Sputum eosinophils predicted more asthma emergency visits, but not more service use in COPD

Design

Retrospective cohort linking tertiary clinic data with Ontario administrative databases, 2006 to 2019

Population

4,821 patients: 3,714 with asthma only and 1,107 with COPD only

Primary outcome

Health service use (hospitalisations, physician and emergency visits) by sputum cell counts

Effect

Asthma ED visits: RR 1.92 (95% CI 1.49 to 2.47) for 3% to 15% and RR 1.88 (95% CI 1.17 to 3.03) above 15% eosinophils

This retrospective cohort linked clinic data from a tertiary airway service in Ontario to health administrative records from 2006 to 2019. It included 4,821 patients, 77% with asthma only and 23% with COPD only. Sputum eosinophil and neutrophil percentages from routine assessment were treated as baseline exposures.

In asthma, sputum eosinophils above 3% were associated with higher asthma-specific health service use. Compared with under 3%, emergency department visits for asthma were increased with eosinophils of 3% to 15% (RR 1.92, 95% CI 1.49 to 2.47) and above 15% (RR 1.88, 95% CI 1.17 to 3.03). In COPD, sputum eosinophils were not significantly associated with use of services, whereas high sputum neutrophil counts were associated with more all-cause hospitalisations (RR 1.71, 95% CI 1.11 to 2.64).

The patients came from a tertiary referral clinic where sputum induction is available, and association does not show that treating eosinophilia lowers use of services.

• Sputum eosinophils above 3% were associated with about 1.9 times the rate of asthma emergency department visits.
• The risk did not rise further above 15% eosinophils (RR 1.88 vs 1.92 at 3% to 15%).
• In COPD, sputum eosinophils were not associated with service use; high neutrophils were linked to hospitalisation.
• Where induced sputum is available, consider it in asthma that is hard to control.
• The cohort was drawn from a tertiary clinic and the data show association only.

Why it matters

It supports measuring airway inflammation directly in asthma and suggests that COPD and asthma behave differently.

Don't overread it

A retrospective tertiary-clinic cohort shows association and not that treating eosinophilia prevents emergency visits.

The statistics, in plain English

A rate ratio of 1.9 means that people with eosinophilia attended emergency departments for asthma at about 1.9 times the rate of those without, after adjustment. The flat relationship above 3% suggests a threshold effect and not a dose-response.

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