Polycythemia, also referred to as erythrocytosis, is characterized by an increased red cell mass. There is no consensus on terminology (ie, primary familial polycythemia but postrenal transplantation erythrocytosis). Polycythemias can be primary or secondary and can be inherited or acquired.
Classification of polycythemic disorders appears in Table 2–2 in Chap. 2.
Primary polycythemias are caused by somatic or germline mutations within hematopoietic stem cells or erythroid progenitors that result in an augmented response to erythropoietin. This response is inappropriate; that is, it is not a compensation for hypoxia.
Secondary polycythemias are caused by either an appropriate (compensatory) or inappropriate increase in the red cell mass as a result of augmented levels of erythropoietin.
The most common primary polycythemia, polycythemia vera, is a clonal acquired multipotential hematopoietic progenitor cell disorder discussed in Chap. 41. It is a myeloproliferative neoplasms.
This disorder is autosomal dominant, with normal leukocyte and platelet counts.
Affected persons may be misdiagnosed as having polycythemia vera.
Low plasma erythropoietin level is a constant feature (see Figure 27–1).
Erythroid progenitors in in vitro cultures are hypersensitive to erythropoietin, but unlike the erythroid progenitors in polycythemia vera, they do not grow in the absence of erythropoietin.
This condition is caused by a truncation of erythropoietin receptor and deletion of the negative regulatory cytoplasmic domain.
Affected individuals may have an increased risk of cardiovascular complications, regardless of control of elevated hematocrit by phlebotomies.