The relationship between atopy, asthma, and eosinophilic inflammation is less clear in early childhood than later in life.

We sought to determine the relationships between asthma, atopy, and serum eosinophil cationic protein (ECP), a biomarker of eosinophil activation, in 6-year-old children.

Serum ECP levels were available from 968 six-year-old children who were part of a longitudinal birth cohort being assessed for asthma and atopy. Detailed clinical history and examination, lung function testing, methacholine challenge, and skin prick testing to 4 common allergens were undertaken. Subgroups of the children were compared by using t tests, ANOVA, χ² tests, and regression analysis.

One hundred ninety-one (19.7%) children had current asthma, with 114 (59.7%) of these being atopic. The mean serum ECP level for the entire group was 18.0 μg/L (range, 2.0-146.0 μg/L), with no difference between male and female patients. Serum ECP was higher in atopic children (20.5 ± 18.4), those with asthma (22.4 ± 19.6), and those with asthma and atopy (26.6 ± 22.4; all P < .001 compared with children with no asthma or atopy [16.1 ± 15.9]). Serum ECP levels were highest in children with severe asthma (P < .001), especially in those with concurrent atopy. Severity of atopy, judged on the basis of wheal size or derived variables combining wheal size and the number of positive skin tests, was a major determinant of serum ECP. Heightened methacholine responsiveness was not associated with increased serum ECP levels.

The higher serum ECP levels seen in 6-year-old children with current asthma and more severe atopy suggest that atopy and eosinophilic inflammation are important in driving this clinical phenotype and that this might represent asthma that persists.