What is benign polycythemia and how does it differ from polycythemia vera?

What is Benign Polycythemia?

"Benign polycythemia" is an outdated, imprecise term that historically referred to either apparent/relative polycythemia (spurious elevation due to reduced plasma volume) or secondary polycythemia (nonclonal increase in red blood cell mass driven by erythropoietin), both of which are fundamentally different from polycythemia vera—a clonal myeloproliferative neoplasm. 1

Understanding the Classification

The term "benign" is misleading because it conflates distinct entities with different pathophysiology and clinical implications:

Apparent (Relative) Polycythemia

• Not a true increase in red blood cell mass—the hematocrit appears elevated due to contracted plasma volume 1
• Causes include:
  • Clinically obvious volume depletion: severe dehydration, diarrhea, vomiting, diuretic use, severe burns 1
  • Smoker's polycythemia: secondary to chronic carbon monoxide exposure, resolves with smoking cessation 1
  • Controversial entities like Gaisböck syndrome (associated with hypertension/nephropathy) and stress polycythemia have little scientific foundation—Mayo Clinic data found zero cases of relative polycythemia in 109 consecutive measurements 1

Secondary Polycythemia (True but Nonclonal)

• Genuine increase in red blood cell mass that is nonclonal and often EPO-mediated 1

• Hypoxia-driven causes:

  • Chronic lung disease, high altitude, sleep apnea, right-to-left cardiac shunts
  • High oxygen-affinity hemoglobinopathies
  • EPO levels initially elevated but may normalize once hemoglobin stabilizes 1

• Hypoxia-independent causes:

  • EPO-secreting tumors: renal cancer, uterine leiomyoma, pheochromocytoma, meningioma 1
  • Congenital disorders: Chuvash polycythemia (von Hippel-Lindau gene mutations), abnormal EPO receptor mutations 1
  • Exogenous erythropoietic drugs: androgens, SGLT2 inhibitors 2
  • Post-renal transplant erythrocytosis 1

Critical Distinction from Polycythemia Vera

Polycythemia vera is a clonal myeloproliferative neoplasm with fundamentally different biology and prognosis 1, 3:

Feature	"Benign" Polycythemia	Polycythemia Vera
Pathophysiology	Nonclonal; reactive or plasma volume contraction	Clonal; JAK2 mutations (V617F in >95%, exon 12 in remainder) [4,3]
EPO levels	Normal to elevated	Subnormal [3]
Other cell lines	Isolated erythrocytosis	Panmyelosis: leukocytosis, thrombocytosis common [2,3]
Splenomegaly	Absent	Present in many cases [3]
Thrombotic risk	Minimal (unless severe hyperviscosity)	High—dominant cause of morbidity/mortality [3,5]
Transformation risk	None	Myelofibrosis, acute leukemia [3]
Symptoms	Usually asymptomatic	Pruritus, erythromelalgia, headache, visual disturbances [2,3]

Diagnostic Approach

When encountering elevated hematocrit, prioritize excluding secondary causes before genetic testing 2:

1. If isolated erythrocytosis:

   • Assess for obvious volume depletion (no specialized testing needed) 1
   • Evaluate for hypoxia: pulse oximetry, arterial blood gas
   • Check EPO level
   • Consider imaging for EPO-secreting tumors if EPO elevated
   • Review medications (androgens, SGLT2 inhibitors)

2. If accompanied by leukocytosis or thrombocytosis:

   • Immediately test for JAK2 V617F mutation and serum EPO level 2
   • Low EPO + JAK2 mutation = polycythemia vera

3. Red flags for PV over secondary causes:

   • Pruritus (especially aquagenic)
   • Erythromelalgia (burning pain in extremities)
   • Splenomegaly
   • Thrombocytosis >400 × 10⁹/L or leukocytosis >12 × 10⁹/L 2

Clinical Pitfalls

• The 2016 WHO lowered diagnostic thresholds for PV (Hb >165 g/L in men, >160 g/L in women), creating substantial overlap with normal values—6.48% of men and 0.28% of women with "normal" CBCs meet these criteria 6
• Do not reflexively order red cell mass measurements for apparent volume depletion or obvious secondary causes 1
• Cytokine profiling shows promise: PV patients have elevated IL-17A, IFN-γ, IL-12p70, and TNF-α compared to secondary polycythemia, potentially aiding differentiation 7

Management Implications

Secondary polycythemia management targets the underlying cause—no clear hematocrit goals exist; phlebotomy is used selectively when hyperviscosity symptoms occur 2. Polycythemia vera requires aggressive management: hematocrit <45%, low-dose aspirin, and cytoreductive therapy (hydroxyurea, ropeginterferon alfa-2b, or ruxolitinib) to prevent thrombosis 3, 5.

The term "benign polycythemia" should be abandoned in favor of precise terminology that reflects the underlying pathophysiology and guides appropriate management.