Diagnostic Value of the Systemic Immune-Inflammation Index in Differentiating Acute Appendicitis From Familial Mediterranean Fever in Children

Objectives: Acute appendicitis and Familial Mediterranean Fever attacks are among the leading causes of acute abdominal pain in children and often present with overlapping clinical features. This study aimed to evaluate the diagnostic utility of the systemic immune-inflammation index in differentiating acute appendicitis from Familial Mediterranean Fever attacks and to assess whether combining this index with other hematological parameters improves diagnostic discrimination. Methods: A retrospective diagnostic accuracy study was conducted at a tertiary pediatric surgery center between January 2019 and December 2024. Pediatric patients aged 1 to 18 years with histopathologically confirmed acute appendicitis or clinically defined Familial Mediterranean Fever attacks were included. Demographic characteristics and complete blood count parameters were recorded. The systemic immune-inflammation index was calculated using neutrophil, platelet, and lymphocyte counts. The neutrophil-to-lymphocyte ratio and platelet-to-lymphocyte ratio were also calculated. Group comparisons were performed using nonparametric tests. Diagnostic performance was evaluated using receiver operating characteristic curve analysis, including a combined hematological model. Results: A total of 90 patients were included, comprising 44 with acute appendicitis and 46 with Familial Mediterranean Fever attacks. Age and sex distributions were comparable between groups. Median systemic immune-inflammation index values were significantly higher in patients with acute appendicitis than in those with Familial Mediterranean Fever attacks. The neutrophil-to-lymphocyte ratio was also significantly higher in the acute appendicitis group, whereas the platelet-to-lymphocyte ratio did not differ significantly between groups. The combined hematological model demonstrated improved discriminative performance compared with the systemic immune-inflammation index alone. Conclusions: The systemic immune-inflammation index is significantly elevated in pediatric patients with acute appendicitis compared with those experiencing Familial Mediterranean Fever attacks and shows good diagnostic performance in this differential setting. The combined use of hematological indices may further enhance diagnostic discrimination and serve as a supportive tool in the evaluation of children presenting with acute abdominal pain.