Revisiting the “1 g/dL Rule:” Physiological Variability in Hemoglobin Increment after Single-Unit Red Blood Cell Transfusion

BACKGROUND: Red blood cell (RBC) transfusion is commonly expected to increase hemoglobin concentration by approximately 1 g/dL/unit transfused. However, considerable variability in posttransfusion hemoglobin response is observed in routine clinical practice. Understanding the physiological factors influencing this variability may improve the interpretation of transfusion response and support patient blood management strategies. METHODS: We conducted a prospective observational cohort study of 518 hemodynamically stable hospitalized adults receiving a single unit of RBCs. Hemoglobin concentrations were measured before transfusion and at 12, 24, and 48 h afterward. The primary outcome was hemoglobin increment at 24 h (ΔHb24). Associations between baseline hemoglobin concentration and hemoglobin response were evaluated using stratified comparisons and multivariable regression models incorporating clinical and physiological covariates. RESULTS: Mean baseline hemoglobin was 8.2 g/dL, and the mean hemoglobin increment at 24 h was 1.41 g/dL. Hemoglobin response demonstrated a graded inverse relationship with baseline hemoglobin concentration, with larger measurable increments observed in patients with more severe anemia. In multivariable analysis, baseline hemoglobin remained the strongest independent determinant of hemoglobin response. The estimated transfused red cell mass was independently associated with posttransfusion hemoglobin levels, whereas splenomegaly showed modest attenuation of response. Renal dysfunction, hepatic dysfunction, and donor unit storage characteristics were not significant predictors. CONCLUSION: Hemoglobin increment following single-unit RBC transfusion varies systematically with baseline anemia severity and transfused red cell dose. These findings support the concept that posttransfusion hemoglobin change reflects physiological distribution rather than a fixed dose effect and reinforce the importance of reassessment following transfusion.