Systemic Immune-Inflammation Index as a Predictive Biomarker for Pre-Eclampsia

Pre-eclampsia, a major obstetric syndrome with an estimated global incidence of 2-8% of all pregnancies, ranks among the foremost causes of adverse maternal and perinatal outcomes worldwide. The Systemic Immune-Inflammation Index (SII)-derived as the product of platelet and neutrophil counts divided by the lymphocyte count-is a composite hematological parameter first established in oncological research that simultaneously captures neutrophil activation, lymphocyte dysfunction, and platelet alterations-three immunohematological disturbances implicated in pre-eclampsia pathophysiology. This narrative review synthesizes the current evidence regarding SII in pre-eclampsia, examining the biological rationale, clinical study findings, comparative performance against established inflammatory biomarkers, practical advantages, and limitations. A comprehensive literature search encompassing PubMed/MEDLINE, Scopus, Web of Science, and Google Scholar was conducted, covering all available records up to January 2026. The available data reveal substantial heterogeneity in both the direction of association (elevated versus paradoxically decreased SII in pre-eclampsia) and diagnostic performance across populations. While some studies report significantly elevated first-trimester SII values in women who subsequently develop pre-eclampsia, others demonstrate lower values or no significant predictive capacity. The only available meta-analysis reported non-significant pooled results for the pre-eclampsia-specific subgroup. Although SII's derivation from routine complete blood count testing presents the advantages of cost-effectiveness and universal accessibility, methodological limitations-including retrospective study designs, the absence of standardized thresholds, the inconsistent discriminatory performance, and the conflicting directionality of association-preclude clinical implementation at present. Integration within multiparametric prediction models may optimize SII's clinical utility. Future research should prioritize prospective validation studies across diverse populations, mechanistic investigations, and randomized controlled trials to establish evidence-based clinical translation.