A multicentre Canadian study challenges the practice of routine lumbar puncture after negative CT in patients with suspected subarachnoid haemorrhage. CT scanning within six hours was highly sensitive, although a few cases of initially misinterpreted CTs “illustrate the importance of having a qualified radiologist with a high level of skill interpreting the head scans in a timely manner“.

Nearly 2% of patients were lost to all follow-up; the authors point out that even if a quarter of these patients could have experienced a subarachnoid haemorrhage, the corresponding negative likelihood ratio for a computed tomography performed within six hours rises to only 0.024 (0.007 to 0.07). They assert:

Such a likelihood ratio could be incorporated into the informed discussion surrounding the risks and benefits of lumbar puncture after negative results on computed tomography for this diagnosis

They point out that when CT imaging is obtained more than six hours after headache onset, clinicians should continue to be cautious because of the decreasing sensitivity for subarachnoid haemorrhage beyond this time.

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

Sensitivity of computed tomography performed within six hours of onset of headache for diagnosis of subarachnoid haemorrhage: prospective cohort study
BMJ. 2011 Jul 18;343:d4277