• Timothy J Coats and Richard Loffhagen.
• Emergency Medicine, Leicester University, Leicester Royal Infirmary, Leicester, UK. t.coats@virgin.net
• Eur J Emerg Med. 2006 Apr 1;13(2):80-3.

ObjectivesAs 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.MethodsLiterature review and Bayesian analysis of the application of the data to clinical practice.ResultsA 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.ConclusionsIn 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.

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