Serial Complete Blood Count-Derived Inflammatory Indices in Acute Pancreatitis: Temporal Associations with Disease Severity, Pancreatic Necrosis, and Prolonged Hospitalization—A Retrospective Single-Center Study

Background: Acute pancreatitis (AP) evolves during the first days of hospitalization, and the prognostic value of complete blood count (CBC)-derived inflammatory indices may depend on measurement timing. We evaluated serial neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), and systemic immune-inflammation index (SII) values and their associations with disease severity, pancreatic necrosis, and prolonged hospitalization. Methods: This retrospective single-center study included 299 adults hospitalized with AP between 2010 and 2020. NLR, PLR, and SII were assessed at admission, 24 h, and 48 h. Temporal changes and between-group differences were analyzed, and discriminatory performance was evaluated using receiver operating characteristic analysis. Results: NLR, PLR, and SII decreased over time in the overall cohort but remained higher in patients with moderate-to-severe AP. Admission PLR had the highest observed AUC for moderate-to-severe AP (0.738). Admission values did not significantly distinguish patients with pancreatic necrosis, whereas 48-h SII had the highest observed AUC for necrosis (0.810; sensitivity 82.35%; specificity 76.95%). For hospitalization > 4 days, 48-h NLR had the highest observed AUC (0.742). Conclusions: Observed discriminatory performance varied according to both measurement timing and clinical outcome. Later measurements showed greater numerical discrimination for pancreatic necrosis and prolonged hospitalization than admission values. These findings are exploratory, and the proposed cut-offs require external validation.