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Airways disease is no longer managed as one condition. A blood eosinophil count, a total IgE and a FeNO reading now determine which inhaled therapy helps, which patient is eligible for a biologic, and which “severe asthma” is actually something else. These are inexpensive, widely available tests that are still under-used — and frequently over-interpreted.
Why this matters more than it used to
Two shifts have moved these tests from allergy clinics into everyday respiratory practice. In asthma, the biologics are targeted at specific inflammatory pathways, and eligibility is defined by biomarkers rather than by symptoms.1,2 In COPD, the blood eosinophil count now sits inside the treatment algorithm itself.3
Both mean that a patient labelled “difficult asthma” without a documented eosinophil count and IgE has not been fully assessed.
Blood eosinophil count
An absolute count from a routine full blood count — no special test required.
| Setting | Threshold | What it indicates |
|---|---|---|
| COPD | <100 cells/µL | Little benefit expected from inhaled corticosteroids3 |
| COPD | ≥300 cells/µL | Favours adding an inhaled corticosteroid in a patient with exacerbations3 |
| Asthma | ≥150–300 cells/µL | Eosinophilic phenotype; supports anti-IL-5/5R and anti-IL-4Rα eligibility, thresholds varying by agent and by PBS criteria1,2 |
| Any | >1500 cells/µL, persistent | Investigate beyond asthma — EGPA, parasitic infection, drug reaction, haematological cause |
Counts also vary day to day and with diurnal rhythm, so serial values are more reliable than one.
Total IgE
Total serum IgE has two distinct uses, and they are often confused.
- Omalizumab dosing. Anti-IgE therapy is dosed from total IgE and body weight, and eligibility sits within a defined range — broadly 30–1500 IU/mL depending on the schedule and weight band.1,2 A total IgE outside that range does not exclude allergic asthma; it excludes that particular drug.
- ABPA. A markedly raised total IgE is central to the diagnosis of allergic bronchopulmonary aspergillosis, and it is used to monitor response — a falling IgE on treatment is the expected trajectory.4,5
A normal total IgE does not exclude allergy, and a raised total IgE on its own proves nothing: it rises with parasitic infection, smoking, eczema and a number of immunodeficiencies.
Specific IgE and skin prick testing
These identify sensitisation to a particular allergen — house dust mite, grass and tree pollens, cat, dog, moulds, cockroach. Skin prick testing is fast, cheap and read in 15 minutes; specific IgE blood testing (formerly “RAST”) is used when antihistamines cannot be withheld, when skin disease prevents testing, or where anaphylaxis risk makes skin testing unwise.
Two specific tests earn their place in respiratory practice regardless of the general panel: Aspergillus fumigatus specific IgE and Aspergillus IgG. Both are required for the diagnosis of ABPA, which should be actively excluded in poorly controlled asthma, in bronchiectasis with recurrent mucus plugging, and in cystic fibrosis.4,5 See allergic bronchopulmonary aspergillosis.
FeNO — exhaled nitric oxide
FeNO is a point-of-care marker of eosinophilic airway inflammation, measured in parts per billion on a simple exhaled manoeuvre. It is now part of the diagnostic pathway for asthma in the UK joint guideline6 and supports biologic selection alongside eosinophils.1
| Adult value | Interpretation7 |
|---|---|
| <25 ppb | Eosinophilic inflammation and corticosteroid responsiveness unlikely |
| 25–50 ppb | Intermediate — interpret with the clinical picture |
| >50 ppb | Eosinophilic inflammation likely; supports corticosteroid responsiveness |
What lowers FeNO: inhaled or oral corticosteroids (which is the point — a high FeNO in a treated patient suggests non-adherence or poor inhaler technique before it suggests treatment failure), smoking, and acute bronchoconstriction. What raises it: allergic rhinitis, atopy and recent allergen exposure. Values in children are lower.
Putting it together
| Picture | Suggests |
|---|---|
| High eosinophils, high FeNO, high IgE with positive aeroallergen tests | Allergic eosinophilic asthma — several biologic options; confirm adherence first |
| High eosinophils, high FeNO, normal IgE, adult onset, nasal polyps | Non-allergic eosinophilic asthma — anti-IL-5/5R territory |
| Very high IgE, Aspergillus IgE and IgG positive, central bronchiectasis, plugging | ABPA until proven otherwise4,5 |
| Normal eosinophils, normal FeNO, poor response to inhaled therapy | Reconsider the diagnosis — breathing pattern disorder, EILO, reflux, cardiac cause, deconditioning |
That final row is the one physiotherapy sees most. A patient with normal type 2 markers and no bronchodilator response is unlikely to improve on more inhaled therapy, and is exactly the patient in whom breathing pattern disorder and exercise-induced laryngeal obstruction are missed.
What this means for physiotherapy
- A documented eosinophil count and IgE belong in the notes of every difficult-airways patient you assess.
- A high FeNO with a poor inhaler technique is a technique problem — and correcting it is a physiotherapy intervention with a measurable biomarker outcome.
- Normal type 2 markers reframe the case: look for the non-inflammatory causes of breathlessness that respond to what we actually do.
- Never advise allergen avoidance on the basis of a positive test alone.
References & evidence base
- Global Initiative for Asthma. Global strategy for asthma management and prevention: 2026 update. GINA; 2026. Available at: ginasthma.org
- Australian Government Department of Health and Aged Care. Pharmaceutical Benefits Scheme: Section 100 Highly Specialised Drugs — severe asthma biologics. PBS Schedule; 2026.
- Global Initiative for Chronic Obstructive Lung Disease. Global strategy for the diagnosis, management and prevention of COPD: 2026 report. GOLD; 2026. Available at: goldcopd.org
- Agarwal R, Chakrabarti A, Shah A, et al. Allergic bronchopulmonary aspergillosis: review of literature and proposal of new diagnostic and classification criteria. Clin Exp Allergy. 2013;43(8):850–873.
- Agarwal R, Sehgal IS, Muthu V, et al. Revised ISHAM-ABPA working group clinical practice guidelines for diagnosing, classifying and treating allergic bronchopulmonary aspergillosis. Eur Respir J. 2024;63(4):2400061.
- National Institute for Health and Care Excellence, British Thoracic Society, Scottish Intercollegiate Guidelines Network. Asthma: diagnosis, monitoring and chronic asthma management. NICE guideline NG245. London: NICE; 2024.
- Dweik RA, Boggs PB, Erzurum SC, et al. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FeNO) for clinical applications. Am J Respir Crit Care Med. 2011;184(5):602–615.
References are numbered in citation order (Vancouver/BMJ style) and were current at the time of writing. Guidelines are living documents — verify against the latest version before clinical use.
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