What blood tests are used to monitor Polycythemia vera?

Blood Tests for Monitoring Polycythemia Vera

The cornerstone blood tests for monitoring PV are hematocrit (target <45%), complete blood count with differential (tracking hemoglobin, white blood cells, and platelets), and periodic serum erythropoietin levels, with additional monitoring of JAK2V617F allele burden in select cases. 1, 2, 3

Essential Routine Monitoring Tests

Hematocrit and Hemoglobin

• Hematocrit must be maintained below 45% in all patients to reduce thrombotic risk, requiring frequent monitoring during phlebotomy therapy 3, 4
• Hemoglobin levels should be tracked alongside hematocrit, with diagnostic thresholds being >16.5 g/dL in men or >16.0 g/dL in women 3, 4
• A sustained increase of ≥2 g/dL hemoglobin from baseline warrants investigation even within normal reference ranges 2

Complete Blood Count (CBC) with Differential

• White blood cell count monitoring is critical as leukocytosis (present in 49% of patients) is a prognostic factor for survival and arterial thrombosis risk 3, 4
• Absolute neutrophil count specifically predicts venous thrombosis risk and should be tracked 4
• Platelet count monitoring identifies thrombocytosis (present in 53% of patients) and extreme thrombocytosis (≥1000 × 10⁹/L), which increases bleeding risk due to acquired von Willebrand disease 3

Iron Studies

• Low MCHC (<32%) suggests iron deficiency that can mask true erythrocytosis, requiring serum ferritin, iron studies, and transferrin saturation assessment 2
• Bone marrow iron stores are typically decreased in PV and should be considered during diagnostic evaluation 5

Specialized Monitoring Tests

Serum Erythropoietin (EPO)

• Low EPO levels have >90% specificity for PV and should be measured periodically, though normal EPO does not exclude PV (sensitivity <70%) 5, 1, 2
• EPO levels remain low even during phlebotomy treatment 5

JAK2 Mutation Monitoring

• JAK2V617F allele burden correlates with venous thrombosis risk and is present in >95% of PV patients 3, 4
• While reduction in allele burden is seen with certain therapies (pegylated interferon, ruxolitinib, busulfan), this should not currently drive treatment decisions unless accompanied by cytogenetic and morphologic remission 4

Additional Monitoring Considerations

Cytogenetic and Molecular Testing

• Abnormal karyotype occurs in 15-20% of patients, with prognostically adverse mutations (SRSF2, IDH2, RUNX1, U2AF1) present in 5-10% 4
• Periodic cytogenetic monitoring helps identify disease progression risk toward post-PV myelofibrosis (16% at 20 years) or acute myeloid leukemia (4% at 20 years) 4

Biochemical Markers

• Elevated leukocyte alkaline phosphatase and increased serum vitamin B12 levels lack sensitivity and specificity but may be tracked 5
• Cytokine levels can alter disease prognosis, though routine monitoring is not yet standard practice 6

Critical Monitoring Pitfalls to Avoid

• Do not rely on hematocrit alone when MCHC is low, as iron deficiency can falsely normalize hematocrit while true erythrocytosis persists 2
• Dehydration is the most common cause of falsely elevated hematocrit and must be excluded before intensifying therapy 2
• Red cell mass (RCM) measurement is not routinely needed for monitoring, as normal RCM does not rule out PV and adds unnecessary cost 5
• Traditional markers like splenomegaly presence alone lack sufficient sensitivity and specificity for monitoring disease activity 5