A 58-year-old man has headache, aquagenic pruritus, and splenomegaly. Hemoglobin is 20.5 g/dL, hematocrit 62%, and red cell mass is elevated. Arterial oxygen saturation is normal. Which single laboratory result best supports a diagnosis of polycythemia vera over secondary erythrocytosis?

• A Elevated serum vitamin B12 level
• B Presence of JAK2 V617F mutation
• C Elevated serum ferritin
• D Low serum erythropoietin level ✓

Explanation

In polycythemia vera the neoplastic erythroid proliferation is erythropoietin independent, so serum EPO is suppressed below normal, whereas any secondary erythrocytosis (chronic hypoxia, renal lesions, EPO-secreting tumors) drives EPO up. JAK2 V617F occurs in about 95% of cases but can rarely be found in other conditions, so the suppressed EPO remains the discriminating value between the two entities.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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