The classification and diagnostic criteria of the erythrocytoses (polycythaemias).

The term 'erythrocytosis' has advantages over 'polycythaemia' to describe patients with a raised haematocrit (PCV) and deserves to be more widely used. Measurement of red cell mass (RCM) and its relation to that expected for an individual's height and weight permits initial subdivision of erythrocytosis into absolute (increased RCM) or apparent normal RCM. Absolute erythrocytosis may be primary (intrinsically abnormal marrow erythropoiesis) or secondary (increased erythropoietin drive in response to pathological events outside the bone marrow). Both primary and secondary erythrocytosis may be either congenital or acquired. Idiopathic erythrocytosis is a third, probably heterogenous, group within the absolute erythrocytoses. Familial abnormalities of the erythropoietin receptor underlie the primary congenital subgroup. Polycythaemia vera (PV), the clonal myeloproliferative disorder, is so far, the only primary acquired disorder. Newer diagnostic investigations such as serum erythropoietin estimation, improved karyotypic analysis, in vitro culture of erythroid colonies and estimation of spleen size before splenomegaly is palpable, have permitted some modification of the traditional diagnostic criteria of polycythaemia vera. This may allow more confident diagnosis and, together with improved testing for causes of secondary erythrocytosis, may reduce the number of patients so far unsatisfactorily consigned to the idiopathic erythrocytosis group.