AbstractBackgroundPreoperative hemoglobin (Hb) level is a predictor of in‐hospital and midterm mortality in patients undergoing coronary artery bypass grafting surgery (CABG). However, the debate about the different hazards across Hb levels and sex differences in outcome occurrence is still on the table.MethodsThis is a registry‐based serial cross‐sectional study at Tehran Heart Center. Nonanemic patients who underwent CABG with complete follow‐up data were included. The Restricted Cubic Splines (RCS) in the Cox model was used to calculate the sex‐specific correlation between in‐hospital, 6‐month, and 1‐year mortalities and normal Hb levels using odds ratio for the in‐hospital and hazard ratios for 6‐month and 1‐year mortality, adjusted for all possible confounders.ResultsFrom 2005 to 2016, a total of 13,557 patients were included, of which 134 had in‐hospital mortality as our primary outcome. Preoperative Hb levels were significantly lower in the deceased. Moreover, dead patients had significantly higher rates of diabetes and hypertension, while lower ejection fraction. Cut‐offs for reference Hb level were higher for males compared with females. The correlation between Hb level and in‐hospital mortality was nearly U‐shaped. Quantitatively, Hb of ≥15.62 and ≤13.25 g/dL for men and that of ≥14.92 and ≤13.4 g/dL for women tended to be associated with higher in‐hospital mortality.ConclusionsThe association between preoperative Hb level and in‐hospital mortality differs in men and women and does not follow a linear correlation among nonanemic patients. Both low and high numbers in the Hb level spectrum are at greater risk.