It has been proposed that serum levels of eosinophil cationic protein (ECP) may be a clinically useful measure in allergic illness. The aim of this report is to describe the distribution and reproducibility of serum ECP levels in a population sample and to examine its relationship with other markers of disease.
The study was conducted in rural areas of Norfolk, UK in a random sample of men aged 20–44 years enriched with subjects drawn from general practice ‘asthma registers’. Asthma symptoms were assessed using the EC Respiratory Health Survey questionnaire. Atopy was measured by skin prick tests and serum IgE. Airway hyperresponsiveness (AHR) was tested by methacholine challenge test. Serum IgE and ECP was measured by fluoroimmunoassay using the Pharmacia CAP system. Reference equations were derived in subjects from the random sample who did not have symptoms of asthma, abnormal lung function or AHR. The relation of serum ECP with various clinical characteristics was examined in the whole study population. Reproducibility of serum ECP measurement was assessed in 57 subjects 4 weeks after the initial test.
The study population comprised 311 from the random sample and 58 from the asthma sample. The reference equation for serum ECP for healthy men was log10 ECP = 1.3966 − [(age − 20) × 0.0057]. The estimated mean serum ECP for a 20-year-old man was 25 μg/L. Current smokers have higher serum ECP levels that non-smokers (P = 0.014). ECP levels were not related to the skin prick test reactivity, serum IgE, a questionnaire-based diagnosis of asthma, or impaired lung function (all P > 0.05). Levels were higher in subjects with AHR (P = 0.003) and those who reported wheeze (P = 0.017) but there was no clinically useful separation in ECP levels between subjects classified by these criteria. The test was moderately reproducible over a 4-week period (intraclass correlation coefficient = 0.62).
Serum ECP levels were higher in this rural English population than reported in a comparable population in Sweden. Serum ECP is a reproducible test but cross-sectionally does not relate in any clinically useful way to markers of asthma. The meaning of between- subject differences in ECP levels requires further exploration.