Eosinophil Count in Sputum, Not Blood, Is Better Predictor of COPD Severity in Smokers

Data from a study of smokers with chronic obstructive pulmonary disease (COPD) indicate that the concentration of eosinophils in the sputum, rather than the blood, is a better predictor of COPD exacerbations and lung function.

The findings, published in The Lancet: Respiratory Medicine, are based on data from the Subpopulations and Intermediate Outcome Measures in COPD Study (SPIROMICS), which analyzes patients with COPD aged 40 to 80 years who smoke. Each patient in this analysis had their blood drawn and tested at baseline as part of the study, which provided their concentration of eosinophils, a type of white blood cell. Some patients also had their eosinophil concentrations measured from their sputum.

Patients with high levels of eosinophils in their sputum had significantly lower lung capacity as measured by FEV_{1 ­}percentage compared with the low sputum eosinophil group. They also experienced worse emphysema and air trapping, as indicated by computed tomography density measures, and they were more likely to experience COPD exacerbations that required treatment with corticosteroids.

Patients with high blood eosinophil levels also had worse FEV₁ percentage than the low blood eosinophil group, but the differences were smaller than those between the sputum groups. High blood eosinophil levels were associated with slightly thicker airway walls and increased wheezing, but not with COPD exacerbations or other measures of COPD severity, like emphysema.

The researchers found a small but significant association between blood eosinophil counts and sputum eosinophil rates, but the false-discovery rate was high at 72%.

“In a large, well characterised cohort of former and current smoking patients with a broad range of COPD severity, high concentrations of sputum eosinophils were a better biomarker than high concentrations of blood eosinophils to identify a patient subgroup with more severe disease, more frequent exacerbations, and increased emphysema,” the authors wrote. “Blood eosinophils alone were not a reliable biomarker for COPD severity or exacerbations, or for sputum eosinophils.”

They suggested that sputum eosinophil levels may be a more useful marker when assessing eosinophilic inflammation in COPD.

“These observations confirm the importance of assessing eosinophils in the airways,” the researchers concluded. “In the future, patients with high sputum eosinophil concentrations should be followed longitudinally to establish whether this factor has long-term effects on the progression of COPD.”