Association between postoperative nadir hematocrit and mortality within 28-day after coronary artery bypass grafting: A retrospective cohort study based on the MIMIC-IV database

Nadir hematocrit (nHCT) has been implicated in postoperative risk, but its association with 28-day mortality after coronary artery bypass grafting (CABG) remains uncertain. We investigated the association between postoperative nHCT and 28-day mortality after CABG. This retrospective cohort study used Medical Information Mart for Intensive Care IV and included adults (≥18 years) who underwent CABG during an ICU stay; patients missing postoperative hematocrit (HCT) or 28-day vital status were excluded. Postoperative nadir hematocrit (nHCT) was defined as the lowest HCT value recorded after CABG surgery and prior to the occurrence of death, or within 28 days postoperatively for patients who survived. Secondary outcomes included acute kidney injury, postoperative delirium, ICU length of stay, and total hospital length of stay. Lower nadir HCT was significantly associated with higher 28-day mortality and worse secondary outcomes. In the fully adjusted model, compared with patients with nHCT < 23%, higher nHCT levels were associated with markedly reduced mortality risk: 23% to 27% (OR = 0.32, 95% CI: 0.16-0.66), 27% to 31% (OR = 0.26, 95% CI: 0.11-0.63), and >31% (OR = 0.40, 95% CI: 0.18-0.89). Curve-fitting analysis demonstrated a nonlinear association with an inflection point at approximately 27%; above this level, survival benefit plateaued. Subgroup findings were consistent. Postoperative nHCT after CABG was independently associated with 28-day mortality. An nHCT range of approximately 27% to 31% was associated with the lowest observed mortality risk. These findings highlight the clinical relevance of postoperative nHCT monitoring and may inform future research on perioperative blood management strategies.