Respiratory medicine
Can a blood test reveal what kind of airway inflammation asthma has?
Open access · cc by · source: Europe PMC
A routine blood eosinophil count identified eosinophilic airway inflammation about as well as exhaled nitric oxide, but blood neutrophils were a poor guide to neutrophilic asthma.
Study at a glance
- Design
- Cross-sectional — Retrospective single-clinic series with same-day sputum induction, blood tests, exhaled NO and lung function; ROC and logistic regression to predict sputum phenotype, then tested in a separate later group of patients.
- N
- N=508 · 508 asthma patients with successful sputum induction (2005-2011); prediction equations were checked in a separate validation group of 178 asthmatics (138 for the eosinophil equation because IgE was missing in 40).
- Population
- Adults with asthma of all severities attending the University Asthma Clinic of Liege, Belgium.
- Outcome
- Sputum inflammatory phenotype (eosinophilic, neutrophilic, mixed, paucigranulocytic) and the accuracy of blood eosinophils, blood neutrophils and exhaled NO for predicting it.
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Key findings
42% of patients had eosinophilic, 40% paucigranulocytic, 16% neutrophilic and 3% mixed inflammation. Blood eosinophils correlated moderately with sputum eosinophils (r = 0.6); a cut-off of 220 per mm3 gave 77% sensitivity and 70% specificity, performing as well as FeNO. Blood eosinophil percentage, FeNO, IgE and a lower FEV1/FVC ratio independently predicted eosinophilic inflammation, and the combined equation showed moderate agreement in the validation group (kappa 0.59). Blood neutrophils correlated only weakly with sputum neutrophils, and older age and higher functional residual capacity were the only independent predictors of neutrophilic asthma; that equation agreed poorly in validation (kappa 0.24).
Methodology
The researchers reviewed 508 asthma patients from a Belgian university clinic who produced a usable induced-sputum sample. On the same day each patient had exhaled nitric oxide (FeNO), spirometry and blood tests. Sputum with at least 3% eosinophils was classed as eosinophilic and at least 76% neutrophils as neutrophilic. They used ROC curves and multiple logistic regression to find blood and clinical markers of each phenotype, then tested the resulting equations in a separate group of 178 later patients.
Limitations
This is a retrospective series from one specialist clinic and includes only patients whose sputum induction succeeded, so it may not represent all people with asthma, particularly in primary care. Phenotype was measured once, although sputum inflammation can change over time and with treatment, and many patients were on inhaled steroids. The study shows which markers track sputum cells but did not test whether treating based on blood counts improves outcomes. The paper does not include a dedicated limitations section, and the neutrophilic threshold of 76% is higher than many other studies use, which affects the phenotype proportions.
How this study connects
Role on claims
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Asthma inflammation comes in types, and a blood test can hint at one type but not the other.
In 508 clinic patients with induced sputum, 42% had eosinophilic, 40% paucigranulocytic, 16% neutrophilic and 3% mixed inflammation; a blood eosinophil cut-off of 220/mm3 identified eosinophilic airways with 77% sensitivity and 70% specificity, about as well as exhaled NO, but blood neutrophils were a poor guide to neutrophilic asthma.
Evidence for the claim as stated.
Asthma inflammation comes in types, and a blood test can hint at one type but not the other.
In 508 clinic patients with induced sputum, 42% had eosinophilic, 40% paucigranulocytic, 16% neutrophilic and 3% mixed inflammation; a blood eosinophil cut-off of 220/mm3 identified eosinophilic airways with 77% sensitivity and 70% specificity, about as well as exhaled NO, but blood neutrophils were a poor guide to neutrophilic asthma.
Scope note — Retrospective single specialist clinic; phenotype measured once.
Limits the claim's scope: a different population, assay, or outcome.
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