In a non‑cirrhotic patient with portal vein thrombosis, which JAK2‑positive myeloproliferative neoplasms should be considered, and what diagnostic work‑up and initial management are recommended?

JAK2-Positive Myeloproliferative Neoplasms in Non-Cirrhotic Portal Vein Thrombosis

In a non-cirrhotic patient with portal vein thrombosis, you must consider polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) as the JAK2-positive myeloproliferative neoplasms, with PV being the most common culprit, found in over 90% of cases with the JAK2 V617F mutation 1, 2.

Which JAK2-Positive MPNs to Consider

The three Philadelphia-negative classical myeloproliferative neoplasms associated with portal vein thrombosis are:

• Polycythemia vera (PV): JAK2 V617F mutation present in >95% of cases 1
• Essential thrombocythemia (ET): JAK2 V617F mutation present in ~60% of cases 1
• Primary myelofibrosis (PMF): JAK2 V617F mutation present in ~60% of cases 1

Critical caveat: Portal vein thrombosis is significantly more common in PV than in ET or PMF 3. Meta-analysis data shows MPNs are present in 40.9% of Budd-Chiari syndrome cases and 31.5% of portal vein thrombosis cases 3.

The "Masked PV" Pitfall

Be alert for iron-deficient or "masked" PV where hemoglobin appears normal or only mildly elevated despite underlying clonal disease 4, 5. In these cases:

• Mean corpuscular volume may be low-normal (80-85 fL)
• Biochemical evidence of iron restriction is present
• Serum erythropoietin is inappropriately low
• Portal vein thrombosis may be the presenting feature before overt erythrocytosis develops

Diagnostic Work-Up

Immediate Laboratory Testing

Order JAK2 V617F mutation testing immediately in all non-cirrhotic portal vein thrombosis patients 2, 3. This single test identifies 41.1% of Budd-Chiari syndrome and 27.7% of portal vein thrombosis cases as MPN-related 3.

Complete the following hematologic panel:

• Complete blood count with differential
• Peripheral blood smear (look for leukoerythroblastosis, teardrop cells)
• Serum erythropoietin level (inappropriately low suggests PV)
• Iron studies (ferritin, transferrin saturation, serum iron)
• Lactate dehydrogenase (elevated in PMF)

Diagnostic Criteria Application

For PV diagnosis (WHO 2016 criteria) 6:

• Hemoglobin >16.5 g/dL (men) or >16 g/dL (women), OR hematocrit >49% (men) or >48% (women)
• Bone marrow showing hypercellularity with trilineage myeloproliferation
• JAK2 mutation present
• Subnormal serum erythropoietin level

For ET diagnosis 1, 6:

• Platelet count ≥450 × 10⁹/L
• Bone marrow with megakaryocyte proliferation, large and mature morphology
• JAK2, CALR, or MPL mutation present
• Exclusion of other myeloid neoplasms

For PMF diagnosis 6:

• Megakaryocyte proliferation and atypia with reticulin/collagen fibrosis
• JAK2, CALR, or MPL mutation present
• At least one of: anemia, leukocytosis ≥11 × 10⁹/L, palpable splenomegaly, elevated LDH, leukoerythroblastosis

Bone Marrow Evaluation

Perform bone marrow aspiration and biopsy with reticulin staining 6, 5. This is essential when:

• JAK2 V617F is positive but blood counts are equivocal
• Splenomegaly is present
• Peripheral smear shows leukoerythroblastosis
• Differentiating between ET and early/prefibrotic PMF

Look specifically for:

• Hypercellularity with panmyelosis (PV)
• Atypical megakaryocytic clustering (all MPNs)
• Reticulin fibrosis grade (PMF: grade 2-3)
• Absent iron stores (masked PV)

Additional Molecular Testing

If JAK2 V617F is negative, test for:

• JAK2 exon 12 mutations (2-4% of PV cases) 1
• CALR mutations (60-80% of JAK2-negative ET and PMF) 7
• MPL mutations (3-5% of ET, 5-8% of PMF) 7

Imaging Assessment

Obtain contrast-enhanced CT abdomen/pelvis to:

• Document extent of thrombosis (portal, splenic, mesenteric veins)
• Assess for splenomegaly (measure splenic size)
• Exclude cirrhosis and hepatocellular carcinoma
• Identify collateral vessels suggesting chronic thrombosis
• Rule out bowel ischemia (rebound tenderness, guarding, fever suggest progression) 2

Initial Management

Anticoagulation Strategy

Initiate anticoagulation immediately unless absolute contraindications exist 2. The presence of MPN does not contraindicate anticoagulation; rather, it mandates it given the high thrombotic risk.

Anticoagulation options:

• Direct oral anticoagulants (DOACs): Edoxaban 30-60 mg daily or apixaban 5 mg twice daily 4, 8
• Low molecular weight heparin bridged to warfarin (INR 2-3) if DOAC contraindicated
• Continue indefinitely given the persistent prothrombotic state from the underlying MPN 8

MPN-Specific Therapy

Risk stratify immediately 1:

High-risk criteria (requiring cytoreduction):

• Age >60 years
• History of thrombosis (which this patient has by definition)

For high-risk PV 1:

• Therapeutic phlebotomy to maintain hematocrit <45% (reduces thrombotic risk)
• Low-dose aspirin 81-100 mg daily (unless contraindicated by bleeding)
• Cytoreduction with hydroxyurea (start 15-20 mg/kg/day, adjust to maintain platelet <400 × 10⁹/L, WBC <10 × 10⁹/L)
• Alternative: Interferon-alpha at any age (preferred in younger patients <40 years)

For high-risk ET 1:

• Cytoreduction with hydroxyurea (first-line at any age)
• Low-dose aspirin unless platelet count >1,500 × 10⁹/L (acquired von Willebrand syndrome risk)

For PMF 1:

• Risk stratify using International Prognostic Scoring System (IPSS)
• Hydroxyurea for splenomegaly
• Consider allogeneic stem cell transplantation if median survival <5 years

Monitoring Response

Use ELN clinicohematologic criteria for PV/ET 1:

Complete response in PV:

• Hematocrit <45% without phlebotomy
• Platelet count <400 × 10⁹/L
• WBC count <10 × 10⁹/L
• No disease-related symptoms

Assess for hydroxyurea resistance/intolerance 1:

• Need for phlebotomy after 3 months on ≥2 g/day hydroxyurea
• Uncontrolled myeloproliferation (platelets >400 × 10⁹/L AND WBC >10 × 10⁹/L) after 3 months
• Cytopenia at lowest effective dose
• Leg ulcers or other unacceptable toxicity

Follow-Up Imaging

Repeat CT abdomen at 6-12 weeks to assess thrombus evolution 4. Expect partial recanalization with appropriate anticoagulation and MPN-directed therapy.

Key Clinical Pearls

JAK2 V617F testing identifies occult MPN in 17.1% of Budd-Chiari and 15.4% of portal vein thrombosis patients without typical hematologic features 3. This validates routine JAK2 testing in all non-cirrhotic splanchnic vein thrombosis.

Young patients (<40 years) with unprovoked or recurrent thrombosis warrant JAK2 testing even with completely normal blood counts 8. Latent MPNs with low allele burden can present with thrombosis as the sole manifestation.

The combination of portal vein thrombosis, splenomegaly, and normal liver function should immediately trigger MPN evaluation 5, 9, not a prolonged search for occult cirrhosis.