Subarachnoid haemorrhage rules in the decision for acute CT of the head: external validation in a UK cohort

Background The Ottawa subarachnoid haemorrhage (SAH) rule and the Emerald SAH rule are clinical decision tools to aid in the decision for computed tomography (CT) of the head in patients attending an emergency department (ED) with acute non-traumatic headache. The objective of this study was to analyse the performance of these rules in a contemporary UK cohort. Methods We performed a retrospective external validation study. Patients undergoing CT of the head for the evaluation and treatment of non-traumatic headaches over a 6-month period in the ED at two tertiary centres were assessed. Each patient's Ottawa rule and Emerald rule were calculated and compared with their final diagnosis. Results The cohort consisted of 366 patients and there were 16 cases of SAH (based on CT findings or the presence of xanthochromia in cerebrospinal fluid). The Ottawa rule identified 288 patients requiring CT of the head. The sensitivity of the Ottawa rule was 100% (95% confidence interval (CI) 71-100%) and the specificity was 22% (95% CI 18-27%). The Emerald rule identified 267 patients who required CT, and achieved a sensitivity of 81% (95% CI 54-96%) and a specificity of 27% (95% CI 23-32%). Conclusions The Ottawa SAH rule correctly identified all patients with SAH in this contemporary cohort. The Emerald rule did not perform as well in this cohort and is unsuitable for clinical use. The Ottawa rule is a useful tool to aid in the decision for CT of the head in patients presenting with acute non-traumatic headache to the ED.