What is Spurious Erythrocytosis (Relative Erythrocytosis)?
Spurious erythrocytosis, also called relative erythrocytosis or apparent polycythemia, is an elevated hemoglobin and hematocrit caused by reduced plasma volume rather than an actual increase in red blood cell mass. 1
Pathophysiology
The red blood cell mass is normal in spurious erythrocytosis, but the plasma volume is decreased, resulting in hemoconcentration that falsely elevates the hemoglobin and hematocrit measurements. 1, 2
This condition represents a contraction of plasma volume rather than true erythroid proliferation, distinguishing it fundamentally from true polycythemia where red cell mass is genuinely elevated. 1, 3
The Mayo Clinic classification system categorizes this as "apparent polycythemia" with two subcategories: relative polycythemia (decreased plasma volume) and extreme "high-normal" values (upper end of normal distribution). 1
Common Causes
Dehydration, diuretic use, and burns are the primary causes of plasma volume depletion leading to spurious erythrocytosis. 2
Hypertension and cardiovascular risk factors are frequently associated with this condition, though they represent comorbidities rather than the cause of the elevated hematocrit. 4
Some cases may represent nothing more than an unusual but normal physiologic state at the extreme end of the normal distribution curve. 4
Diagnostic Approach
Red blood cell mass measurement using radionuclide methods (Cr-51) is the gold standard for distinguishing spurious erythrocytosis from true polycythemia, showing normal red cell mass (<125% of predicted) with reduced plasma volume in spurious cases. 3
In a Mayo Clinic series of 109 consecutive red cell mass and plasma volume measurements, no patients with relative polycythemia were identified, suggesting this diagnosis may be overused when modern diagnostic tests are not employed. 1
Hemoglobin and hematocrit levels alone are unreliable for diagnosis because they do not account for variations in plasma volume and can lead to misclassification. 3
Studies show that even with hemoglobin >185 g/L in males, only 50% actually have absolute erythrocytosis, while the remainder have spurious elevation. 3
Clinical Significance and Management
Spurious erythrocytosis is not a primary disease process and requires no hematologic treatment—phlebotomy and myelosuppression are inappropriate because there is no abnormal erythroid proliferation. 4
The elevated hematocrit itself bears no relation to morbidity in spurious erythrocytosis. 4
The major clinical concern is managing associated cardiovascular risk factors, particularly hypertension, which accounts for the substantial cardiovascular morbidity in these patients and demands aggressive treatment. 4
Rehydration with oral fluids or intravenous normal saline is the appropriate first-line intervention when spurious erythrocytosis is suspected. 1
Critical Distinction from True Polycythemia
Unlike polycythemia vera, spurious erythrocytosis shows normal or elevated serum erythropoietin levels (not low), absence of JAK2 mutations, and no bone marrow panmyelosis. 2, 5
The condition does not progress to myelofibrosis or acute leukemia, unlike polycythemia vera which carries these risks as part of its natural history. 1
Smoker's polycythemia should not be confused with spurious erythrocytosis—smoking causes true secondary erythrocytosis from chronic carbon monoxide exposure, which resolves with smoking cessation. 1