SAH takes another hit

August 15, 2016

Short…ish Attention Span Summary

This one is longer.  It’s complicated…and important.
If a patient with suspected SAH has a negative head CT, when should we perform LP?  Last month we covered a meta-analysis that found LP would only be of benefit if the post-CT probability of disease was 5%. This study, a decision analysis, found that number to be 4.3% (95% CI, 1.4-9.3%).

So do they need LP?
Let’s take the example of the classic BMJ study by Jeffrey Perry and Ian Steill.  In that study, the prevalence of SAH in the 3132 study patients was 7.7%.  The post-test probability of SAH in those who got a CT within 6 hours was 0.6% (negative likelihood ratio 0.07).  But what about patients who had CT >6 hours from headache onset?  Sensitivity was lower, but the negative likelihood ratio was still solid at 0.14, making the post-test probability of SAH in this group only 1.2%.

That means that even after a delayed (>6 hours) negative head CT, the probability of SAH is less than the testing threshold of 4.3% and even less than the lower range of 95% confidence, 1.4%.  The authors conclude, “considering the low probability of aneurysmal SAH after a negative CT, classical teaching and current guidelines addressing testing for subarachnoid hemorrhage should be revisited.”

Spoon Feed
Unless your patient population has a much higher prevalence of disease than the Perry study and the meta-analysis last month, the risk of missing SAH with a negative head CT in < 6 hours is extremely low. I would not do additional testing.

A negative head CT >6 hours is still under the testing threshold of this decision analysis.  Based on this new information, I would recommend having an informed discussion with your patients about the real risk of missing SAH vs. that of additional testing. Both LP and CTA are tests with complications and potential unintended consequences.

This one needs some discussion and debate.  Feel free to use the comments section on the website, below this post.  I don’t see anyone else in the #FOAMed community discussing this article, so please use this as a forum for discussion.


Acad Emerg Med. 2016 Jul 5. doi: 10.1111/acem.13042. [Epub ahead of print]

Determination of a Testing Threshold for Lumbar Puncture in the Diagnosis of Subarachnoid Hemorrhage after a Negative Head CT: A Decision Analysis.

Taylor RA1, Singh Gill H2, Marcolini EG1, Meyers HP3, Faust JS4, Newman DH4.

Author information:

1Department of Emergency Medicine, Yale University School of Medicine, 464 Congress Avenue, Suite 260.

2Departments of Emergency Medicine and Critical Care, Cleveland Clinic, Cleveland, OH.

3Duke University School of Medicine, Durham, NC.

4Department of Emergency Medicine, Icahn School of Medicine at Mount Sinai, One Gustave Levy Place, NYC, NY, 10029.



To determine the testing threshold for lumbar puncture (LP) in the evaluation of aneurysmal subarachnoid hemorrhage (SAH) after a negative head CT. As a secondary aim we sought to identify clinical variables that have the greatest impact on this threshold.


A decision analytic model was developed to estimate the testing threshold for patients with normal neurologic findings, being evaluated for SAH, after a negative CT of the head. The testing threshold was calculated as the pretest probability of disease where the two strategies (LP or no LP) are balanced in terms of quality adjusted life years (QALYs). Two-way and probabilistic sensitivity analyses (PSA) were performed.


For the base case scenario the testing threshold for performing an LP after negative head CT was 4.3%. Results for the two-way sensitivity analyses demonstrated that the test threshold ranged from 1.9%-15.6%, dominated by the uncertainty in the probability of death from initial missed SAH. In the PSA the mean testing threshold was 4.3% (95%CI, 1.4-9.3). Other significant variables in the model included: probability of aneurysmal versus non-aneurysmal SAH after negative head CT, probability of long-term morbidity from initial missed SAH, and probability of renal failure from contrast induced nephropathy.


Our decision analysis results suggest a testing threshold for LP after negative CT to be approximately 4.3%, with a range of 1.4% to 9.3% on robust PSA. In light of these data, and considering the low probability of aneurysmal SAH after a negative CT, classical teaching and current guidelines addressing testing for subarachnoid hemorrhage should be revisited. This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

PMID: 27378053 [PubMed – as supplied by publisher]

What are your thoughts?