Eosinophils sit at the centre of asthma treatment decisions, and a study published at the end of August 2026 in ERJ Open Research has just made that link harder to ignore. Researchers followed more than 280,000 people with asthma or chronic obstructive pulmonary disease (COPD) in Belgium and found that heavy use of quick-relief inhalers went hand in hand with more cardiovascular disease and more deaths. The message from the authors was not that the blue inhaler is dangerous in itself. It is that reaching for it often is a signal that the underlying inflammation is not controlled. And the single lab value that most often decides what to do next is a white blood cell you already have on your report. In this article you will learn what the study showed, how eosinophils are read, and which blood tests are worth discussing.
What the new study reported
The Belgian team looked at real-world pharmacy and health records rather than a trial. They separated two families of quick-relief inhalers: short-acting beta-agonists, the familiar blue rescue inhalers, and short-acting muscarinic antagonists, used more often in COPD. They then adjusted for age, sex, smoking, disease severity, income and other illnesses.
Nearly one in five people with asthma and roughly one in three people with COPD were using more of these inhalers than guidelines consider acceptable. Compared with people who used none, those using short-acting beta-agonists had about a 12% higher risk of dying with asthma and about 14% with COPD. The risk climbed further as the yearly number of inhalers rose, and it was linked to heart failure, heart attack and ischaemic stroke. People with no previous heart problems appeared especially exposed.
Two cautions matter. This is an observational study, so it shows an association rather than proof of cause. And frequent relief use is itself a marker of poorly controlled disease, which already raises cardiovascular risk on its own.
Why a rescue inhaler is a treatment marker
Quick-relief inhalers relax the muscle around the airways. They do nothing about the inflammation inside the airway wall. That is why international guidance stopped recommending rescue-only treatment for asthma: the symptoms are relieved whilst the disease keeps progressing.
The practical threshold clinicians use is three or more reliever canisters a year. Above that, the usual response is not a stronger reliever but a review of maintenance treatment, inhaler technique and adherence. Understanding the difference between the two treatment families helps, and readers often start with a plain-language guide to asthma symptoms, causes and treatment.
How eosinophils guide asthma treatment
Eosinophils are white blood cells involved in allergy and in parasite defence. In the airways they drive a pattern of inflammation that responds well to inhaled corticosteroids and, in severe cases, to injectable biologic drugs. Their number in blood is measured on the differential of a routine sample, so your report already carries the value. Anyone who wants the underlying reference ranges can read the dedicated page on eosinophils and their normal levels, and the wider panel is explained in the guide to the complete blood count.
| Blood eosinophil level | What clinicians usually take from it |
|---|---|
| Below 100 cells per microlitre | In COPD, little benefit expected from inhaled steroids; other options are weighed first |
| 150 cells per microlitre or more | In asthma, an allergy-type (type 2) inflammation is considered plausible |
| 300 cells per microlitre or more | Eosinophilic pattern; in severe disease, targeted injectable treatment may be discussed |
These figures are decision aids, not diagnoses. A single value can be pushed down by a recent course of steroid tablets or by an inhaled steroid already in use, which is why doctors look at several results over time rather than one snapshot.
Which blood tests are worth discussing
If you are emptying a reliever every few weeks, a review appointment is more useful than a new prescription. Alongside the eosinophil count, several other results often come up in that conversation.
- Potassium. Beta-agonists can push potassium down, which matters for heart rhythm. Doctors therefore check the potassium blood test.
- Magnesium. Low levels can accompany low potassium and are assessed through the magnesium blood test.
- Allergy markers. When triggers are unclear, allergists may order an allergy blood test.
- Inflammation. A general marker is described in the page covering the CRP blood test, and a markedly raised value is discussed in the guide to high CRP levels.
- Broader chemistry. Some readers compare the CBC and the CMP to see which panel covers what.
When to see a doctor
Book a review if you use three or more reliever inhalers in a year, if you need relief more than twice a week, if symptoms wake you at night, or if a reliever no longer works as long as it used to. Seek urgent care for breathlessness at rest, chest pain, blue lips or speech broken into single words. Persistent breathlessness on mild exertion also has non-respiratory causes, and doctors may then consider heart failure and its treatment. Never stop a maintenance inhaler on your own.
Latest scientific advances
Research over the past three years has steadily strengthened the case for using this one blood value to personalise treatment.
A 2025 systematic review pooling 27 studies found that people overusing rescue inhalers had roughly twice the rate of death and of severe flare-ups compared with those who did not. Pooling means combining many studies to get a steadier answer. What this means for you: the three-inhaler threshold is not an arbitrary number, it tracks real outcomes.
A 2025 analysis of UK primary care records added a nuance. Even among people who had not been having frequent flare-ups, quarters with frequent reliever prescriptions carried more acute heart events than quarters with none, and the effect grew with the number of inhalers. What this means for you: overuse is worth flagging even when your asthma feels “not that bad”.
On the treatment side, an individual-patient analysis presented in 2026 pooled thirteen randomised trials covering more than 21,000 people with COPD. The higher the eosinophil count, the more inhaled steroids reduced flare-ups, with a survival benefit appearing at the higher levels and essentially no protection against severe flare-ups at very low counts. What this means for you: the same inhaler is not equally useful for everyone, and your blood count helps predict who benefits.
