Christine Cserti‐Gazdewich

Background Leading digit bias is a heuristic whereby humans overemphasize the left‐most digit when evaluating numbers (e.g., 9.99 vs. 10.00). The bias might affect the interpretation of hemoglobin results and influence red cell transfusion in hospitalized patients. Study Design and Methods Adults who received a red cell transfusion while registered at the University Health Network (Toronto, Canada) between January 1, 2016 and January 1, 2022 ( n = 6 years) were included. The primary analysis excluded apheresis, red cell disorders, radiology suites, and operating rooms. The primary comparison was a regression discontinuity analysis of transfusion occurrence above and below the hemoglobin threshold of 79 g/L (local units). Additional analyses tested other leading digit and control thresholds (71, 81, and 91 g/L). Secondary analyses explored temporal covariates and clinical subgroups. Results A total of 211,872 red cell transfusions were identified over the study period (median pre‐transfusion hemoglobin 76 g/L; interquartile range = 69–92 g/L), with 107,790 inpatient transfusions in the primary analysis. The 79 g/L threshold showed 815 fewer red cell units above the threshold (95% confidence interval [CI]: −1215 to −415). The 69 g/L threshold showed 2813 fewer transfused units (95% CI: −4407 to −1220), and 89 g/L showed 40 fewer units (95% CI: −408 to 328). The effect was accentuated during daytime, weekday, and May–June months, persisted in analyses including all transfusions, and was absent at control thresholds. Conclusion Leading digit bias might have a modest influence on the decision to transfuse red cells. The findings may inform practice guidelines and quasi‐experimental study design in transfusion research.