Introduction: This study aimed to investigate the use of the systemic immune-inflammation index obtained by hemogram parameters in determining the clinical severity of pneumonia.

Material and method: This study was conducted retrospectively with a total of 6,802 patients diagnosed with viral and bacterial pneumonia from January 1, 2013, through January 1, 2023, at the emergency department of a tertiary hospital. The patients’ age, gender, white blood cell, neutrophil, lymphocyte, and platelet counts, clinical outcomes (mortality and discharge), and mechanical ventilator requirements during treatment were obtained from the electronic patient files.

Results: The mean age of the patients was 62.3 ± 17.3 years, and 57.8% (n = 3,928) were male. The systemic immune-inflammation index was found to predict mortality in patients with a sensitivity of 77.9% and a specificity of 36.2% at a cut-off value of 114.72 [area under the curve (AUC): 0.654]. The receiver operating characteristic (ROC) curve analysis showed that the systemic immune-inflammation index was statistically significant in determining mortality among the patients (p < 0.001, 95% confidence interval: 0.639–0.669). The systemic immune-inflammation index was found to predict the mechanical ventilator requirement with a sensitivity of 70.0% and a specificity of 47.5% at a cut-off value of 137.88 (AUC: 0.629). According to the ROC curve analysis, the systemic immune-inflammation index was also statistically significant in determining the mechanical ventilator requirement among the patients (p < 0.001, 95% CI: 0.599–0.658).

Conclusion: The systemic immune-inflammation index was found to be valuable in determining clinical severity in patients with pneumonia.