What are the clinical indications for JAK 2 (Janus kinase 2) testing in adult patients with suspected myeloproliferative neoplasms (MPNs)?

Clinical Indications for JAK2 Testing

JAK2 mutation testing should be performed in any adult patient presenting with unexplained erythrocytosis, thrombocytosis, leukocytosis, splenomegaly, or unexplained thrombosis, as these findings strongly suggest a myeloproliferative neoplasm. 1

Primary Indications for JAK2 Testing

Elevated Blood Counts

• Test immediately when hemoglobin exceeds 16.5 g/dL in women or 18.5 g/dL in men, as JAK2V617F is present in approximately 95% of polycythemia vera cases 1, 2
• Persistent thrombocytosis with platelet count >450 × 10⁹/L on repeated measurements warrants JAK2V617F testing, as the mutation is found in approximately 60% of essential thrombocythemia cases 1, 3
• Unexplained leukocytosis in the absence of infection or other reactive causes should prompt testing 2

Clinical Presentations

• Unexplained splenomegaly detected on physical examination or imaging requires JAK2 testing as part of the diagnostic workup for myeloproliferative neoplasms 2, 1
• Unexplained thrombosis, particularly in unusual sites (splanchnic, cerebral), should trigger JAK2 mutation analysis even with normal blood counts 1
• Constitutional symptoms including unexplained fever >37.5°C, night sweats, or >10% weight loss over 6 months in the context of abnormal blood counts 2

Diagnostic Algorithm

Sequential Testing Approach

JAK2V617F mutation analysis should be the first-line molecular test when myeloproliferative neoplasm is suspected 2, 1

• If JAK2V617F is negative in patients with suspected essential thrombocythemia or primary myelofibrosis, proceed with sequential testing for CALR and MPL mutations 2
• If JAK2V617F is negative in patients with suspected polycythemia vera, test for JAK2 exon 12 mutations, which account for 2-4% of cases 2
• Philadelphia chromosome (BCR-ABL1) must be excluded via FISH or RT-PCR before attributing findings to a JAK2-related myeloproliferative neoplasm 2

Testing Methodology

• Whole blood or purified granulocytes can be used, though purified granulocytes are preferred when low mutation burden is suspected (particularly for JAK2 exon 12 mutations) 2
• Testing methods include conventional sequencing, qualitative and quantitative PCR, and high-resolution melting analysis with sensitivity of 1% or higher 2

When NOT to Test

Routine Monitoring

• Serial JAK2 mutation burden measurement is NOT recommended during routine follow-up or to assess treatment response in most clinical scenarios 2
• The exceptions where monitoring JAK2V617F allele burden is appropriate include:
  • Following allogeneic stem cell transplantation (detection limit ≤0.1% recommended) 2, 1
  • Possibly during interferon treatment 2

Family Screening

• Routine genotyping for JAK2 mutations or JAK2 46/1 (GGCC) haplotype in asymptomatic relatives is NOT indicated in the absence of hematologic or clinical abnormalities 2
• However, clinicians should maintain heightened awareness that first-degree relatives have a 5-7 fold increased risk of developing myeloproliferative neoplasms 1, 3

Special Populations

Pediatric Patients

• Diagnostic criteria for myeloproliferative neoplasms in children are the same as in adults 2
• Family screening is recommended in JAK2V617F-negative essential thrombocythemia to differentiate from rare familial disorders caused by mutations of TPO or MPL (particularly MPLS505N) 2

Pregnancy

• JAK2 testing should be performed in pregnant women with unexplained erythrocytosis or thrombocytosis, as JAK2V617F mutation may be associated with increased risk of pregnancy complications including miscarriage (3-4 fold higher than general population), abruptio placentae, pre-eclampsia, and intrauterine growth retardation 2, 1, 3

Common Pitfalls to Avoid

• Do not assume a negative JAK2V617F test excludes myeloproliferative neoplasm: approximately 10-15% of essential thrombocythemia and primary myelofibrosis patients are "triple negative" for JAK2, CALR, and MPL mutations 2
• Do not order JAK2 testing in isolation: it must be integrated with complete blood count, peripheral blood smear examination, bone marrow biopsy with reticulin staining, and cytogenetics for accurate diagnosis 2
• The presence of JAK2 mutation excludes reactive causes of erythrocytosis, thrombocytosis, or myelofibrosis but does not by itself specify which myeloproliferative neoplasm subtype is present 2