C-Reactive protein and the CALLY index for predicting complicated appendicitis: a retrospective cohort study of 5,428 patients

Distinguishing complicated from uncomplicated appendicitis before surgery remains a critical yet unresolved clinical challenge. Delayed recognition of complicated appendicitis, particularly in cases of perforation or abscess formation, is associated with increased morbidity and mortality. Although various laboratory markers and inflammatory indices have been proposed as markers associated with disease severity, their relative diagnostic performance remains unclear. This study aimed to evaluate and compare the discriminatory performance of conventional laboratory parameters, leukocyte-derived indices, and composite biomarkers for complicated appendicitis. In this retrospective single-center cohort study, 5,428 patients who underwent emergency appendectomy between 2015 and 2025 were included. Patients were classified as uncomplicated or complicated appendicitis based on intraoperative findings. Preoperative laboratory parameters, including C-reactive protein, white blood cell count, total bilirubin, leukocyte-derived indices, and composite biomarkers such as the C-reactive protein–albumin–lymphocyte (CALLY) index and the hemoglobin–albumin–lymphocyte–platelet (HALP) index, were analyzed. Receiver operating characteristic analysis was used to assess diagnostic performance, and optimal cut-off values were determined using the Youden index. Multivariate logistic regression was performed to identify variables independently associated with complicated appendicitis. Of the 5,428 patients, 707 (13.1%) had complicated appendicitis. C-reactive protein demonstrated the highest diagnostic accuracy (AUC: 0.781) and demonstrated the strongest independent association (OR: 5.29). Total bilirubin was also independently associated with complicated appendicitis (OR: 1.21). Leukocyte-derived indices showed limited discriminatory ability, with only the systemic immune-inflammation index remaining significant in multivariate analysis. The CALLY index demonstrated relatively strong discriminatory performance (AUC: 0.747), with lower values associated with increased disease severity. C-reactive protein remains the most robust laboratory marker for identifying complicated appendicitis. Composite biomarkers such as the CALLY index may provide additional value in risk stratification. These findings support the use of routinely available laboratory parameters in clinical risk assessment, although prospective validation is warranted.