A serial mediation analysis of preoperative anemia, postoperative anemia burden, and RBC transfusion on AKI risk in cardiac surgery

Abstract Objective To explore the association between preoperative anemia and postoperative acute kidney injury (AKI) in adults undergoing cardiac surgery, whether this association is channeled through the postoperative anemia burden and red blood cell (RBC) transfusion. Methods This single-center retrospective cohort study enrolled 44,810 adults who underwent elective aortic, valvular, coronary artery bypass grafting (CABG), or combined surgeries. Preoperative anemia was defined as the exposure, AKI during hospitalization as the outcome, and postoperative anemia burden and RBC transfusion as mediators. Multiple linear/logistic regression was used to construct a serial mediation model. After adjustment for confounders, direct, specific indirect, and total effects were estimated using 1000 bootstrap replicates, followed by sensitivity and subgroup analyses. Results After adjustment, preoperative anemia remained associated with increased AKI risk, and 33.8% (95%CI, 22.9%–44.7%) of the effect was statistically channeled through the mediators. Preoperative anemia was significantly associated with greater postoperative anemia burden (β = 0.676) and transfusion risk (OR = 1.069, 95%CI 1.055–1.082). Postoperative transfusion was independently associated with AKI (OR = 1.081, 95%CI 1.071–1.092). Findings were generally consistent across most subgroups, with higher mediation proportions in male patients, those aged < 65 years, and those with hypertension; in patients with chronic kidney disease, the effect of preoperative anemia on AKI was almost entirely mediated through the pathways. Conclusion Preoperative anemia is an important risk marker for postoperative AKI after cardiac surgery. Its effect appears to be partially mediated through the serial pathway, with postoperative transfusion as a potentially key mediating link.