What bone marrow flow cytometry findings would indicate classic hairy cell leukemia versus hairy cell leukemia variant in a patient with BRAF‑V600E–negative mutations?

Flow Cytometry Findings in BRAF-Negative Hairy Cell Leukemia

In a BRAF-negative patient, flow cytometry showing CD25-negative, CD123-negative, and annexin A1-negative cells indicates hairy cell leukemia variant (HCLv), while cells that are CD25+, CD123+, and annexin A1+ suggest either classic HCL with IGHV4-34 rearrangement or a rare BRAF-negative classic HCL case. 1

Key Immunophenotypic Distinctions

Classic HCL (BRAF-negative subset)

When BRAF V600E is absent but classic HCL is suspected, look for:

• CD25: Positive (bright)
• CD123: Positive
• Annexin A1: Positive
• CD11c: Positive
• CD20: Bright positive
• CD103: Positive
• CD200: Bright positive
• Cyclin D1: Positive
• CD5, CD10: Negative
• Monocytopenia present 1

This pattern suggests classic HCL with IGHV4-34 rearrangement, which behaves more aggressively like HCLv despite having classic morphology and immunophenotype. These cases typically lack BRAF V600E mutations and respond poorly to purine analog therapy. 1

HCL Variant (HCLv)

The distinguishing flow cytometry pattern shows:

• CD25: Negative (critical distinguishing feature)
• CD123: Negative (critical distinguishing feature)
• Annexin A1: Negative (critical distinguishing feature)
• CD11c: Positive
• CD20: Positive
• CD103: Positive
• BRAF V600E: Negative 1

Clinical Algorithm for BRAF-Negative Cases

1. Perform comprehensive flow cytometry panel including CD19, CD20, CD5, CD10, CD11c, CD22, CD25, CD103, CD123, cyclin D1, CD200, and annexin A1 1

2. Assess the CD25/CD123/annexin A1 triad:

   • If all three are negative → Diagnose HCLv
   • If all three are positive → Consider classic HCL with IGHV4-34 rearrangement

3. Order IGHV4-34 molecular testing when flow cytometry suggests classic HCL but BRAF is negative, as this identifies a prognostically important subset 1

4. Check for MAPK1 mutations in confirmed HCLv or IGHV4-34 cases, as these often harbor this alternative mutation 1

Critical Pitfalls to Avoid

Do not rely solely on morphology in BRAF-negative cases. The presence of "hairy" cells alone is insufficient—immunophenotyping is essential to distinguish between classic HCL and HCLv, as they require different treatment approaches. 1

Recognize that CD19 negativity can rarely occur in classic HCL as an aberrant finding, so don't exclude the diagnosis based on this alone. 2

Remember that monocytopenia is characteristic of classic HCL but not HCLv—check the absolute monocyte count as an additional distinguishing feature. 1

Prognostic Implications

The distinction matters significantly for treatment planning. HCLv and IGHV4-34-positive classic HCL both show:

• Primary refractoriness to purine analog monotherapy
• More rapid disease progression
• Poorer prognosis compared to BRAF-positive classic HCL
• Need for combination therapy rather than single-agent purine analogs 1

Unmutated IGHV status (found in 54% of HCLv vs. 17% of classic HCL) serves as an additional poor prognostic marker when detected. 1