Systemic Immune-Inflammation Index (SII) as a Predictor of Treatment Response in Primary Immune Thrombocytopenia

Primary immune thrombocytopenia (ITP) is an autoimmune disorder characterized by isolated thrombocytopenia resulting from immune-mediated platelet destruction and impaired platelet production. Although corticosteroids and intravenous immunoglobulin (IVIG) are standard first-line therapies, many patients fail to achieve or maintain an adequate response, emphasizing the need for early predictors of treatment resistance. This study aimed to identify clinical and laboratory factors associated with unresponsiveness to corticosteroid and IVIG therapy in primary ITP and to explore the predictive value of systemic immune-inflammation index (SII). This retrospective study included 109 adults newly diagnosed with primary ITP between January 2008 and December 2023. Demographic, clinical, and laboratory data were collected from patient records, and SII was calculated for each patient. Receiver operating characteristic (ROC) analysis determined the optimal SII cut-off for predicting treatment response, while logistic regression identified independent predictors of corticosteroid and IVIG resistance. The mean age was 51.48 ± 16.9 years, and 72.5% were female. All patients received systemic steroids, and 32.1% also received IVIG. The complete response rate to steroids was 50%, while 31.3% were non-responders; the IVIG non-response rate was 45.7%. ROC analysis identified an optimal SII cut-off of 48.33, with lower SII associated with significantly higher response rates to both steroids and IVIG (p < 0.05). Multivariate analysis confirmed low SII as an independent predictor of IVIG responsiveness (OR = 8.25, 95% CI 1.15–59.00; p = 0.036). In conclusion, SII at the time of diagnosis independently predicts response to first-line therapy, and early assessment may facilitate timely second-line interventions