Systemic immune-inflammation index as a prognostic biomarker in critically ill dogs

Rapid and accurate prognostic assessment in critically ill dogs with systemic inflammation remains challenging. The Systemic Immune-Inflammation Index (SII), calculated as (neutrophil count × platelet count)/lymphocyte count, has been recognized as a biomarker for systemic inflammation and immune dysregulation in human medicine. However, its clinical relevance in veterinary emergency and critical care settings remains unclear. This study aimed to evaluate the clinical significance of the SII in critically ill dogs and establish its reference interval in healthy control dogs. This retrospective study analyzed 96 healthy control dogs and 61 critically ill dogs that presented to the intensive care unit (June 2023-January 2025). Complete blood count (CBC) parameters collected within 1 h of presentation were used to calculate the SII. CBC parameters and CBC-derived indices including SII were compared between critically ill dogs and healthy control dogs. Furthermore, CBC parameters, CBC-derived indices, and the Acute Patient Physiologic and Laboratory Evaluation (APPLE) score were compared between survivors and non-survivors. Receiver operating characteristic (ROC) curve analysis was performed to determine an optimal SII cutoff for predicting mortality and a 95% reference interval was established for healthy control dogs. Critically ill dogs had significantly higher median SII compared to healthy control dogs (1,997 vs. 516, p < 0.001). Non-survivors showed significantly elevated white blood cell count, neutrophil count, SII, neutrophil-to-lymphocyte ratio (NLR), and APPLEfast score compared to survivors. ROC curve analysis demonstrated that SII predicted mortality with moderate accuracy (area under the curve [AUC]: 0.806; cutoff: 2,628; sensitivity: 90%; specificity: 74.5%), with performance at least comparable to the APPLEfast score (AUC: 0.759; sensitivity: 90%; specificity: 64.7%) in this cohort. The 95% reference interval for SII in healthy control dogs was 180-1,392. In conclusion, SII was significantly increased in critically ill dogs, and values above the cutoff were associated with an increased mortality risk. These findings suggest that the SII is a valuable, novel indicator for prognostic evaluation in critically ill canine patients; however, further validation in larger cohorts is warranted.