Systemic inflammation response index as an independent predictor of unfavorable prognosis and its application in risk stratification in patients with aneurysmal subarachnoid hemorrhage

Background Aneurysmal subarachnoid hemorrhage (aSAH) is a devastating cerebrovascular disease associated with high rates of mortality and long-term disability. Early risk stratification is essential to guide personalized management. Systemic inflammation plays a key role in secondary brain injury after aSAH. The systemic inflammation response index (SIRI), a novel inflammatory marker combining neutrophil, monocyte, and lymphocyte counts, has shown prognostic value in multiple disorders, but its long-term prognostic role in aSAH remains unclear. Objectives This study aimed to investigate the association between admission SIRI and 12-month unfavorable functional outcomes (modified Rankin Scale [mRS] ≥ 3) in patients with aSAH, verify its independent prognostic value, and construct a clinically useful prediction nomogram. Methods A retrospective cohort study was performed including 258 patients with aSAH admitted between January 2021 and December 2024. Patients were divided into a favorable prognosis group (mRS 0–2, n = 158) and an unfavorable prognosis group (mRS ≥ 3, n = 100). Baseline characteristics, imaging indices including modified Fisher scale, laboratory parameters, and treatment data were collected. Multivariate logistic regression with forced entry was used to identify independent prognostic factors. Restricted cubic spline (RCS) analysis was applied to explore the non-linear relationship between SIRI and prognosis. A prediction nomogram was constructed and validated using temporal validation (training cohort n = 170; validation cohort n = 88). Model performance was evaluated using discrimination, calibration, and decision curve analysis. Results SIRI was significantly higher in the unfavorable prognosis group ( p < 0.001). Multivariate analysis confirmed that SIRI (OR = 1.20, 95% CI: 1.08–1.34, p = 0.001), age, hypertension, GCS score ≤ 8, modified Fisher scale, and treatment modality were independent prognostic factors. RCS analysis demonstrated a non-linear relationship (P for nonlinearity = 0.020), with a clear threshold at SIRI = 4.36; the risk of unfavorable outcomes rose steeply above this cutoff. The nomogram showed excellent discrimination (AUC = 0.881 in training; 0.919 in validation) and satisfactory calibration. Decision curve analysis confirmed favorable clinical utility. Conclusion Admission SIRI is an independent predictor of 12-month unfavorable functional outcomes in patients with aSAH. A threshold value of 4.36 can effectively identify high-risk patients. The SIRI-integrated nomogram provides accurate and individualized prognosis prediction across both training and temporal validation cohorts. This validated tool provides robust evidence to support clinical risk stratification and personalized decision-making.