Should whole-body red cell mass be measured or calculated?

The whole-body volume of red blood cells must be known for correct diagnosis of polycythemia vera. Since the venous hematocrit may not correctly reflect the absolute amount of red blood cells, the red cell mass (RCM) is usually determined by radioisotope labeling of red blood cells according to recommendations of the International Committee for Standardization in Haematology. We examined whether the radioisotope labeling procedure can be replaced by a simple calculation of RCM from the values of venous blood hematocrit and plasma volume, using an empirical factor (Ratio f) equal to the mean ratio between whole-body and venous hematocrit. A retrospective study was performed of 264 cases in which the RCM was assayed using (51)Cr or (99m)Tc for red cell labeling, and (125)I-labeled albumin was used for estimation of plasma volume. The results showed wide scattering of Ratio f (mean value 0.911; range from 0.76 to 1.15) that was responsible for substantial differences between the measured and calculated values of RCM. We also tested whether the calculated RCM may be used as an appropriate marker for polycythemia vera according to recommendations of the International Council for Standardization in Haematology. Our data showed that 146 patients had measured RCM values more than 25% above the mean normal predicted value. Using the calculated RCM, 17 of these patients would be lost from the polycythemia vera group, whereas 29 subjects with measured RCM levels equal to or lower than 125% of predicted values would incorrectly meet the RCM criterion for polycythemia vera. Thus, a total of 46 patients would be misclassified by using the calculated RCM. We conclude that the radioisotope labeling of red blood cells remains mandatory for correct determination of the whole-body red cell volume.