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Sensitivity of computed tomography performed within six hours of onset of headache for diagnosis of subarachnoid haemorrhage: prospective cohort study

BMJ 2011; 343 doi: (Published 18 July 2011) Cite this as: BMJ 2011;343:d4277
  1. Jeffrey J Perry, associate professor of emergency medicine and of epidemiology and community medicine1,
  2. Ian G Stiell, professor and chair department of emergency medicine1,
  3. Marco L A Sivilotti, associate professor of emergency medicine and of pharmacology and toxicology2,
  4. Michael J Bullard, professor of emergency medicine3,
  5. Marcel Émond, assistant professor4,
  6. Cheryl Symington, research coordinator1,
  7. Jane Sutherland, research coordinator5,
  8. Andrew Worster, associate professor6,
  9. Corinne Hohl, assistant professor7,
  10. Jacques S Lee, assistant professor8,
  11. Mary A Eisenhauer, associate professor9,
  12. Melodie Mortensen, research coordinator1,
  13. Duncan Mackey, associate professor3,
  14. Merril Pauls, associate professor10,
  15. Howard Lesiuk, consulting neurosurgeon11,
  16. George A Wells, professor biostatistics and epidemiology12
  1. 1Department of Emergency Medicine, University of Ottawa, Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, ON, Canada
  2. 2Departments of Emergency Medicine and of Pharmacology and Toxicology, Queen’s University, Kingston, ON
  3. 3Department of Emergency Medicine, University of Alberta, Edmonton, AB
  4. 4Department of Emergency Medicine, Université Laval, QC
  5. 5University of Ottawa, Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, ON
  6. 6Department of Emergency Medicine, University of British Columbia, Vancouver, BC
  7. 7Division of Emergency Medicine, McMaster University, Hamilton, ON
  8. 8Division of Emergency Medicine, University of Toronto, Toronto, ON
  9. 9Division of Emergency Medicine, University of Western Ontario, London, ON
  10. 10Department of Emergency Medicine, University of Manitoba, Winnipeg, MB
  11. 11Division of Neurosurgery, University of Ottawa, the Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, ON
  12. 12Department of Epidemiology and Community Medicine, University of Ottawa, Clinical Epidemiology Program, Ottawa Hospital Research Institute, Ottawa, ON
  1. Correspondence to: J J Perry, Clinical Epidemiology Unit, F647, Ottawa Hospital, 1053 Carling Avenue, Ottawa, ON, Canada K1Y 4E9 jperry{at}
  • Accepted 13 May 2011


Objective To measure the sensitivity of modern third generation computed tomography in emergency patients being evaluated for possible subarachnoid haemorrhage, especially when carried out within six hours of headache onset.

Design Prospective cohort study.

Setting 11 tertiary care emergency departments across Canada, 2000-9.

Participants Neurologically intact adults with a new acute headache peaking in intensity within one hour of onset in whom a computed tomography was ordered by the treating physician to rule out subarachnoid haemorrhage.

Main outcome measures Subarachnoid haemorrhage was defined by any of subarachnoid blood on computed tomography, xanthochromia in cerebrospinal fluid, or any red blood cells in final tube of cerebrospinal fluid collected with positive results on cerebral angiography.

Results Of the 3132 patients enrolled (mean age 45.1, 2571 (82.1%) with worst headache ever), 240 had subarachnoid haemorrhage (7.7%). The sensitivity of computed tomography overall for subarachnoid haemorrhage was 92.9% (95% confidence interval 89.0% to 95.5%), the specificity was 100% (99.9% to 100%), the negative predictive value was 99.4% (99.1% to 99.6%), and the positive predictive value was 100% (98.3% to 100%). For the 953 patients scanned within six hours of headache onset, all 121 patients with subarachnoid haemorrhage were identified by computed tomography, yielding a sensitivity of 100% (97.0% to 100.0%), specificity of 100% (99.5% to 100%), negative predictive value of 100% (99.5% to 100%), and positive predictive value of 100% (96.9% to 100%).

Conclusion Modern third generation computed tomography is extremely sensitive in identifying subarachnoid haemorrhage when it is carried out within six hours of headache onset and interpreted by a qualified radiologist.


  • We thank the hundreds of physicians who completed our data collection forms and the emergency department nurses and clerks at the 11 study sites for their cooperation with the study; Albert E Lauwers, deputy coroner, and the Office of the Coroner of Ontario for their assistance verifying outcomes; and the following research staff at the study hospitals: Jan Buchanan (Vancouver General Hospital, Vancouver, BC), Evelyn Gilkinson (London Health Sciences Centre, London, ON), Erica Battram, Juanita Wilzer, Nicole Fortin, Renée Labreche (Ottawa Hospital-Civic Campus and General Campuses, Ottawa, ON), Harris Lari, Leslie Saunders, Ginny Willis, Sandy Sandilands (University of Alberta, Edmonton, Alberta), Deborah Wright, Johanna Pak (Sunnybrook and Women’s College Health Sciences Centre, Toronto, ON), Kathy Bowes, Julie Richard, Deborah Crosby, Jane Reid, Nicholas Martin (Kingston General Hospital, Kingston, Ontario), Patricia Chabot (Hôpital de L’Enfant-Jésus, Quebec City, QC), Christina Brean (Hamilton Health Sciences Centre, Hamilton, ON), Marlene Myles (Lethbridge Regional Hospital, Lethbridge, AB), Irene Osinchuk (Winnipeg Health Sciences Centre, Winnipeg, MB). We also thank our colleagues at the Ottawa Hospital Research Institute (Sarai Cohn-Kalter, Malaika Mvungi, Sheryl Domingo, My-Linh Tran, Irene Harris, and Angela Marcantonio) for their assistance with this project.

  • The study was presented at the Society for Academic Emergency Medicine, New Orleans, Louisiana, May 2009, and the Canadian Association of Emergency Physicians, Calgary, Alberta, June 2009.

  • Contributors: JJP developed and contributed substantially to study concept and design and secured research funding. All authors participated in the study design, supervised recruitment of patients and management of data, supervised the conduct of the trial and data collection, drafted the manuscript and/or contributed to its revision, and approved the final version. JJP, IGS, and GAW provided statistical advice and data analysis. JJP is guarantor.

  • Funding: The study was funded by the Canadian Institutes for Health Research (grants 67107, 153742), the Ontario Ministry of Health and Long Term Care, and the physicians of Ontario through the Physician’s Services Incorporated Foundation (01-39). JJP is supported by a Canadian Institutes for Health Research new investigator award and was previously supported as a career scientist by the Ontario Ministry of Health. IGS is a distinguished professor and university health research chair, University of Ottawa. CH is supported by a mentored clinician scientist award from the Vancouver Coastal Health Research Institute.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at (available on request from the corresponding author) and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: The study protocol was approved by the research ethics boards at each centre without the need for written informed consent as patients continued to be investigated as per current practice. Participants were informed that they could be contacted by telephone for an update on their status, and verbal consent was obtained at the time of the telephone call.

  • Data sharing: No additional data available.

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