Elevated preoperative red cell distribution width and incident anemia after metabolic and bariatric surgery: a cohort...

Postoperative anemia is a common complication of metabolic and bariatric surgery (MBS), particularly among female patients. The predictive value of elevated preoperative red cell distribution width (RDW), a marker of erythropoietic stress, for incident postoperative anemia in nonanemic patients remains unclear.

Data from the TriNetX Global Collaborative Network were used to classify adult female patients without preoperative anemia who underwent primary MBS between 2010 and 2024 into elevated RDW (>14.5%) and normal RDW (11.5-14.5%) groups. The primary outcome was incident anemia (hemoglobin <12 g/dL) over 1 year, assessed from a 30-day postoperative landmark to reduce perioperative bias. Secondary outcomes included incident iron deficiency anemia (IDA), non-IDA nutritional anemia, ferritin <30 ng/mL, subsequent RDW elevation, and hospital readmissions. Multivariable Cox regression was additionally performed in the unmatched cohort.

After matching, 8,809 pairs were analyzed. Incident anemia occurred in 17.9 and 13.6% of patients in the elevated and normal RDW groups (hazard ratio [HR] 1.35, 95% confidence interval [CI]: 1.25-1.45; p < 0.001), with an absolute risk difference of 4.3% (approximate number needed to screen, 23). Incident IDA was also higher in the elevated RDW group (HR 1.93, 95% CI: 1.64-2.28; p < 0.001). Ferritin <30 ng/mL (HR 1.51; p < 0.001) and subsequent RDW elevation (HR 3.38; p < 0.001) were significantly more frequent, whereas non-IDA nutritional anemia and hospital readmission did not differ. In multivariable analyses, elevated RDW remained independently associated with anemia (HR 1.60) and IDA (HR 2.13), with effect sizes comparable to those of ferritin <30 ng/mL. The associations were consistent over 3 years, across subgroups, and at a higher RDW threshold (>15%). In the smaller male cohort, elevated RDW was associated with iron-related markers (Ferritin <30 ng/mL) but not with incident anemia, possibly reflecting limited statistical power.

Elevated preoperative RDW is independently associated with an increased risk of incident anemia and IDA in non-anemic female patients undergoing MBS. As a routinely available laboratory parameter, RDW may provide complementary value for preoperative risk stratification and identification of patients who may benefit from closer postoperative hematologic monitoring in the future.