Erythrocytosis is defined as an abnormally increased red blood cell count, and its occurrence may be seen in various conditions. Laboratory workup confirms the diagnosis.
One of the most important conditions in which erythrocytosis is seen is polycythemia vera (PV), characterized by constitutional symptoms (malaise, fatigue, lethargy), weight loss, night sweats and unexplained low-grade fevers . More severe cases present with severe bleeding, such as gastrointestinal or central nervous system hemorrhage . In fact, thrombotic events are an important complication of erythrocytosis and PV, as they can lead to vision loss, swelling and pain in the lower extremities as well . In rare cases, occlusion of the hepatic vein, known as Budd-Chiari syndrome, is encountered in PV patients . Sweating, splenomegaly, bruising, dyspnea, lightheadedness, and pruritus after bathing with hot water are other notable symptoms of PV, while less common complaints are tinnitus, dizziness, paresthesia, and hepatomegaly  . A similar clinical presentation may be observed in 10-15% of patients who underwent renal transplantation (post-transplantation erythrocytosis), most frequently after a period of 8-24 months . Individuals who develop erythrocytosis on the grounds of any other condition, including malignant diseases, genetic mutations or heavy smoking, have a similar clinical presentation . Physical examination can reveal red, warm distal extremities and fingers that are painful and exhibit signs of ischemia, known as erythromelalgia  .
Physical examination may not always provide sufficient information to make the diagnosis, given the fact that symptoms are nonspecific, which is why patient history and laboratory workup are important constituents when determining the cause of erythrocytosis. Recent travel or permanent residence on high altitudes is one of the physiological causes of increased RBC count, while cigarette smoking or the abuse of erythropoietin (EPO) in professional athletes, especially in cyclists, are also the other risk factors. To confirm erythrocytosis, a complete blood count should show one of the following: an increase in red cell mass of more than 25% of normal values; haemoglobin values > 18 mg/dL for men and > 16 mg/dL for women or hematocrit above the 99th percentile of reference range for age, gender or altitude of residence . Additional tests may be necessary to define the etiology. Both leukocytosis and thrombocytosis, in addition to erythrocytosis, makes PV a definite cause . In addition, EPO levels are a very useful tool in recognizing the nature of erythrocytosis  . If EPO levels are subnormal, it is associated with genetic mutations of erythroid progenitor cells that induce a higher rate of red blood cell production, which occurs in the groups of erythropoietin receptor and JAK 2 mutations . Normal or elevated levels of EPO, on the other hand, suggest a secondary cause, and subsequent testing to determine hemoglobin affinity for oxygen (p50) is necessary . Normal p50 values suggest either 2,3 biphosphoglycerate (2,3 BPG) deficiency, while Von Hippel-Lindau (VHL) and hypoxia-inducible factor 1-alpha (HIF-1α) mutations are possible causes of increased p50 values .