Limitations of the hematocrit level to assess the need for red blood cell transfusion in hypovolemic anemic patients

BACKGROUND: The transfusion trigger that physicians use to determine whether a patient requires a red blood cell (RBC) transfusion is the peripheral venous hematocrit (Hct) value. Although this measurement is an indicator of the concentration of RBCs in the blood, it does not reveal the RBC volume, plasma volume, or total blood volume, nor does it give any indication of whether the patient is hypovolemic, normovolemic, or hypervolemic. STUDY DESIGN AND METHODS: Two patient populations were studied: 41 consecutive patients subjected to elective vascular surgery and 20 consecutive patients subjected to cardiopulmonary bypass surgery. The RBC volume was measured with 51Cr- or 99mTc-labeled autologous fresh RBCs, and the plasma volume and total blood volume were estimated from the measured RBC volume and the total body Hct level. Measurements made 1 to 2 and 24 hours after surgery were compared to the preoperative values for these two groups of patients. RESULTS: During the 24-hour postoperative period, the RBC, plasma, and total blood volumes were reduced compared to the preoperative volumes. These patients were hypovolemic and anemic, and their Hct values during the 24-hour postoperative period were increased by a mean of 4 to 5 volume-percent compared to values that would be expected if they were normvolemic and anemic. CONCLUSIONS: The Hct values in hypovolemic anemic patients are elevated because the plasma volume does not increase to achieve the normovolemic anemic state.