Design: Retrospective cohort linked to health administrative data
Population: 4,821 patients with asthma or COPD at a Canadian tertiary airways clinic
Primary outcome: Health service use (admissions, physician and emergency visits)
Effect: Asthma, eosinophils 3 to 15%: ED visits RR 1.92 (1.49 to 2.47); COPD, high neutrophils: admissions RR 1.71

A Canadian retrospective cohort linked sputum cell counts from 4,821 patients at a tertiary airways clinic (3,714 with asthma, 1,107 with COPD) to provincial health records from 2006 to 2019.

In asthma, sputum eosinophils of 3% to 15% and above 15% were each associated with about twice the rate of asthma-specific emergency visits compared with less than 3% (RR 1.92 and 1.88). In COPD, sputum eosinophils were not associated with health service use, but high sputum neutrophils were associated with more all-cause hospitalisations (RR 1.71, 95% CI 1.11 to 2.64).

Induced sputum counts are available in few centres, but the findings support measuring airway inflammation in difficult asthma and underline that eosinophilic and neutrophilic patterns carry different meanings in asthma and COPD.

In difficult asthma, sputum eosinophils above 3% flag higher risk of emergency visits
Blood eosinophils are a practical, if imperfect, substitute where induced sputum is unavailable
In COPD, a neutrophilic sputum pattern may signal admission risk; eosinophils were not predictive here
Use inflammatory phenotype to guide steroid and biologic decisions in specialist clinics

Why it matters

It shows the same cell count can mean different things in asthma and COPD.

The statistics, in plain English

An RR of about 1.9 means nearly twice the rate of emergency visits, after adjustment for demographic and clinical factors. These are associations in tertiary clinic patients, who may differ from those seen in general practice.