Background/Objectives: Postoperative bleeding remains a major complication following coronary artery bypass grafting (CABG), contributing to transfusion requirements, reintervention, and morbidity. The objective of this study was to assess the clinical significance of preoperative platelet function testing and postoperative viscoelastic testing in patients undergoing elective isolated on-pump CABG.

Methods: This prospective observational study included 708 patients undergoing surgery between January 2023 and January 2025. Preoperative platelet function was assessed with Multiplate^® impedance aggregometry (ADPHS and ASPI), and postoperative coagulation was assessed with ClotPro^® after cardiopulmonary bypass. The prespecified Multiplate thresholds were population- and assay-specific and were not intended as universal cutoffs. Postoperative bleeding was defined by cumulative chest-tube drainage during the first 24 h.

Results: In unadjusted analyses, low ADPHS values (≤602.5 AU·min) were associated with higher platelet transfusion, cryoprecipitate use, and overall transfusion, and ADPHS showed modest discrimination for blood loss > 500 mL/24 h (AUC = 0.61, p = 0.041). Low ASPI values (≤453 AU·min) were associated with increased cryoprecipitate use and showed modest discrimination for blood loss > 1000 mL/24 h (AUC = 0.62, p = 0.005). After multivariable adjustment for baseline differences, neither ADPHS ≤ 602.5 (adjusted OR 1.01, 95% CI 0.74–1.39, p = 0.936) nor ASPI ≤ 453 (adjusted OR 0.89, 95% CI 0.64–1.23, p = 0.475) was independently associated with postoperative bleeding > 500 mL/24 h. Postoperative ClotPro^® parameters showed statistically significant associations with transfusion and bleeding, but their discriminatory performance was limited.

Conclusions: Perioperative platelet function and viscoelastic test results were associated with bleeding and transfusion outcomes in unadjusted analyses, but discrimination was modest, and the platelet function cutoffs were not independently associated with bleeding > 500 mL after adjustment. These findings support a complementary hemostatic assessment role for combined POC testing but do not establish a standalone predictive model or an implementable transfusion algorithm.