title: Diagnosis of subarachnoid haemorrhage following a negative computed tomography for acute headache: a Bayesian analysis.
abstract: OBJECTIVE
  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.
authors.affiliation: Emergency Medicine, Leicester University, Leicester Royal Infirmary, Leicester, UK. t.coats@virgin.net
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Coats
authors.forename: Richard
authors.initials: R
authors.lastname: Loffhagen
mesh_terms: D000208:Acute Disease; D001499:Bayes Theorem; D006261:Headache / Q000000981:diagnostic imaging* / Q000209:etiology; D006801:Humans; D016013:Likelihood Functions; D012680:Sensitivity and Specificity; D013129:Spinal Puncture; D013345:Subarachnoid Hemorrhage / Q000150:complications / Q000175:diagnosis* / Q000000981:diagnostic imaging; D014057:Tomography, X-Ray Computed