Eosinophilic Asthma: Understanding and Treating the Most Common Form of Severe Asthma

What Is Eosinophilic Asthma?

Eosinophils are a type of white blood cell that plays a role in the immune system's response to parasites and allergens. In healthy individuals, eosinophils circulate in low numbers. In eosinophilic asthma, however, these cells are produced in excess and accumulate in the airways, where they drive chronic inflammation, mucus overproduction, and airway remodeling — the gradual structural changes that make the airways increasingly narrow and reactive over time.

Eosinophilic asthma falls under the broader category of Type 2-High (T2) inflammation, a term that describes immune responses driven by specific cytokines — signaling molecules including interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-13 (IL-13). IL-5 is particularly important because it is the primary driver of eosinophil production, survival, and activation. This is why therapies targeting IL-5 have proven so effective for this patient population.

An estimated 50 to 70 percent of patients with severe asthma have an eosinophilic phenotype, making it the single most common driver of difficult-to-control asthma. Understanding whether your asthma is eosinophilic changes your treatment trajectory entirely.

How Is Eosinophilic Asthma Diagnosed?

Eosinophilic asthma is not diagnosed by symptoms alone — it requires specific biomarker testing. At Advanced Asthma Clinic in Plantation, FL, Dr. Frank Hull uses a comprehensive diagnostic approach that includes:

• Complete blood count (CBC) with differential: The most accessible test. A blood eosinophil count of 150 cells per microliter (cells/μL) or higher suggests eosinophilic involvement. Counts above 300 cells/μL are strongly associated with eosinophilic asthma and typically meet the eligibility thresholds for biologic therapies.
• Fractional Exhaled Nitric Oxide (FeNO) testing: FeNO measures nitric oxide in exhaled breath, which correlates with eosinophilic airway inflammation. A reading of 25 parts per billion (ppb) or higher in adults is considered elevated. FeNO testing is non-invasive and provides results in minutes.
• Total serum IgE: While IgE is more closely associated with allergic asthma, elevated IgE in combination with high eosinophils helps characterize the full Type 2 inflammatory profile and guides biologic selection.
• Spirometry: Lung function testing establishes your baseline airflow and degree of obstruction. Many eosinophilic asthma patients show reduced FEV1 (forced expiratory volume in one second) that improves partially with bronchodilator use.
• Sputum analysis (when indicated): Direct measurement of eosinophils in sputum provides the most specific evidence of airway eosinophilia, though this test is not required for most treatment decisions in clinical practice.

Importantly, eosinophil counts can fluctuate — they may be temporarily suppressed by oral corticosteroids or vary with season and allergen exposure. Dr. Hull may order repeat testing or interpret results in the context of your full clinical picture. A single blood draw is never the whole story.

Signs Your Asthma May Be Eosinophilic

While definitive diagnosis requires biomarker testing, certain clinical patterns strongly suggest eosinophilic asthma:

• Asthma symptoms that persist despite regular use of high-dose inhaled corticosteroids and a second controller medication
• Frequent exacerbations (two or more per year) requiring oral steroids, ER visits, or hospitalization
• Dramatic improvement in symptoms when taking oral prednisone, followed by relapse when the course ends — this "steroid-responsive" pattern is a hallmark of eosinophilic disease
• Nasal polyps or chronic rhinosinusitis — these conditions share the same Type 2 inflammatory pathway and co-occur with eosinophilic asthma in a significant proportion of patients
• Adult-onset asthma (developing asthma after age 25) — eosinophilic asthma commonly presents in adulthood rather than childhood
• Aspirin sensitivity or aspirin-exacerbated respiratory disease (AERD), also known as Samter's triad

If you recognize several of these patterns, consult your physician about eosinophilic asthma testing. Early identification leads to earlier access to targeted therapies.

Biologic Therapies for Eosinophilic Asthma

The advent of biologic therapies has transformed the outlook for patients with eosinophilic asthma. Unlike broad-spectrum anti-inflammatory medications, biologics are precision medicines — each one targets a specific molecule in the inflammatory pathway driving eosinophil production and activity.

Anti-IL-5 and Anti-IL-5 Receptor Therapies

Because IL-5 is the primary cytokine responsible for eosinophil maturation, survival, and release from bone marrow, blocking it is highly effective for eosinophilic asthma:

• Mepolizumab (Nucala): A subcutaneous injection administered every four weeks. In clinical trials, mepolizumab reduced exacerbations by approximately 50% in patients with severe eosinophilic asthma and significantly reduced oral steroid dependence.
• Benralizumab (Fasenra): Targets the IL-5 receptor alpha on eosinophils, leading to rapid and near-complete eosinophil depletion. Administered by subcutaneous injection every four weeks for the first three doses, then every eight weeks. Studies have shown exacerbation reduction of up to 51% and a 75% reduction in oral steroid use.
• Reslizumab (Cinqair): An intravenous infusion administered every four weeks. Particularly studied in patients with blood eosinophils of 400 cells/μL or higher, with demonstrated improvements in lung function and exacerbation rates.

