Polycythemia Vera: The Packed Cell Volume and The Curious Logic of The Red Cell Mass

Since the first systematic blood volume studies of polycythemia in the 1920s, measurement of blood volume and red cell mass (RCM) has become routine. However, the radionuclide-labeling methods promulgated by the International Committee for Standardization in Haematology (ICSH) remain complex and poorly understood. Many hematologists and other clinicians err in the belief that these methods permit "direct measurement" of RCM, whereas the ICSH method is indirect: it requires calculation of RCM from (PCV) X (whole blood volume). The use of an elevated value of PCV to calculate RCM in order to evaluate the same elevated value of PCV is a curiously circular logic that is embraced by most clinicians and most hematologists. Analysis of published data in 186 cases of polycythemia vera indicates that RCM is an exponential function of PCV. In most cases, PCV alone suffices to document normal or increased RCM. Relative polycythemia results from dehydration, not from stress. Clinicians need to be aware of the range of physiologic fluctuations that normally occur in plasma volume. Realistic criteria for normal ranges of PCV, Hb concentration and RCM should be adopted in clinical laboratories so that clinicians will not be misled to undertake futile and costly investigations of results that are in the upper percentiles of the normal distribution, as exemplified by the Ulysses Syndrome.