What is the recommended diagnostic work‑up and treatment approach for hypereosinophilic syndrome?

Hypereosinophilic Syndrome: Diagnostic Work-Up and Treatment

For suspected hypereosinophilic syndrome, immediately obtain peripheral blood eosinophil count, bone marrow aspirate/biopsy with molecular testing for tyrosine kinase fusion genes (particularly FIP1L1-PDGFRA), and cardiac evaluation, then initiate imatinib if PDGFRA-rearranged or corticosteroids for all other subtypes while evaluating for secondary causes. 1

Diagnostic Algorithm

Initial Laboratory Evaluation

The diagnostic work-up must distinguish between secondary (reactive), primary (clonal/neoplastic), and idiopathic causes of hypereosinophilia (defined as eosinophils ≥1.5 × 10⁹/L) 2:

First-tier testing:

• Complete blood count with differential (confirm persistent eosinophilia ≥1.5 × 10⁹/L)
• Serum tryptase, vitamin B12 (markedly elevated in myeloproliferative neoplasms)
• Serum IgE levels (elevated in allergic/parasitic causes and lymphocytic variant)
• Troponin, NT-proBNP (cardiac damage markers)
• Liver and renal function tests
• Stool examination for parasites (particularly Strongyloides)
• HIV and HTLV-1 serology 1, 3

Critical Molecular and Bone Marrow Testing

Bone marrow evaluation is mandatory and should include 1:

• Aspirate and biopsy with morphologic review
• Immunohistochemistry for CD117, CD25, tryptase (to detect systemic mastocytosis)
• Reticulin/collagen stains for fibrosis
• Conventional cytogenetics
• FISH for CHIC2 deletion (detects cryptic FIP1L1-PDGFRA fusion—the most common tyrosine kinase rearrangement, present in ~10-15% of cases)
• Nested RT-PCR or RT-qPCR for FIP1L1-PDGFRA (more sensitive than FISH)
• FISH break-apart probes or RT-PCR for PDGFRB, FGFR1, JAK2, and other tyrosine kinase fusions
• Next-generation sequencing panel for myeloid mutations 1, 2

Critical distinction: Standard cytogenetics cannot detect FIP1L1-PDGFRA because it results from a cryptic interstitial deletion; FISH with CHIC2-specific probes is essential 1.

Flow Cytometry for Lymphocytic Variant

If lymphocytic-variant HES (L-HES) is suspected based on elevated IgE or clinical features 1:

• Flow cytometry (preferred over IHC) to identify aberrant T-cell populations
• Look for CD3⁻CD4⁺CD7⁻CD5⁺ phenotype (most common)
• Alternative phenotypes: CD3⁺CD4⁺CD7⁻ or CD3⁺CD4⁻CD8⁻
• Molecular analysis for clonal TCR gene rearrangements if flow cytometry equivocal
• STAT3 mutation testing (recently identified in L-HES) 1

Organ Damage Assessment

Cardiac evaluation is the highest priority due to risk of life-threatening endomyocardial fibrosis and thromboembolism 4, 2:

• Electrocardiogram
• Troponin and NT-proBNP
• Echocardiogram (all patients)
• Cardiac MRI if troponin elevated or clinical signs of cardiac involvement 1

Other organ-specific testing based on symptoms:

• Pulmonary: Chest X-ray, pulmonary function tests, bronchoscopy with BAL if respiratory symptoms
• Gastrointestinal: Endoscopy with mucosal biopsy (IHC for CD25, CD117, tryptase)
• Neurologic: Electromyography and nerve biopsy for peripheral neuropathy
• Dermatologic: Deep skin biopsy including fascia; MRI for eosinophilic fasciitis
• ENT: Evaluation for sinusitis, nasal polyposis, sensorineural hearing loss 1, 3

Classification Framework

After excluding secondary causes (parasites, drugs, allergic diseases, malignancies), classify into 2:

1. Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase fusions (MLN-eo-TK) - PDGFRA, PDGFRB, FGFR1, JAK2, others
2. Chronic eosinophilic leukemia (CEL) - clonal eosinophilia with blast excess or myeloid mutations
3. Lymphocytic-variant HES - aberrant T-cell clone driving reactive eosinophilia
4. Idiopathic HES - diagnosis of exclusion after comprehensive work-up

Treatment Approach

PDGFRA/PDGFRB-Rearranged Disease

Imatinib is first-line therapy with extraordinary efficacy 5, 2:

• Start imatinib 100-400 mg daily (most patients respond to 100 mg)
• Response typically occurs within days to weeks
• Achieve complete hematologic and molecular remission in >95% of cases
• Continue indefinitely; relapses common upon discontinuation
• Monitor with RT-qPCR for molecular response 1, 2

Critical pitfall: Always test for FIP1L1-PDGFRA before starting corticosteroids, as steroids can mask the diagnosis and delay appropriate imatinib therapy.

FGFR1-Rearranged Disease

• Pemigatinib (FGFR inhibitor) recently approved for relapsed/refractory FGFR1-rearranged neoplasms
• Consider allogeneic hematopoietic stem cell transplantation for eligible patients (only curative option) 2

Non-Clonal HES (Lymphocytic-Variant and Idiopathic)

Corticosteroids remain first-line therapy 4, 5, 6, 2:

• Prednisone 1 mg/kg/day (or equivalent) for acute/severe disease
• Taper to lowest effective dose once control achieved
• Many patients require long-term low-dose maintenance

For steroid-refractory or steroid-dependent disease:

• Mepolizumab (anti-IL-5 antibody) is FDA-approved for idiopathic HES 2

  • 300 mg subcutaneously every 4 weeks
  • Reduces eosinophil counts and prevents flares
  • Steroid-sparing effect demonstrated in clinical trials

• Benralizumab (anti-IL-5 receptor antibody) under investigation, may offer more complete eosinophil depletion 6, 2

• Hydroxyurea 500-2000 mg daily as steroid-sparing agent 5, 2

• Interferon-α (pegylated formulations preferred) for steroid-refractory cases 5, 2

Aggressive/Blast-Phase Disease

• Cytotoxic chemotherapy (acute leukemia-type regimens)
• Allogeneic hematopoietic stem cell transplantation for eligible patients 2

Risk Stratification and Monitoring

Watch-and-wait approach acceptable only for:

• Eosinophil count <1.5 × 10⁹/L
• No symptoms or signs of organ involvement
• Close follow-up mandatory 2

All other patients require treatment given the risk of irreversible organ damage, particularly cardiac fibrosis and thromboembolism 4, 7.

Common Pitfalls

1. Failing to test for FIP1L1-PDGFRA before starting steroids - this is the single most important test as it directs curative imatinib therapy
2. Relying on standard cytogenetics alone - will miss cryptic PDGFRA rearrangements
3. Inadequate cardiac evaluation - cardiac involvement is a major cause of morbidity/mortality
4. Misinterpreting myeloid mutations as evidence of primary disease - must distinguish from clonal hematopoiesis of indeterminate potential (CHIP), which occurs in 20-25% of idiopathic HES patients but may not indicate true myeloid neoplasm 8
5. Premature diagnosis of idiopathic HES - this is a diagnosis of exclusion requiring comprehensive evaluation for secondary causes 3, 4