Anti-IL-4/IL-13 Therapy

• Dupilumab (Dupixent): Blocks both IL-4 and IL-13 signaling by targeting the IL-4 receptor alpha subunit. Effective for patients with eosinophilic asthma, particularly those with concurrent atopic dermatitis (eczema) or nasal polyps. Also FDA-approved for these comorbid conditions, making it an efficient choice for patients with overlapping Type 2 diseases.

Anti-TSLP Therapy

• Tezepelumab (Tezspire): Targets thymic stromal lymphopoietin (TSLP), a cytokine that sits upstream of the entire Type 2 inflammatory cascade. By blocking TSLP, tezepelumab reduces eosinophils, IgE, and FeNO simultaneously. It is the first biologic approved for severe asthma regardless of inflammatory phenotype, though it is particularly effective in eosinophilic disease.

Selecting the right biologic depends on your specific biomarker profile, comorbidities, and treatment history. This is a decision Dr. Hull makes collaboratively with each patient after a thorough evaluation. For a complete comparison, visit our Biologic Therapies page.

Beyond Biologics: Comprehensive Eosinophilic Asthma Management

Biologic therapy is transformative, but optimal management of eosinophilic asthma also requires attention to:

• Trigger identification and avoidance: Environmental allergens (dust mites, mold, pollen) and occupational exposures can sustain eosinophilic inflammation even with biologic therapy. Allergy testing and targeted environmental modifications are part of a complete treatment plan.
• Comorbidity management: Nasal polyps, chronic rhinosinusitis, GERD, and obstructive sleep apnea frequently co-occur with eosinophilic asthma and can undermine asthma control if untreated.
• Steroid reduction protocols: Once a biologic is established and disease control is achieved, Dr. Hull works with patients to systematically taper oral corticosteroids. This must be done gradually and under medical supervision — abrupt discontinuation carries risks.
• Asthma action plan development: Even patients on biologics benefit from a written action plan that outlines daily management, early warning signs, and emergency steps.
• Ongoing monitoring: Regular follow-up with repeat eosinophil counts, FeNO, and spirometry ensures that your biologic is maintaining control and allows for treatment adjustments if needed.

Eosinophilic Asthma and Clinical Research

The biologics available today are the result of clinical trials, and the next generation of treatments is already in development. Through our research affiliate Lung Research Florida, patients with eosinophilic asthma may have the opportunity to access investigational therapies before they reach the market.

Dr. Frank Hull has over 20 years of experience in pulmonary research and has been involved in numerous clinical trials investigating novel asthma therapies. Participation in a clinical trial is always voluntary, conducted under strict regulatory oversight, and provided at no cost to the participant. Learn more about our clinical trial opportunities.

Eosinophilic Asthma in South Florida

Patients in Plantation, Broward County, and the greater South Florida area face environmental conditions that can intensify eosinophilic asthma. Year-round warmth and humidity promote indoor mold growth, dust mite proliferation, and extended pollen seasons — all of which drive Type 2 inflammation and eosinophil activation. Understanding how the South Florida environment interacts with your specific asthma phenotype is a key part of the care we provide at Advanced Asthma Clinic.

Frequently Asked Questions

What eosinophil count indicates eosinophilic asthma?

A blood eosinophil count of 150 cells per microliter or higher is generally considered suggestive of eosinophilic inflammation. Counts above 300 cells per microliter are strongly associated with eosinophilic asthma and typically meet eligibility thresholds for biologic therapies. However, eosinophil counts can fluctuate — particularly during oral steroid use — so repeat testing and clinical context are essential. Consult your physician for interpretation of your specific results.

Can eosinophilic asthma be cured?

Eosinophilic asthma cannot currently be cured, but it can be effectively managed. Modern biologic therapies have enabled many patients to achieve clinical remission — meaning minimal symptoms, no exacerbations, and elimination of oral steroids. Consult your physician to discuss whether biologic therapy may be appropriate for your situation.

What is the difference between eosinophilic asthma and allergic asthma?

Allergic asthma is triggered by specific allergens (dust mites, pollen, pet dander) and is mediated by IgE antibodies. Eosinophilic asthma is defined by elevated eosinophil levels in the blood and airways. These two categories overlap significantly — many patients have both allergic and eosinophilic features — but some patients have eosinophilic asthma without identifiable allergic triggers. The distinction matters because it influences which biologic therapy is most appropriate.

Do I need to see a specialist for eosinophilic asthma?

Yes. Eosinophilic asthma requires biomarker-guided treatment decisions that go beyond standard primary care asthma management. A pulmonologist or asthma specialist with experience in biologic therapies can provide the diagnostic evaluation and ongoing monitoring that eosinophilic asthma demands. You do not always need a referral — you can contact our clinic directly at 954-522-7226.

Is eosinophilic asthma the same as severe asthma?

Not exactly. Eosinophilic asthma describes the type of inflammation driving your asthma. Severe asthma describes the degree of disease burden. Most severe asthma is eosinophilic, but mild or moderate asthma can also have an eosinophilic component. Identifying eosinophilic inflammation at any severity level helps guide treatment decisions. Learn more about the different types of asthma.