A 2023 Chest analysis refined how to read the number, showing that a count measured whilst already on an inhaled steroid is less informative than one measured off treatment. And a 2023 randomised trial in UK general practice found that using a finger-prick eosinophil result to decide whether to give steroid tablets during a COPD flare was as safe as giving them to everyone, whilst cutting steroid exposure. What this means for you: the goal is fewer unnecessary steroids, not more.
All of this remains guidance for a clinician rather than a self-service rule, and none of it replaces a consultation.
Glossary
| Term | Definition |
|---|---|
| Eosinophil | A white blood cell involved in allergic reactions and in defence against parasites. Counted on a routine blood differential. |
| Short-acting beta-agonist (SABA) | A quick-relief inhaler that relaxes airway muscle within minutes. It does not treat inflammation. |
| Short-acting muscarinic antagonist (SAMA) | Another family of quick-relief inhalers, used more often in COPD. |
| Inhaled corticosteroid (ICS) | A daily maintenance inhaler that reduces airway inflammation over weeks. |
| Exacerbation | A flare-up of asthma or COPD needing extra treatment, often steroid tablets. |
| Type 2 inflammation | An allergy-driven inflammatory pattern often marked by higher eosinophils. |
| Biologic therapy | An injectable antibody treatment targeting one step of the inflammatory process in severe disease. |
| Observational study | Research that watches what happens in real life without assigning treatment, so it shows links rather than causes. |
| Microlitre (µL) | A very small volume of blood used as the reference unit for counting cells. |
Frequently asked questions
Does a high eosinophil count mean I have asthma?
No. Eosinophils rise in allergies, eczema, drug reactions, parasitic infections and several other conditions. In someone already diagnosed with asthma or COPD, the number helps characterise the type of inflammation and guide treatment. On its own it does not diagnose anything, and a doctor reads it alongside symptoms, lung function tests and history.
How many reliever inhalers a year is too many?
Guidelines commonly use three canisters a year as the point at which treatment should be reviewed, and the studies discussed above track outcomes from that level upwards. Needing relief more than twice a week is another practical trigger for a review. The number is a prompt for a conversation, not a verdict.
Should I stop using my reliever inhaler?
No. A reliever is there for symptoms and can be essential in a flare-up. What the research suggests is that needing it often should lead to a review of the maintenance treatment underneath, not to abandoning the reliever. Stopping any inhaler without medical advice can be dangerous.
Can medication change my eosinophil count?
Yes. Steroid tablets and, to a lesser degree, inhaled steroids lower the count, sometimes considerably. That is why one result taken during treatment is less informative than several results over time, and why doctors note what you were taking when the sample was drawn.
Is a low eosinophil count a problem?
A low count is usually of no concern in itself and can simply reflect steroid treatment or a recent infection. In COPD it mainly signals that inhaled steroids are less likely to help, which points the treatment choice elsewhere.
Which test measures eosinophils?
The blood differential, part of a full blood count, reports each type of white blood cell as a percentage and usually as an absolute number. The absolute number, expressed in cells per microlitre, is the one used for treatment thresholds.
Sources
- National Library of Medicine, MedlinePlus — Blood Differential, medical test overview, 2024. medlineplus.gov
- Centers for Disease Control and Prevention — Most Recent Asthma Data, national and state statistics, 2026. cdc.gov
- National Heart, Lung, and Blood Institute — Asthma: what asthma is, causes and treatment. nhlbi.nih.gov
- Van Vaerenbergh F. et al. — Cardiovascular effects and mortality of short-acting bronchodilators in asthma and COPD — ERJ Open Research, 2026. doi.org/10.1183/23120541.00462-2026
- Tsao C.-L. et al. — Adverse Outcomes Associated With Short-Acting Beta-Agonist Overuse in Asthma: A Systematic Review and Meta-Analysis — Allergy, 2025. consensus.app
- Pfeffer P. et al. — Association of Frequent Short-Acting Beta-Agonist Inhaler Prescriptions with Acute Cardiovascular Events — Pragmatic and Observational Research, 2025. consensus.app
- Janson C. et al. — High use of short-acting beta2-agonists in COPD is associated with an increased risk of exacerbations and mortality — ERJ Open Research, 2023. consensus.app
- Mathioudakis A. et al. — Blood Eosinophil Count Predicts Inhaled Corticosteroids Response in COPD: ICS-RECODE Individual Participant Data Meta-Analysis — American Journal of Respiratory and Critical Care Medicine, 2026. consensus.app
- Mathioudakis A. et al. — Rethinking Blood Eosinophils for Assessing Inhaled Corticosteroids Response in COPD — Chest, 2023. consensus.app
- Ramakrishnan S. et al. — Blood eosinophil-guided oral prednisolone for COPD exacerbations in primary care (STARR2) — The Lancet Respiratory Medicine, 2023. consensus.app
Further reading
- Eosinophils: high, low and normal levels
- Full blood count: read your results
- Asthma: symptoms, causes, diagnosis and treatment
- Allergy blood test: IgE and panels explained
- C3 complement blood test: aging and inflammation
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