Diagnostic Performance of Red Blood Cell Indices, Serum Erythropoietin, and JAK2 Mutation Testing for the Evaluation of Polycythemia Vera at High Altitude

Introduction:

Polycythemia vera (PV) is a clonal myeloproliferative neoplasm associated with mutation of Janus kinase 2 ( JAK2) in virtually all patients. Patients with PV typically present with constitutional symptoms and thrombotic complications related to myeloproliferation but can present with only mild or no symptoms. In 2016, the World Health Organization (WHO) diagnostic criteria for PV were revised to use lower hemoglobin and hematocrit cut-off levels in order to avoid missing masked polycythemia. Accordingly, a hemoglobin concentration of more than 16 g/dL (HCT>48%) for females and 16.5 g/dL (HCT>49%) for males can be considered for PV evaluation (previously >18.5 g/dL for men and >16.5 g/dL for women). In conjunction, concerns arise about the unnecessary evaluation of individuals with otherwise high-normal hemoglobin levels, more so for those living at high altitude. Altitude exposure, amongst others, decreases plasma volume to increase [Hb] which may accordingly muddle the diagnosis of PV at altitude.

Nonetheless, Red Cell Mass (RCM) is seldom assessed in clinical practice. In addition, red blood cell count, related indices, and serum erythropoietin may also be affected by high altitude, and their diagnostic performance in the evaluation of PV at altitude has not been prospectively assessed.

Method: We collected lab records for all JAK2 requests over three years (2020-2022) in two referral hospitals serving a population residing at an altitude of 2,270-3,000 meters above sea level. Indications for requests, as well as laboratory data and imaging, were gathered. Complete blood count (CBC) and other laboratory values were collected when ordered within one week of JAK2 testing.

For CBC, EDTA anticoagulated samples were processed using Sysmex XN-3100. For JAK2 testing, RNA isolated from peripheral blood samples (using the MagNA Pure System) was reverse-transcribed, amplified, and sequenced for any mutations involving exons 12, 13, 14, or 15 using an ABI 3730 XL genetic analyzer.

For serum EPO, frozen serum samples were outsourced to Bio Scientia Labs, Germany. Reference values for serum erythropoietin were lab generated at 4.3-29 mIU/ml and were not validated for our altitude.

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Results:Of the 208 JAK2 tests performed during the reviewed period, 132 (63.4%) were for polycythemia (Table 1). The JAK2 V617F mutation was found in 22 (16.7%) patients, with 10 (45.5%) of them being men. No JAK2 mutation other than the exon 14 V617F substitution was found. JAK2-positive patients had a median age of 61 years, while JAK2-unmutated patients had a median age of 47 years. Overt polycythemia, according to the WHO 2016 classification, was identified in 15 (68.2%) , while the remainder had masked PV ( defined as increased RBCs with normal hemoglobin/ HCT).

Patients with PV had significantly higher RBCs and lower MCV/MCH/MCHC compared to those with JAK2-unmutated polycythemia. They also exhibited significant elevation of RDW-CW and significant neutrophilia, basophilia, thrombocytosis, and splenomegaly. All PV patients with available serum EPO tests had low values (median 2 mIU/ml).

Conclusion: At altitude, patients with PV show the same age and sex characteristics as those living at low elevations. Patients with PV have higher RBCs and lower MCV/MCH/MCHC compared to those with JAK-unmutated (secondary) polycythemia. RDW is significantly increased in patients with PV, reflecting an associated iron deficiency. Serum EPO performed well at altitude to suggest the presence of PV.

The low rate of positive JAK2 mutation in our high-altitude cohort may reflect poor selection of subjects and over-workup of otherwise altitude-associated erythrocytosis.

Isolated mild polycythemia at altitude remains common. Using hemoglobin/hematocrit alone to select patients for PV evaluation has a low positive predictive value and may be associated with unnecessary workup. RBC indices and RDW, along with an evaluation for iron status, might help select those more likely to have PV and could decrease unnecessary workups.