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ERYTHROCYTOSIS (polycythemia), an excess of red blood cells with a resultant elevation of the hemoglobin and hematocrit. Polycythemia may be classified as relative (hemoconcentration secondary to dehydration) or absolute (an actual increase in the total red cell mass). Relative polycythemia is also seen in an uncommon condition called Gaisbocks syndrome or stress polycythemia. Absolute polycythemia is further classified as primary (increased RBC mass occurs as a result of an intrinsic abnormality of myeloid stem cells) or secondary (increased RBC mass occurs in response to increased erythropoietin levels). The most common cause of polycythemia is a response to smoking. The pathophysiology of smokers polycythemia is believed to be multifactorial. Suspected causes include a reduced plasma volume and an elevated red cell mass secondary to increased levels of carboxyhemoglobin. The reduced plasma volume secondary to cigarette smoking is easily reversible within a few days; therefore, a CBC should be rechecked after the patient has abstained from smoking for several days. It is also prudent to obtain a carboxyhemoglobin level to prove that polycythemia is secondary to smoking in appropriate patients. However, unlike the reduced plasma volume seen with smokers, an increased RBC mass secondary to elevated carboxyhemoglobin levels may take months to correct after the patient abstains from smoking.
Polycythemia vera should be suspected when there is associated leukocytosis or thrombocytosis or both, a change in the hemoglobin/hematocrit from a prior known normal value, or associated symptoms of polycythemia vera. Polycythemia vera is generally associated with a low erythropoietin level. Normal erythropoietin levels may be associated with polycythemia vera but a high level makes the diagnosis unlikely. The finding of the Janus kinase 2 (JAK-2) mutation in peripheral blood is seen in over 95% of cases and establishes the diagnosis in suspected cases. If the diagnosis is still in doubt, then bone marrow biopsy examination is helpful.
Causes of secondary polycythemia include chronic high altitude exposure, chronic obstructive pulmonary disease, chronic cigarette smoking (diagnosed by history and a high carboxyhemoglobin level), cardiovascular right-to-left shunting of blood, high affinity hemoglobinopathies (diagnosed by a low Pa50), 2,3-diphosphoglycerate mutase deficiency (also associated with a low Pa50), malignancy (renal cell carcinoma, ovarian tumors, adrenal tumors, cerebellar hemangioblastoma, hepatocellular carcinoma, pheochromocytoma, uterine leiomyoma, meningioma, etc.), post renal transplantation (responds to treatment with ACEI's or ARB's), and the abuse of erythropoietic drugs by high performance athletes. Erythropoietin levels are usually elevated or normal with secondary erythrocytosis except in cases of erythropoietin receptor mutations in which secondary erythrocytosis is associated with low levels of erythropoietin. Once secondary erythrocytosis is suspected, arterial blood gas measurement with Pa50 and carboxyhemoglobin levels is the next step. If the process is secondary to hypoxia, then the work-up should proceed so as to determine the cause of the hypoxia with chest radiography, pulmonary function testing, or bubble contrast echocardiography. Hypoxia independant secondary erythrocytosis should prompt abdominal CT imaging to detect renal or hepatic cysts or malignancies and/or neuroimaging with MRI.