Polycythemia Classification and Diagnosis

Introduction to Polycythemia

Polycythemia must be classified into three major categories: apparent (relative) polycythemia due to plasma volume depletion, primary polycythemia (polycythemia vera), and secondary polycythemia driven by either hypoxia-dependent or hypoxia-independent mechanisms 1, 2

Apparent polycythemia represents a false elevation in hemoglobin/hematocrit without true increase in red cell mass, which can be caused by plasma volume depletion from dehydration, severe diarrhea, vomiting, diuretic use, burns, or capillary leak syndrome 2, 3
Smoker's polycythemia is caused by chronic carbon monoxide exposure, which binds hemoglobin with 200-250 times greater affinity than oxygen, creating functional hypoxia 2, 4

Polycythemia vera (PV) is a JAK2-mutated myeloproliferative neoplasm with clonal erythrocytosis, characterized by low or inappropriately normal serum EPO levels 1
Familial polycythemia is an autosomal-dominant condition with activating mutations of the EPO receptor (EPOR) 1, 3

Chronic lung disease (COPD, pulmonary fibrosis) can trigger compensatory erythropoiesis through tissue hypoxia 1, 2
Right-to-left cardiopulmonary shunts can cause secondary polycythemia due to hypoxia 1, 2
High-altitude habitation can lead to secondary polycythemia due to hypoxia 1, 3
Hypoventilation syndromes, including obstructive sleep apnea, can cause secondary polycythemia due to hypoxia 1, 2

Renal cell carcinoma can produce erythropoietin (EPO) independently of hypoxia, leading to secondary polycythemia 1, 2
Hepatocellular carcinoma can produce EPO independently of hypoxia, leading to secondary polycythemia 1, 2
Cerebellar hemangioblastoma can produce EPO independently of hypoxia, leading to secondary polycythemia 1, 3

Serum EPO levels are the key discriminator between primary and secondary polycythemia: low or inappropriately normal in PV, elevated in most secondary causes 1, 2
JAK2 V617F mutation is present in up to 97% of PV cases and should be tested when EPO is low or normal 2
Bone marrow examination remains the cornerstone for confirming PV diagnosis, showing characteristic morphologic features 5, 6

Failing to distinguish relative from true polycythemia leads to unnecessary workup and misdiagnosis 2
Overlooking smoking as a cause of polycythemia, as smoker's polycythemia resolves with cessation, with risk reduction beginning within 1 year 2, 4
Misinterpreting normal EPO levels in chronic hypoxic states where levels may have normalized after compensatory hemoglobin elevation 2
Performing unnecessary red cell mass measurements when clinical context is obvious (e.g., hematocrit >60% without hemoconcentration) 1