ORIGINAL ARTICLESDiagnosis of subarachnoid haemorrhage following a negative computed tomography for acute headache: a Bayesian analysisCoats, Timothy J.a; Loffhagen, RichardbAuthor Information aEmergency Medicine, Leicester University, Leicester Royal Infirmary, Leicester bNorth East Thames Emergency Medicine Training Rotation, UK Correspondence and requests for reprints to T.J. Coats, Emergency Medicine Department, Leicester University, Leicester Royal Infirmary, Infirmary Square, Leicester LE1 5WW, UK Tel: 0116 258 5646; fax: 0116 204 7935; e-mail: email@example.com Sponsorship: Anthony Hopkins Memorial Fund. Received 30 August 2005 Accepted 4 October 2005 Contributions: Tim Coats developed the underlying ideas and contributed to writing the paper. Richard Loffhagen participated in the discussion of ideas, undertook the literature search and contributed to the writing of the paper. European Journal of Emergency Medicine: April 2006 - Volume 13 - Issue 2 - p 80-83 doi: 10.1097/01.mej.0000190277.92731.52 Buy Metrics Abstract Objectives As access to computed tomography scanning has become easier, patients with a lower probability of serious pathology are being scanned. We asked how many lumbar punctures need to be performed to detect each subarachnoid haemorrhage in these lower-risk patients. Methods Literature review and Bayesian analysis of the application of the data to clinical practice. Results A computed tomography scan for acute headache has a negative likelihood ratio of 0.02 if the computed tomography is performed at <12 h, 0.07 at <24 h and 0.18 at >24 h. A low pre-test probability, for example 1 in 20 (5%), and a negative computed tomography at <12 h means that more than 1000 lumbar punctures would be required to detect each subarachnoid haemorrhage. Conclusions In patients who have a low pre-computed tomography probability of subarachnoid haemorrhage and undergo an early computed tomography scan, the risk/benefit ratio of lumbar puncture is unclear. A decision rule (risk stratification system) might improve our ability to help the patient make an informed choice. © 2006 Lippincott Williams & Wilkins, Inc.