Clinical significance of the systemic immune-inflammation index as a minimally invasive biomarker reflecting histological activity in IgA nephropathy

Background The systemic immune-inflammation index (SII) is a prognostic biomarker in IgA nephropathy (IgAN), but its relationship with acute versus chronic histological lesions remains unclear.Methods This retrospective study included 71 adults with biopsy-proven primary IgAN. To distinguish systemic inflammation drivers, histopathological severity was evaluated using continuous variables: the proportions of glomeruli with acute (cellular/fibrocellular crescents, necrosis) and chronic (global/segmental sclerosis, fibrous crescents) lesions. Associations between SII and these lesions were assessed using multiple linear regression. An additional sensitivity analysis adjusting for age and eGFR was performed.Results Median SII was 508.87. In the linear regression model adjusted for age, sex, estimated glomerular filtration rate, and proteinuria, acute lesions showed a positive but statistically nonsignificant association with SII (β = 10.615, 95% CI −0.629–21.860, p = 0.064), whereas chronic lesions also showed no statistically significant association (p = 0.871). In the sensitivity analysis adjusting for age and eGFR, the proportion of acute lesions was significantly associated with SII (p = 0.043). Receiver operating characteristic analysis showed an area under the curve of 0.725 (95% CI, 0.554–0.848) for identifying severe active lesions (>10% acute lesions; n = 10), with an exploratory SII cutoff of 618.8 (70.0% sensitivity, 67.2% specificity).Conclusions Higher SII may be associated with acute glomerular lesions in IgAN, although the association did not reach statistical significance after full adjustment. As a minimally invasive index, SII may have potential as a supplementary biomarker of histological activity, but its clinical utility requires validation in larger cohorts.