JAK2 Unmutated Erythrocytosis: 2026 Update on Diagnosis and Management

JAK2 unmutated erythrocytosis encompasses a heterogeneous spectrum of hereditary and acquired entities.

The foremost step is excluding polycythemia vera (PV) with JAK2 mutation screening (exons 12-15). Apparent polycythemia such as physiological outliers or relative polycythemia secondary to volume contraction should be considered. A historical overview of hematocrit (Hct) and hemoglobin (Hgb) levels helps distinguish longstanding from acquired erythrocytosis. Serum erythropoietin (Epo) levels are variably informative.

Hereditary erythrocytosis should be considered in longstanding erythrocytosis with a positive family history; causes include EPOR mutations (subnormal Epo), high oxygen affinity hemoglobin variants, PIEZO1 mutations, 2,3-bisphosphoglycerate deficiency, methemoglobinemia, and germline oxygen sensing pathway mutations (HIF2A-PHD2-VHL).

Acquired erythrocytosis results from central (cardiopulmonary disease) or peripheral (renal artery stenosis) hypoxia, Epo-producing tumors (renal cell carcinoma) or drugs (testosterone, sodium glucose co-transporter-2 inhibitors (SGLT2-i), erythropoiesis stimulating agents).

Idiopathic erythrocytosis is an ill-defined terminology that presumes the existence of an increased Hgb/Hct level without an identifiable etiology.

Cytoreductive therapy should be avoided. Phlebotomy should be considered for symptom control. Cardiovascular risk optimization and low-dose aspirin are advised, while the role of HIF2A inhibitors remains unclear.

EPO mutations which produce hyperactive, hepatic-like Epo were identified. In cases with negative workup but high clinical suspicion, an expanded next generation sequencing panel for hereditary erythrocytosis is recommended. Among drugs, SGLT2-i-associated erythrocytosis is increasingly recognized.

Advances in molecular hematology are expected to improve the characterization of "idiopathic erythrocytosis". Results from prospective studies are needed to elucidate the underlying pathology and guide management.