Relative polycythaemia

Overview

Relative (apparent/spurious) polycythaemia - raised haematocrit due to reduced plasma volume, NOT increased red cell mass. Bone marrow and erythropoietin are entirely normal.

Presentation

•Headaches, dizziness, blurred vision - raised blood viscosity

•Facial plethora, conjunctival plethora

•Hypertension - both cause and consequence

•No splenomegaly, no aquagenic pruritus, no raised WBC/platelets

⚠️

Aquagenic pruritus (itch after warm bath) is a hallmark of polycythaemia vera, NOT relative polycythaemia - its presence should prompt JAK2 testing.

Investigations

•Threshold for investigation: haematocrit >0.52 in males, >0.48 in females (persistent)

•FBC - raised Hb/haematocrit; normal WBC and platelets (raised counts suggest PV)

•Serum erythropoietin - normal in relative polycythaemia; elevated in secondary erythrocytosis; suppressed in PV

•JAK2 V617F - negative in relative polycythaemia; positive in ~95% of PV

🏆 Gold standard

•red cell mass studies (isotope dilution) - confirms normal red cell mass, reduced plasma volume

📌

Take blood samples without a tourniquet - prolonged tourniquet use causes local haemoconcentration and can artifactually raise the measured haematocrit.

Differential diagnosis

Distinguishing causes of raised haematocrit

Feature	Relative polycythaemia	Polycythaemia vera	Secondary erythrocytosis
Red cell mass	Normal	Raised	Raised
EPO	Normal	Suppressed	Elevated
JAK2	Negative	Positive (~95%)	Negative
WBC/platelets	Normal	Raised	Normal
Splenomegaly	Absent	Present	Absent

Management

🥇 First-line

•address reversible causes - adequate hydration, smoking cessation, reduce/stop diuretics if appropriate, reduce alcohol, weight loss

•Review antihypertensive regimen - if diuretic causing plasma contraction, consider switching to ACE inhibitor or calcium channel blocker

•Recheck haematocrit after 2 months of addressing risk factors

🥈 Second-line

•haematology referral if haematocrit fails to normalise after 2 months, or features suggest absolute polycythaemia

•No role for venesection or cytoreductive therapy - these are reserved for absolute polycythaemia

Complications

•Thrombotic events - stroke, TIA, MI, DVT, PE; most clinically important complication due to raised blood viscosity

•Hypertension - self-perpetuating cycle especially when managed with diuretics

🎯

Unlike polycythaemia vera, relative polycythaemia does NOT carry risk of transformation to myelofibrosis or acute myeloid leukaemia - myeloid transformation is a complication of PV (a myeloproliferative neoplasm), not plasma volume contraction.

Transfusion thresholds (NICE 2015)

•Transfuse packed red blood cells if Hb <70 g/L (without acute coronary syndrome)

•Threshold rises to 80 g/L in patients with acute coronary syndrome