Definitional Difference Between Polycythemia and Erythrocytosis
Erythrocytosis is the broader, more precise term that refers specifically to an elevated red blood cell mass (hemoglobin >16.5 g/dL in men or >16.0 g/dL in women), while polycythemia technically means an increase in all three blood cell lines (red cells, white cells, and platelets), though the terms are often used interchangeably in clinical practice. 1, 2
Terminology and Clinical Usage
Erythrocytosis is the preferred modern terminology when describing isolated elevation of red blood cells:
- Erythrocytosis specifically denotes increased red blood cell concentration above age- and sex-specific normal levels (hemoglobin >16.5 g/dL in men, >16.0 g/dL in women; hematocrit >49% in men, >48% in women). 1, 2, 3
- This term is more accurate when only the red cell line is elevated. 2
Polycythemia historically implies a multi-lineage increase:
- By strict definition, polycythemia means elevation of all three myeloid cell lines (erythrocytes, leukocytes, and platelets), as evidenced by the classic presentation of polycythemia vera where erythrocytosis is accompanied by thrombocytosis (53% of cases) and leukocytosis (49% of cases). 4, 1
- However, in clinical practice, "polycythemia" is commonly used synonymously with "erythrocytosis" to describe any condition with elevated red cell mass. 4
Classification Framework
Both terms can be categorized into three major types:
Primary (Polycythemia Vera):
- A JAK2-mutated myeloproliferative neoplasm with clonal erythrocytosis, present in >95% of cases, characterized by low or inappropriately normal serum EPO levels. 1, 5, 6
- Represents a clonal stem cell disorder with panmyeloid hyperplasia. 4
Secondary (Reactive):
- Results from external stimuli to bone marrow causing compensatory red cell production, with elevated or normal EPO levels. 2, 7
- Includes hypoxia-driven causes (chronic lung disease, sleep apnea, high altitude, smoking) and hypoxia-independent causes (EPO-secreting tumors, post-renal transplant). 4, 7, 6
Apparent (Relative):
- False elevation without true increase in red cell mass, caused by plasma volume depletion from dehydration, diuretics, or burns. 6, 7
Key Clinical Distinction
The critical diagnostic discriminator is serum EPO level: low or inappropriately normal EPO suggests primary polycythemia vera (requiring JAK2 mutation testing), while elevated EPO points toward secondary erythrocytosis (requiring evaluation for hypoxic or tumor-related causes). 8, 6, 5
Common Pitfall
Clinicians often use "polycythemia" and "erythrocytosis" interchangeably in practice, but when precision matters—particularly in distinguishing isolated red cell elevation from true multi-lineage myeloproliferation—"erythrocytosis" is the more accurate term. 2, 3 The presence of accompanying leukocytosis and thrombocytosis strengthens the diagnosis of polycythemia vera as a true polycythemia rather than isolated erythrocytosis. 4, 1