Serum Calcium and Hospital Stay in Acute Pancreatitis: A Nonlinear Association With an Inflection Point

Background: Acute pancreatitis (AP) is a critical inflammatory condition associated with significant morbidity and mortality. Calcium ions (Ca2+) have been implicated in the pathophysiology of AP, with potential predictive value for disease severity and patient outcomes, including hospital length of stay (LOS). Objective: The study aimed to investigate the relationship between serum calcium levels and LOS in patients with AP, considering the non-linear trends and potential threshold effects of Ca2+. Methods: We conducted a retrospective cohort study including 717 patients with AP admitted to the gastroenterology departments of Ningbo Second Hospital from July 2020 to June 2023. Data on demographic characteristics, medical history, and laboratory parameters were collected. The relationship between Ca2+ levels and LOS was analyzed using univariate and multivariate regression models, with adjustments for potential confounders. Non-linear trends were explored using a Generalized Additive Model (GAM) and piecewise linear regression. Results: The study revealed a significant non-linear relationship between Ca2+ levels and LOS, with distinct inflection points at Ca2+ concentrations of 1.7 mmol/L and 2.5 mmol/L. Below 1.7 mmol/L, Ca2+ showed no significant association with LOS, whereas between 1.7 and 2.5 mmol/L, a significant negative effect was observed. Above 2.5 mmol/L, a significant positive effect emerged. The study also identified significant effect size variations across different subgroups, including sex, age, BMI, and comorbidities.