Association of Neutrophil-to-Lymphocyte Ratio and Systemic Immune-Inflammation Index With Mortality in Patients With Pericarditis: A Retrospective Dual-Cohort Study Using Two Independent Databases

Background: Risk stratification in pericarditis relies mainly on clinical presentation, suspected etiology, imaging findings, and conventional inflammatory biomarkers. Whether complete blood count-derived inflammatory indices are associated with mortality in pericarditis and whether these associations are directionally consistent across independent real-world datasets remain unclear. Methods: We conducted a retrospective dual-cohort study of hospitalized adults with pericarditis using a Hong Kong cohort from the Clinical Data Analysis and Reporting System (CDARS) as the primary analysis cohort and the Medical Information Mart for Intensive Care IV (MIMIC-IV) cohort as an independent reproducibility cohort. Baseline neutrophil-to-lymphocyte ratio (NLR) and systemic immune-inflammation index (SII) were analyzed as continuous variables and cohort-specific tertiles. The primary outcome was long-term all-cause mortality in the Hong Kong cohort. Secondary and reproducibility outcomes included 90-day mortality in the Hong Kong cohort and 30-day, 90-day, and observable follow-up mortality in MIMIC-IV. Cox models were adjusted for age, sex, renal disease, diabetes mellitus, hypertension, ischemic heart disease, and malignancy. Results: Among 504 patients in the Hong Kong cohort and 464 patients in MIMIC-IV, all-cause mortality occurred in 241 and 113 patients during cohort-specific follow-up, respectively. In the Hong Kong cohort, higher NLR was associated with long-term all-cause mortality after full adjustment. Compared with NLR tertile 1, the adjusted hazard ratio was 1.60 for tertile 3. Higher SII was also associated with long-term mortality, with an adjusted hazard ratio of 1.55 for tertile 3 versus tertile 1. NLR and SII showed directionally consistent associations with 90-day mortality in the Hong Kong cohort and with 30-day, 90-day, and observable follow-up mortality in MIMIC-IV. Sensitivity analyses yielded broadly consistent findings. Conclusions: In two independent real-world cohorts of hospitalized patients with pericarditis, higher baseline NLR and SII were associated with increased all-cause mortality, with NLR showing the more consistent prognostic signal. These complete blood count-derived indices may provide simple adjunctive information for mortality risk stratification, although prospective validation is needed before incorporation into formal management algorithms.