Blood Eosinophil Count and Asthma: Testing, Thresholds, and Biologic Eligibility

Reviewed by Dr. Frank Hull, MD — Board-certified pulmonologist, 20+ years pulmonary research — Advanced Asthma Clinic, Plantation, FL — Updated June 2026

A single routine blood test — the complete blood count with differential — yields one of the most clinically important numbers in modern severe asthma management: the absolute blood eosinophil count. This figure determines whether a patient has an eosinophilic asthma phenotype, guides selection among FDA-approved biologic therapies, and predicts likelihood of response to corticosteroids. Understanding your eosinophil count is increasingly essential for patients in Plantation, Fort Lauderdale, and Broward County who are being evaluated for advanced asthma treatment.

This guide explains what blood eosinophils are, how the test works, how to interpret results, and how eosinophil levels map to specific biologic therapies. For the companion biomarker, see our FeNO testing guide. For a full side-by-side comparison of all five biologic agents, see our asthma biologic comparison page. Always consult your physician before making any treatment decisions.

What Are Eosinophils?

Eosinophils are white blood cells that develop in the bone marrow under the influence of interleukin-5 (IL-5), a cytokine produced primarily by T helper 2 (Th2) cells. They are a key effector cell of type 2 immunity — the immune pathway that underlies allergic disease, eosinophilic asthma, atopic dermatitis, and related conditions.

In the lungs, eosinophils cause direct airway damage through release of toxic granule proteins: major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase, and eosinophil-derived neurotoxin. These proteins damage the airway epithelium, trigger bronchospasm, promote mucus secretion, and drive airway remodeling over time. Elevated airway and blood eosinophils are associated with:

• Higher exacerbation frequency and severity
• Greater corticosteroid responsiveness
• Eligibility for multiple biologic agents
• Worse lung function decline if untreated

Blood eosinophil count is not a perfect surrogate for airway eosinophilia, but it is the most practical and cost-effective biomarker available. Studies show a meaningful correlation between peripheral blood eosinophils and bronchial biopsy eosinophil counts, particularly at higher blood levels.

How the Blood Eosinophil Count Is Measured

Blood eosinophil count is measured as part of a complete blood count (CBC) with differential (CPT code 85025). This is a standard laboratory test requiring only a routine blood draw from a vein in the arm. No fasting is required. Results are typically available within 24 hours from most clinical laboratories.

What the Report Shows

The laboratory report provides two eosinophil values:

• Absolute eosinophil count (AEC): The number of eosinophils per microliter of blood (cells/μL). This is the primary number used in biologic eligibility criteria.
• Eosinophil percentage: Eosinophils as a percentage of the total white blood cell count. Less useful for biologic decisions than the absolute count, because it is affected by overall WBC fluctuation.

For asthma biologic eligibility, always reference the absolute eosinophil count, not the percentage.

When to Draw the Sample

Blood eosinophil levels show diurnal variation: counts are typically lowest in the morning (6–8 AM) and highest in the afternoon (4–8 PM). For consistent serial monitoring, some clinicians prefer morning draws. More important, however, is timing relative to recent corticosteroid use — addressed in detail below.

Reference Ranges and Clinical Thresholds

The standard laboratory reference range for blood eosinophils in adults is approximately 100–500 cells/μL, representing 1–5% of the white blood cell differential. However, for asthma management, clinicians use clinical thresholds that differ from these population reference ranges.

These thresholds are guidelines for clinical decision-making. Individual patient factors — symptom burden, exacerbation history, prior steroid use, and comorbidities — are always considered alongside the absolute number.

