How Biomarkers Pick a Biologic for Severe Asthma

Choosing a biologic for severe asthma starts with three blood and breath tests, not with the drug's brand. Blood eosinophils, fractional exhaled nitric oxide (FeNO), and immunoglobulin E (IgE) together describe whether a person's airway inflammation is "type 2," the immune pattern every currently approved asthma biologic was built to interrupt. A high eosinophil count points toward the interleukin-5 pathway, an elevated FeNO reflects interleukin-13 activity, and allergen-specific IgE marks allergic disease, so the pattern across all three narrows a field of four mechanisms to the one most likely to work. When the biomarkers stay stubbornly low, one newer target sits far enough upstream to help anyway.

What "type 2 inflammation" means

Type 2 (T2) inflammation is a coordinated immune response driven by cytokines including interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-13 (IL-13). The 2024 Global Initiative for Asthma (GINA) severe asthma guidance treats it as the gateway question in severe asthma, because the biologics that exist today all work inside this pathway. GINA considers airway inflammation likely T2-high when, on high-dose inhaled corticosteroids, a patient shows a blood eosinophil count of at least 150 cells/µL, a FeNO of at least 20 parts per billion (ppb), sputum eosinophils of at least 2 percent, or clinically allergen-driven symptoms. Because steroids suppress these markers and the numbers fluctuate, GINA advises measuring while the treatment is documented and repeating the tests, up to three times, before calling asthma non-T2.

Reading the three biomarkers

Blood eosinophils

The eosinophil count is the most actionable number. A review in Allergy, Asthma and Immunology Research describes a count at or above 300 cells/µL as a practical marker of eosinophilic inflammation and the best available predictor of response to drugs that target IL-5. GINA uses at least 150 cells/µL as the threshold for suspecting T2 disease and higher counts as evidence of a stronger eosinophilic signal. The caveat, stressed in the same review, is that blood eosinophils only loosely mirror what is happening in the airway itself.

FeNO

FeNO is a breath test that rises under IL-13 stimulation of the airway lining, so it reports on a different arm of the T2 response than eosinophils do. It can be elevated when the eosinophil count is only moderate, which is why the two are read together rather than as substitutes. Higher FeNO has been linked to steroid responsiveness and to future exacerbation risk.

Total and allergen-specific IgE

IgE identifies the allergic phenotype. The AAIR review notes that a high total IgE (broadly in the 30 to 1,500 IU/mL range) combined with a positive skin or specific-IgE test to a perennial aeroallergen is the selection biomarker for anti-IgE therapy. Total IgE alone is not enough; the sensitization pattern and the patient's body weight also factor into eligibility.

Matching the pattern to a mechanism

Anti-IgE

Omalizumab binds circulating IgE and is chosen for allergic severe asthma with documented sensitization and a total IgE within the approved range. Here IgE and allergy testing, not the eosinophil count, drive the decision.

Anti-IL-5 and anti-IL-5 receptor

Mepolizumab and reslizumab neutralize IL-5, while benralizumab targets the IL-5 receptor to deplete eosinophils directly. The AAIR review reports that a blood eosinophil count at or above 300 cells/µL, rather than FeNO, best predicts who responds. These agents are the natural choice for the high-eosinophil, exacerbation-prone patient.

Anti-IL-4/13

Dupilumab blocks the shared IL-4 receptor alpha subunit and therefore dampens both IL-4 and IL-13 signaling, which lowers FeNO as well as eosinophil-driven inflammation. GINA frames eligibility around a blood eosinophil count of at least 150 cells/µL or a FeNO of at least 25 ppb, and dupilumab is also used in oral-corticosteroid-dependent asthma. A high FeNO alongside a moderate eosinophil count is a pattern that points here.

Anti-TSLP

Tezepelumab blocks thymic stromal lymphopoietin, an epithelial "alarmin" that sits upstream of IL-4, IL-5, and IL-13. Because it acts before the pathway branches, its benefit does not depend on a high baseline biomarker. The FDA approved it in December 2021 as the first asthma biologic without a phenotype or biomarker restriction in its label, and its pivotal trial showed reduced exacerbations across the full range of eosinophil counts, including patients below the usual T2 thresholds.

When the biomarkers overlap or stay low

Many patients qualify for more than one biologic, and the numbers rarely point to a single answer. GINA frames the remaining choice around exacerbation history, oral steroid use, coexisting conditions such as nasal polyps or eczema that a given drug may also treat, dosing interval, and patient preference, worked through together with the treating clinician. If the first choice does not help after an adequate trial, GINA supports switching to another agent for which the person qualifies. Genuinely low biomarkers, confirmed on repeat testing, are the setting where the upstream anti-TSLP mechanism is most relevant, and also a prompt to reconsider whether severe asthma is even the right diagnosis.

This article is educational and is not medical advice; decisions about asthma biologics belong with a qualified clinician who can weigh the full clinical picture.