Introduction
Hypophosphataemia is common in critically ill patients, but neither its prevalence nor its association with outcome have been investigated specifically in patients with aneurysmal subarachnoid haemorrhage (aSAH).

Methods
Patients with aSAH and at least one phosphate measurement were included from two independent cohorts; an American cohort extracted from two open-access databases (Medical Information Mart for Intensive Care-III and eICU Collaborative Research Database v. 2.0) and a Danish cohort consisting of patients with aSAH admitted to Rigshospitalet, Denmark over a 4-year period. In each cohort, we calculated the prevalence of mild (0.32–0.80 mmol/L) and severe (<0.32 mmol/L) hypophosphataemia and their association with in-hospital mortality before and after propensity-score matching.

Results
Hypophosphataemia occurred in 72.4% (95% CI: 68.1–76.3) of patients in the American cohort (n = 471) and 54.9% (50.0–59.7) in the Danish cohort (n = 419). However, it was not associated with mortality in neither full (American: Mild, Odds ratio (OR) 0.99 (0.91–1.07), Severe OR 1.20 (0.95–1.51); Danish: Mild, OR 1.01 (0.95–1.08), Severe OR 1.20 (0.95–1.51)) nor propensity-score matched cohorts (American (n = 168): Mild, OR 1.06 (0.88–1.28), Severe OR 1.46 (0.96–2.12); Danish (n = 44): Mild, OR 1.16 (0.82–1.65), Severe OR 0.45 (0.13–1.55)).

Conclusion
In this retrospective study of patients with aSAH, hypophosphataemia was common.