PBV = k1 x height (m)3 + k2 x weight (kg) + k3 When k1= 0.3669, k2 = 0.03219, k3 = 0.6041 for men; k1= 0.3561, k2 = 0.03308, k3 = 0.18331 for women Then, multiplying the PBV by the hematocrit (Hct) will give the total RBC volume. The change in RBC volume can be calculated from the change in Hct [20]. In this study, total RBC volume loss at postoperative 48 hours is calculated using the following formula; Total RBC volume loss = PBV x (Preoperative Hct - Postoperative Hct at 48 hours) At 24 hours after surgery, the volume of drained blood is measured and then multiplying by the preoperative Hct will give the drainage RBC blood volume at 24 hours. During 24-48 hours after surgery, the volume of drained blood is measured and then multiplying by the postoperative Hct at 24 hours will give the drainage RBC blood volume at 24-48 hours. The summation of drainage RBC blood volume at 24 hours and at 24-48 hours is total drained RBC volume. If the patient receives blood transfusion, the volume of packed red cell is also recorded. Finally, invisible blood loss is calculated using the following formula; Invisible blood loss = Total RBC volume loss - Total drained RBC volume + Transfused blood volume