Eosinophilic Asthma: The Phenotype Defined by Eosinophils

Eosinophilic asthma is the most common severe asthma phenotype, accounting for approximately 50–60% of severe asthma cases. It is defined by persistent airway and/or blood eosinophilia despite adequate inhaled corticosteroid therapy. Key characteristics include:

• Blood eosinophils ≥150–300 cells/μL on multiple measurements
• Elevated FeNO (≥25 ppb), indicating active IL-4/IL-13 driven airway inflammation
• Frequent exacerbations, often triggered by respiratory infections or allergen exposure
• Sputum eosinophilia ≥3% on induced sputum (when available)
• Often accompanied by allergic rhinitis, nasal polyps, or atopic dermatitis

Patients with eosinophilic asthma are the most likely to respond to biologic therapy targeting the IL-5 pathway (mepolizumab, benralizumab) or the type 2 cytokine axis (dupilumab, tezepelumab). Eosinophilic asthma is also associated with aspirin-exacerbated respiratory disease (AERD) and eosinophilic granulomatosis with polyangiitis (EGPA) — conditions that require careful evaluation before initiating biologic therapy.

Distinguishing Eosinophilic from Non-Eosinophilic Asthma

Not all severe asthma is eosinophilic. A substantial minority of patients have a non-eosinophilic or neutrophilic phenotype, driven by different inflammatory pathways. These patients typically show:

• Blood eosinophils persistently <150 cells/μL
• Low FeNO (<25 ppb)
• Obesity-associated asthma or occupational asthma presentation
• Relatively poor response to corticosteroids
• Smoking history or air pollution exposure as primary driver

For these patients, tezepelumab (which targets the upstream TSLP alarmin regardless of inflammatory phenotype) is currently the most evidence-supported biologic option. See our tezepelumab guide for full details.

Biologic Eligibility Thresholds by Agent

Each FDA-approved asthma biologic has distinct blood eosinophil criteria. These thresholds are drawn from pivotal clinical trial eligibility criteria and FDA label requirements.

For a full comparison including dosing schedules, mechanism, and all eligibility criteria, see the asthma biologic comparison hub.

Factors That Affect Blood Eosinophil Count

Several common clinical factors can artificially lower or raise eosinophil counts, making accurate interpretation essential.

Corticosteroids (the most important confounder)

Systemic corticosteroids are the most significant suppressor of blood eosinophil counts:

• Oral prednisone/prednisolone: Can reduce blood eosinophils by 80–90% within 6 hours of a single dose. A short burst (5–10 days) can suppress counts for 1–3 weeks after completion.
• IV methylprednisolone: Similarly rapid and profound suppression -- counts drawn during or immediately after an exacerbation treated with IV steroids will be falsely low.
• Inhaled corticosteroids (ICS): Modest suppressive effect on blood eosinophils; generally does not invalidate testing at standard doses.

Clinical implication: For biologic eligibility assessment, blood eosinophil testing should be timed at least 2–4 weeks after completion of any oral or IV corticosteroid course. If a patient has frequent exacerbations requiring repeated OCS bursts, prior-year eosinophil records (taken during stable periods) are critical for establishing the true baseline. This is why mepolizumab's label allows a prior-year count of ≥300 to substitute for the current measurement.

Other Factors

Monitoring Eosinophil Count During Biologic Therapy

After initiating biologic therapy, blood eosinophil counts are monitored to assess treatment response and guide ongoing management.

Expected Changes by Biologic

• Mepolizumab (Nucala): Reduces blood eosinophils by 75–85% from baseline within 4 weeks. Counts typically stabilize in the 50–150 cells/μL range during maintenance therapy.
• Benralizumab (Fasenra): More profound and rapid depletion via ADCC mechanism -- blood eosinophils typically fall to near-undetectable levels (<10 cells/μL) within 24 hours of the first dose. This is expected and does not indicate an adverse event.
• Dupilumab (Dupixent): A transient increase in blood eosinophils is commonly observed in the first 4–12 weeks of treatment (as eosinophils are displaced from tissue back into circulation). Counts typically normalize by 16–24 weeks. This early rise is generally not clinically significant and does not indicate treatment failure.
• Tezepelumab (Tezspire): Reduces blood eosinophils by approximately 70% from baseline; also reduces IgE and FeNO, reflecting broad upstream inflammation suppression.
• Omalizumab (Xolair): Primarily reduces IgE; modest secondary reduction in eosinophils mediated through downstream allergic cascade suppression.

