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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. The most common cause of erythrocytosis 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 cigarettes 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 (this will be mildly elevated in smokers) to prove that erythrocytosis is secondary to smoking in appropriate patients. However, unlike the reduced plasma volume seen with smokers, the increased RBC mass secondary to elevated carboxyhemoglobin levels may take months to correct after the patient abstains from smoking.
The initial workup of erythrocytosis is dependant on the probability of polycythemia vera as the underlying cause. A serum erythropoietin level is usually the first step. Polycythemia vera is associated with a low erythropoietin level but the diagnosis is still possible even with a normal level and the work-up should proceed if there is associated leukocytosis and/or thrombocytosis, thrombotic events, microcytosis with a normal hemoglobin/hematocrit, a new increase in a previously normal hemoglobin/hematocrit, splenomegaly, or symptoms of polycythemia vera.
If a secondary cause for erythrocytosis is suspected (associated with either a normal or elevated erythropoietin level) then arterial blood gas measurement with a PaO2-50 is the next step as secondary erythrocytosis is either hypoxia dependant or hypoxia independant. If the patient is a smoker, be sure to measure the carboxyhemoglobin level on arterial blood gas sampling. Hypoxia dependant cases should proceed so as to determine the cause of the hypoxia with chest radiography, pulmonary function testing, and possibly bubble contast cardiac echocardiography. High oxygen-affinity hemoglobinopathy or 2,3-diphosphoglycerate mutase deficiency are possible when the PaO2-50 is depressed. Hypoxia independant cases are associated with malignancies (renal cancer, uterine leiomyoma, pheochromocytoma, meningioma, adrenal tumors, cerebellar hemangioblastoma, hepatocellular carcinoma, etc.), or congenital cases of erythrocytosis secondary to abnormalities of the erythropoietin receptor such as Chuvash polycythemia which is seen in patients of Russian heritage. Some athletes abuse erythropoietic drugs to enhance performance which result in erythrocytosis. Patients who have received a renal transplantation may develop a secondary erythrocytosis associated with a low erythropoietin level that is responsive to therapy with either an ACEI or an ARB. When there is a low erythropoietin level and polycythemia vera has been excluded, then mutations of the erythropoietin receptor should be considered.