Given mildly elevated lactate dehydrogenase, markedly elevated vitamin B12, upper‑normal alkaline phosphatase, borderline high hemoglobin and hematocrit, mild leukocytosis, normal serum tryptase, and a negative JAK2 V617F mutation, what is the likely diagnosis and recommended next steps?

Evaluation of Borderline Erythrocytosis with Elevated B12 and Negative JAK2 V617F

The most likely diagnosis is a JAK2-negative myeloproliferative neoplasm (MPN), specifically polycythemia vera (PV) or early primary myelofibrosis (PMF), and the next critical step is bone marrow biopsy with testing for JAK2 exon 12 mutations, CALR, and MPL mutations.

Diagnostic Reasoning

Your laboratory findings present a constellation suggestive of an MPN despite the negative JAK2 V617F:

• Borderline erythrocytosis: Hemoglobin 16.4 g/dL and hematocrit 49.1% approach the WHO diagnostic thresholds for PV (Hgb ≥16.5 g/dL in women or ≥18.5 g/dL in men; Hct ≥48% in women or ≥49% in men) 1
• Elevated LDH (250): A minor criterion for primary myelofibrosis 1
• Markedly elevated B12 (1799): Commonly seen in MPNs due to increased production by granulocytes
• Mild leukocytosis (11.7): Consistent with myeloproliferative process
• Normal tryptase (4.9): Effectively excludes systemic mastocytosis

Critical Next Steps

1. Bone Marrow Evaluation (Essential)

Obtain bone marrow aspirate and biopsy with 1:

• Reticulin and trichrome staining to assess for fibrosis (grade 0-3)
• Morphologic assessment for megakaryocyte proliferation and atypia (small to large megakaryocytes with aberrant nuclear/cytoplasmic ratio, hyperchromatic, bulbous, or irregularly folded nuclei) 1
• Cytogenetics (karyotype ± FISH) to exclude BCR-ABL1 and identify clonal markers 1
• Assessment for hypercellularity with trilineage growth (panmyelosis) characteristic of PV 1

2. Expanded Molecular Testing (Critical)

Since JAK2 V617F is negative, test for 1:

• JAK2 exon 12 mutations: Found in 2-5% of JAK2 V617F-negative PV cases, typically presenting with isolated erythrocytosis 2, 3
• CALR mutations: Present in approximately 20-30% of JAK2-negative ET and PMF 1
• MPL mutations (MPL W515K/L): Found in small proportion of JAK2-negative PMF and ET 1

3. Additional Laboratory Studies

Complete the diagnostic workup with 1:

• Serum erythropoietin level: Expected to be low in PV (minor criterion) 1
• Serum iron studies: To exclude iron deficiency that could mask higher hemoglobin levels 1
• Peripheral blood smear examination: Look for leukoerythroblastosis (minor criterion for PMF) 1
• BCR-ABL1 testing (FISH or RT-PCR): Mandatory to exclude chronic myeloid leukemia 1

4. Clinical Assessment

Document 1:

• Spleen size by palpation: Splenomegaly is a minor criterion for PMF 1
• History of thrombotic/hemorrhagic events
• Constitutional symptoms: Weight loss, night sweats, fever (minor criterion for PMF) 1
• Symptom burden assessment using MPN-SAF (MPN Symptom Assessment Form) 1

Diagnostic Algorithm Based on WHO 2016 Criteria

If Bone Marrow Shows Hypercellularity with Trilineage Growth:

Polycythemia Vera diagnosis requires 1:

• Both major criteria (elevated Hgb/Hct + JAK2 mutation) AND one minor criterion, OR
• First major criterion (elevated Hgb/Hct) AND two minor criteria:
  • Minor criteria: (1) Bone marrow panmyelosis, (2) Low serum EPO, (3) Endogenous erythroid colonies

If Bone Marrow Shows Megakaryocyte Proliferation with Fibrosis:

Primary Myelofibrosis diagnosis requires 1:

• All 3 major criteria:
  1. Megakaryocyte proliferation and atypia with reticulin/collagen fibrosis
  2. Not meeting WHO criteria for other myeloid neoplasms
  3. JAK2/CALR/MPL mutation OR absence of reactive fibrosis
• Plus 2 of 4 minor criteria: Leukoerythroblastosis, increased LDH (present in your case), anemia, splenomegaly

Important Caveats

JAK2 V617F absence does not exclude MPN: The mutation is found in >90% of PV but only ~50% of PMF and ET 1. Your patient requires testing for alternative driver mutations 1.

Elevated B12 is not diagnostic but supportive: While markedly elevated B12 (>1000 pg/mL) is commonly seen in MPNs, it can also occur in liver disease, renal failure, and malignancies. However, in the context of borderline erythrocytosis and elevated LDH, it strongly supports an MPN diagnosis.

Avoid misdiagnosis of B12 deficiency: Although severe B12 deficiency can cause elevated LDH and hemolysis 4, 5, 6, your patient has elevated (not low) B12 and elevated (not low) hemoglobin/hematocrit, making this diagnosis incompatible.

Consider splanchnic vein thrombosis risk: Even without overt MPN diagnosis, JAK2 mutations (including exon 12) are associated with increased risk of abdominal vein thrombosis 1. If splanchnic thrombosis is present, this further supports MPN evaluation.

Referral Recommendation

Refer to a specialized center with MPN expertise for definitive diagnosis and management planning, particularly if bone marrow findings confirm myelofibrosis or if molecular testing reveals driver mutations 1.