The relationship between total red cell volume, plasma volume and venous haematocrit

Total red cell volume (TRCV) and plasma volume (PV) have been measured in 130 cases including 65 polycythaemic patients and 65 patients with a wide variety of disorders. Patients with gross splenomegaly were excluded from the study. The regression of TRCV on venous haematocrit (VHct) was found to be curvilinear, a single exponential function providing a good approximation ( r=0.92, P less than 0.001). No significant difference was found between the mean PVs of the polycythaemic and control groups (40.97 and 42.94 ml/kg respectively) although a slight but highly significant negative correlation was observed between PV and VHct (r = 0.47, P less than 0.001). This most marked for patients with VHcts below 0.30, for whom a separate plot yielded a correlation coefficient of 0.75. Total blood volume (TBV) and VHct were found to be positively correlated (r = 0.57, P less than 0.001) as would be expected from the TRCV and PV data. The significance of these findings is discussed.