Anemia following acute kidney injury after noncardiac surgery and long-term outcomes: the NARA-AKI cohort study

BACKGROUND: This study was conducted to investigate whether acute kidney injury (AKI) is an independent predictor of anemia and whether anemia following AKI is a mediator of mortality after AKI. METHODS: This is a retrospective cohort study. Adults with noncardiac surgery from 2007 to 2011 were included. Obstetric or urological surgery, missing data or preoperative dialysis were excluded. Subjects were followed until the end of 2015 or lost to follow-up. Exposures of interest were postoperative AKI. Outcome variables were hematocrit values at 3, 6 and 12 months postoperatively and mortality. Associations between AKI and hematocrit or association between AKI and mortality were examined by multivariable linear regression or Cox regression, respectively. RESULTS: Among 6692 subjects, 445 (6.6%) developed AKI. Among those with postoperative data, AKI was independently associated with lower hematocrit at 3, 6 and 12 months postoperatively, with coefficients of −0.79 [95% confidence interval (CI) −1.47 to −0.11; n = 1750], −1.35 (−2.11 to −0.60; n = 1558) and −0.91 (−1.59 to −0.22; n = 2463), respectively. Higher stages or longer duration of AKI were associated with more severe anemia. AKI was associated with higher mortality after 3 months postoperatively with a hazard ratio of 1.54 (95% CI 1.12–2.12). Further adjustment with hematocrit at 3 months attenuated the association. The mediation effect was significant (P = 0.02) by mediation analysis. CONCLUSIONS: AKI was an independent predictor of anemia following AKI. Higher mortality associated with AKI was at least partially mediated by anemia following AKI. Whether correction of anemia following AKI improves mortality requires further research.