The level of circulating blood eosinophils in healthy adults is much lower than currently believed and can be influenced by co-morbidities. The findings could have implications for the management of chronic respiratory diseases, experts say.

Writing in the European Respiratory Journal [see paper here] Dr Alvar Agusti from the University of Barcelona, and colleagues said that the Global Initiative for Asthma (GINA) and the Global Strategy for the Diagnosis, Management and Prevention of Chronic Obstructive Lung Disease (GOLD) recommended the use of eosinophil counts to guide therapy but the advised thresholds often attracted criticism.

“The typical range of blood eosinophil counts in the general population is not well established, since the role of potential confounding factors other than atopy, such as age, sex, environmental exposures or presence of disease is unclear,” they wrote.

To address this gap in knowledge they analysed the blood eosinophil counts of over 11,000 Austrians participating in the observational LEAD trial.

In line with previous research, they found that eosinophil counts were higher in children and atopic patients. However, they also found that male gender (OR 1.36), current smoking (OR 1.72), obesity (OR 1.16), metabolic syndrome (OR 1.41) and a diagnosis of asthma (OR 2.05) were all associated with higher eosinophil counts, with the morbidities having an additive effect.

Conversely, former smoking, hypertension, cardiovascular disease and diabetes were not linked to higher eosinophil levels.

When the researchers excluded patients with co-morbidities known to raise eosinophil levels they found that circulating eosinophil levels were lower that than generally used in the clinic (median 120 cells [30-330] /µL in males and 100 cells [30-310] /µL in females.

The research team suggested that that the additive effect of factors associated with high eosinophil counts could impact the predictive role of blood eosinophils for treatment efficacy in some subgroups of patients with asthma or COPD.

“A threshold of 300 cells/µL to support the use of ICS in patients with COPD is often questioned because it is believed to be within the ‘normal’ range of blood eosinophil counts. Yet, our results suggest that average values, especially in those over the age of 40 years, are lower than this value, with medians of 100-120 cells µL in the ‘healthy’ sub-population,” the study authors wrote.

According to the authors, their results showed that age and sex needed more attention when defining ‘normal’ values and that co-morbidities had an additive effect on blood esoinophil counts.

“Overall, this novel information will contribute to better interpretation of blood eosinophil counts, both in general population cohorts and in patients with airways diseases,” they concluded.