Monitoring Schedule

At Advanced Asthma Clinic, we typically monitor blood eosinophils at:

• Baseline (before biologic initiation)
• 4–8 weeks after first dose (early response check)
• Every 3–6 months during stable maintenance therapy
• At any exacerbation (to assess inflammatory status)
• Before any consideration of switching or stopping biologic therapy

Blood Eosinophil Testing at Advanced Asthma Clinic, Plantation FL

Advanced Asthma Clinic serves patients from Plantation, Fort Lauderdale, Coral Springs, Davie, Weston, Miramar, and across Broward County. Our on-site capabilities include same-visit blood draw for CBC with differential, FeNO testing (CPT 95012), spirometry, and allergy panel testing — meaning the complete biomarker profile needed for biologic evaluation can typically be obtained in a single appointment.

Dr. Frank Hull's 20+ years of pulmonary research experience includes direct involvement in clinical trials for anti-eosinophil therapies. Our team interprets eosinophil results in the context of your complete clinical picture: steroid exposure history, exacerbation pattern, comorbidities, and South Florida's unique year-round aeroallergen environment.

If you have been told your asthma is severe or difficult to control, a blood eosinophil count is one of the first steps toward determining whether a biologic is right for you. Contact our office to schedule an evaluation.

Frequently Asked Questions

What is a normal blood eosinophil count?

The general population reference range is 100–500 cells/μL, or roughly 1–5% of the white blood cell differential. In the context of asthma biologic eligibility, clinically relevant thresholds are: less than 150 (low/non-eosinophilic), 150–299 (borderline), 300–499 (elevated), and 500 or above (high eosinophilia). Always consult your physician to interpret results in the context of your individual clinical picture.

Does corticosteroid use affect eosinophil count?

Yes, significantly. Systemic corticosteroids (oral prednisone, IV methylprednisolone) suppress eosinophil counts by 80–90% within hours and can keep counts depressed for 2–4 weeks after a short course. Blood eosinophil testing for biologic eligibility should ideally be drawn when the patient is not on a recent oral steroid burst. Inhaled corticosteroids have a modest, generally non-invalidating suppressive effect. Your pulmonologist will account for recent steroid use when interpreting results.

What blood eosinophil count qualifies for biologic therapy?

Thresholds vary by biologic: mepolizumab requires ≥150 cells/μL at initiation or ≥300 in the prior year; benralizumab requires ≥300 at screening; dupilumab recommends ≥150 or FeNO ≥25 ppb. Tezepelumab has no minimum eosinophil threshold. Consult your physician regarding your specific eligibility.

How is the blood eosinophil count measured?

It is measured as part of a complete blood count (CBC) with differential (CPT 85025) — a standard blood draw requiring no fasting. Results report the absolute eosinophil count in cells/μL (the key number for biologic decisions) and the eosinophil percentage of the white cell differential.

Can eosinophil count fluctuate over time?

Yes. Blood eosinophils show diurnal variation (lower AM, higher PM), seasonal fluctuation with allergen exposure, and acute changes with infection, steroid use, or parasitic exposure. For biologic eligibility assessment, multiple measurements over time provide a more reliable picture than a single reading. Your pulmonologist reviews your eosinophil trajectory alongside clinical history.

My eosinophil count is normal but my asthma is severe. What are my options?

Low blood eosinophils do not mean biologics are off the table. Tezepelumab (Tezspire) is FDA-approved for severe uncontrolled asthma with no minimum eosinophil threshold. Dupilumab may also be appropriate if FeNO ≥25 ppb. Additionally, eosinophil suppression from frequent corticosteroid use may mask underlying eosinophilia — prior-year measurements are important. Consult your physician for a complete evaluation.