Polycythemia

Erythrocytosis (or polycythemia) is the presence of an excessive number of red blood cells in the circulation. It can be primary or arise secondarily to another pathology.

Definition

Absolute erythrocytosis is defined as a red cell mass which is >125% predicted for sex and age .

Although the diagnosis of erythrocytosis is often put forward when the hemoglobin and/or hematocrit are elevated, they can both be raised, especially the former, yet the red cell mass not be abnormal. Nevertheless if the hematocrit is >0.60 in men or 0.56 in women, then an absolute erythrocytosis is certain .

Causes

Following is a list of primary and secondary causes of erythrocytosis.
Secondary etiologies are much more common :

Primary 

Congenital

• erythropoietin receptor (EPOR) mutations

Acquired

• polycythemia vera
• genetic (LNK) mutations

Secondary

Congenital

• high oxygen affinity hemoglobins
  • >100 mutations described
• abnormal 2,3 bisphosphoglycerate mutase
• methemoglobinemia
• dysfunctional oxygen-sensing pathway
  • e.g. VHL mutations

Acquired

Hypoxia-driven

• central hypoxia
  • ​chronic lung disease
  • right-to-left cardiopulmonary shunts
  • carbon monoxide toxicity
  • smoking-related polycythemia
  • sleep apnea (and other hypoventilation states)
  • living at high altitude
• local renal hypoxia
  • renal artery stenosis
  • end-stage renal failure
  • hydronephrosis
  • polycystic kidney disease (autosomal dominant/recessive)
  • post-renal transplant erythrocytosis

Pathological erythropoietin secretion

• neoplasms
  • cerebellar hemangioblastoma
  • meningioma
  • parathyroid adenoma/carcinoma
  • hepatocellular carcinoma
  • renal cell carcinoma
  • pheochromocytoma

Exogenous erythropoietin

• drug administration (physician-directed or illicit)
  • erythropoietin (EPO)
  • androgens