The interpretation of measured red cell mass and plasma volume in patients with elevated PCV values

Various formulae using both height and weight have been proposed to interpret measured red cell mass (RCM) and plasma volume (PV). This paper compares four different methods for evaluating these measurements in 50 males with PCV values above 0.51 and three different methods for evaluating results in 50 females with PCV values above 0.49. Overall there was close agreement in the RCM result expressions by all methods for both males and females except at extremes of height and/or weight. The incidence of absolute polycythaemia in both sexes was influenced by the width of the normal range. When this was taken into account overall the incidence of polycythaemia was similar, but in a small number of individuals there were discrepancies of interpretation. These individual patient differences establish that decisions regarding further investigations of a patient must not rest on a strict interpretation of a measured RCM. The various methods of measured PV interpretation gave dissimilar results--in particular by one of the male PV interpretation methods. The differences in the methods almost certainly reflect methodological discrepancies in the original techniques used to establish the normal values. The differences in result expression for PV would however lead to a different incidence of relative, low plasma volume, polycythaemia by the various techniques.