The laboratory diagnosis of polycythemia depends on the demonstration of an elevation in hematocrit or in the hemoglobin content of the peripheral blood. Since these determinations are really expressions of concentrations, observed elevations may be induced by either an increase in the red cell mass or a decrease in the plasma in which the cells are suspended. Although states of hemoconcentration are frequently encountered in acute dehydration from any cause, the presence of a chronic, relatively asymptomatic state of hemoconcentration was not appreciated until the clinical use of blood volume determinations. Recent literature has applied the term "relative polycythemia" to discussions of patients suspected of polycythemia but found to have a normal red cell mass and a smaller than normal plasma volume. The term "pseudopolycythemia" probably is more descriptive of the current view of such patients.
Examples of this type of patient were recognized by Keith, Rowntree, et al.,1,2
KAUNG DT, PETERSON RE. "Relative Polycythemia" or "Pseudopolycythemia". Arch Intern Med. 1962;110(4):456–460. doi:10.1001/archinte.1962.03620220048008