Association Between the Ionized-to-Total Calcium Ratio and the Risk of Acute Kidney Injury in Patients With Severe Autoimmune Diseases: A Retrospective Analysis From the MIMIC-IV Database.

Aim: To investigate the relationship between the ionized-to-total calcium ratio and the incidence of acute kidney injury (AKI) in critically ill patients with autoimmune diseases.

Methods: A retrospective study was conducted utilizing data from the Medical Information Mart for Intensive Care IV database. In-hospital AKI was the primary outcome. Patients were classified into four distinct categories depending on the calcium ratio. Cox proportional hazards model and restricted cubic spline (RCS) analysis were applied to assess the association between the calcium ratio and AKI risk. Subgroup analyses were conducted to evaluate the consistency of this association across various patient characteristics.

Results: A total of 1054 critically ill patients with autoimmune diseases were enrolled and stratified by calcium ratio quartiles into four groups (< 0.515; 0.515-0.546; 0.546-0.576; > 0.576). Cumulative incidence curves showed significant difference in AKI risk among the four groups. Patients in the highest calcium ratio quartile experienced a markedly reduced likelihood of AKI versus those in the lowest, especially in the fully adjusted model (HR = 0.66, 95% CI 0.51-0.84, p < 0.001). RCS analysis demonstrated an obvious nonlinear relationship, showing a steep decline in AKI risk with increasing calcium ratio, which eventually plateaued. This protective effect of higher calcium ratios against AKI was consistent across subgroups, except in those with chronic kidney disease at baseline.

Conclusion: The ionized-to-total calcium ratio proves to be significantly associated with the development of AKI in critically ill patients with autoimmune diseases, with its elevation indicating a reduced AKI risk.