Association of neutrophil-to-lymphocyte ratio with outcomes in aneurysmal subarachnoid hemorrhage: a systematic review and meta-analysis

The neutrophil-to-lymphocyte ratio (NLR) is a readily available inflammatory marker with potential prognostic relevance in aneurysmal subarachnoid hemorrhage (aSAH). This systematic review and meta-analysis evaluated its association with poor functional outcome and other adverse outcomes in patients with aSAH. Following PRISMA guidelines, we searched PubMed, Embase, Web of Science, the Cochrane Library, and Scopus from inception to May 1, 2026 (PROSPERO: CRD420261401613). Observational studies reporting associations between NLR and outcomes in aSAH were included. Poor functional outcome was the primary endpoint. Separate analyses evaluated individual complications, and an exploratory synthesis combined study-level estimates across different adverse endpoints. Random-effects models using restricted maximum likelihood estimation were applied. Subgroup analyses, univariate meta-regression, sensitivity analyses, and assessments of publication bias were performed. Evidence certainty was evaluated using GRADE. Twenty-eight studies involving 8,892 patients were included. Elevated NLR was associated with poor functional outcome (22 studies; OR = 1.08, 95% CI 1.04–1.13, P < 0.001), delayed cerebral ischemia (9 studies; OR = 1.52, 95% CI 1.15–2.01, P = 0.003), and postoperative pneumonia (4 studies; OR = 1.07, 95% CI 1.04–1.10, P < 0.001). The exploratory overall synthesis also showed a positive association (OR = 1.08, 95% CI 1.05–1.12, P < 0.001). Associations with mortality, cerebral vasospasm, and cerebral infarction were not statistically significant. Substantial heterogeneity was observed for most outcomes. NLR exposure modeling was the only significant meta-regression moderator, accounting for an estimated 49.9% of between-study variance. The overall association remained significant throughout leave-one-out analyses. Funnel plot asymmetry suggested small-study effects, and evidence certainty ranged from low to very low. Elevated NLR was associated with poor functional outcome, delayed cerebral ischemia, and postoperative pneumonia in patients with aSAH. These findings support further evaluation of NLR as a low-cost adjunct to prognostic assessment. Standardized prospective studies and external validation are needed to establish its incremental predictive value and clinical utility.