How Biomarkers Pick a Biologic for Severe Asthma

For severe asthma, the biologic is chosen by the biomarker, not the other way around. Three inexpensive measurements, a blood eosinophil count, fractional exhaled nitric oxide (FeNO), and immunoglobulin E (IgE), together read out how much of your airway inflammation runs through the "type 2" immune pathway that every approved asthma biologic was designed to block. Once that pattern is clear, a field of four drug mechanisms usually collapses to the one most likely to help you. Even when the numbers stay low, one newer target sits high enough in the cascade to still make a difference.

Key points#

• Match the mechanism to the marker: high eosinophils favor anti-IL-5, high FeNO favors anti-IL-4/13, allergic sensitization favors anti-IgE, and persistently low markers point toward anti-TSLP.
• Read the three tests as a set, because eosinophils and FeNO report on different arms of the same type 2 response.
• Steroids suppress these markers, so the 2024 GINA guidance recommends measuring on treatment and repeating, up to three times, before labeling asthma non-type-2.
• Many patients qualify for more than one biologic, and the final call weighs coexisting conditions, oral-steroid dependence, dosing, and patient preference.

The panel behind the decision#

Before any biologic is named, a clinician is really asking one question: is this airway inflammation type 2 (T2) or not? T2 inflammation is an orchestrated immune program driven by the cytokines interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-13 (IL-13). The 2024 Global Initiative for Asthma (GINA) severe-asthma guidance makes this the gateway step, because every biologic on the market today works somewhere inside that program.

GINA treats airway inflammation as likely T2-high when, on high-dose inhaled corticosteroids, a patient shows any of the following: 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 symptoms clearly driven by allergens. Because corticosteroids push all of these numbers down and the values swing over time, GINA advises documenting the steroid dose at the time of testing and repeating the panel, up to three times, before concluding that asthma is non-T2.

Three markers, three windows into the airway#

Each of the three tests looks at a different part of the T2 response, which is why they are interpreted together rather than swapped for one another.

Blood eosinophils: the workhorse number#

The eosinophil count is the most decision-ready value on the panel. A review in Allergy, Asthma and Immunology Research describes a count at or above 300 cells/µL as a practical signal of eosinophilic inflammation and the best available predictor of response to drugs aimed at IL-5. GINA uses 150 cells/µL as the threshold for suspecting T2 disease, with higher counts reflecting a stronger eosinophilic signal. The same review adds an honest caveat: blood eosinophils track what is happening in the airway only loosely.

FeNO: a breath test for IL-13 activity#

FeNO rises when IL-13 acts on the airway lining, so it reports on a separate arm of the T2 response than the eosinophil count does. It can be high even when eosinophils are only moderate, which is exactly why the two are read side by side. Higher FeNO has been tied to steroid responsiveness and to a greater risk of future flares.

IgE: the allergic signature#

Total IgE and allergen-specific IgE together mark 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 year-round aeroallergen, is the selection biomarker for anti-IgE therapy. Total IgE by itself is not enough; the sensitization pattern and your body weight also determine eligibility.

From pattern to mechanism#

Read together, the panel usually favors one of four drug families.

Allergic pattern, anti-IgE. Omalizumab binds circulating IgE and fits allergic severe asthma with documented sensitization and a total IgE inside the approved range. Here it is the IgE level and the allergy testing, not the eosinophil count, that carry the decision.

High eosinophils, anti-IL-5. Mepolizumab and reslizumab neutralize IL-5, while benralizumab targets the IL-5 receptor to strip out 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 suit the high-eosinophil, flare-prone patient.

High FeNO with moderate eosinophils, anti-IL-4/13. Dupilumab blocks the shared IL-4 receptor alpha subunit and so dampens both IL-4 and IL-13 signaling, lowering FeNO along with 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 next to a moderate eosinophil count is the pattern that points here.

Low or ambiguous markers, 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 hinge on a high baseline number. The FDA approved it in December 2021 as the first asthma biologic with no phenotype or biomarker restriction in its label, and its pivotal trial cut flares across the full range of eosinophil counts, including patients below the usual T2 thresholds.

When the answer is not clean#

In practice the numbers rarely point to a single drug. Many patients meet the criteria for more than one biologic, and GINA settles the remaining choice on factors beyond the panel: flare history, oral-steroid dependence, coexisting conditions such as nasal polyps or eczema that a particular agent may also treat, dosing interval, and your own preference, all worked through with the treating clinician. If the first choice does not deliver after an adequate trial, GINA supports switching to another agent you qualify for.

Genuinely low biomarkers, confirmed on repeat testing, are both the clearest case for the upstream anti-TSLP mechanism and a cue to step back and ask whether severe asthma is even the right diagnosis.