Background The beneficial effect of corticosteroids and anti-interleukin (IL)-5 on exacerbations of airway disease becomes apparent at blood eosinophil counts above 0.15 × 10⁹/L, well within the normal range. One potential explanation is that the upper limit of the normal range is artificially high because studies have included patients with allergic disease and eosinophilic inflammation. We have assessed the normal range for blood eosinophil counts in volunteers with no self-reported history of allergic disease and compared this with the findings from a more traditional control population.

Methods We recruited 78 volunteers (15 male) with a mean age of 38.8 years. Volunteers with a self-reported history of asthma, allergic rhinitis and/or eczema were excluded. The differential cell count was carried out using sysmex XN analyser and serum IgE measured using automated enzyme immunoassay by Phadia Immunocap equipment. Result were compared with an unscreened population (n = 120) used to calculate our local normal ranges.

Results One outlier value of 0.79 × 10⁹/L, (>5 SD above the mean) was excluded from further analysis. In the remainder the mean blood eosinophil count was 0.15 × 10⁹/L with an upper limit of normal range of 0.27 × 10⁹/L. Volunteers with no self-reported history of allergic disease but an IgE >120 iu/L and/or positive specific IgE to house dust mites or grass were not statistically different. The mean blood eosinophil count in the laboratory population was 0.19 × 10⁹/L (p 0.018 vs our population) and the upper limit of normal range 0.42 × 10⁹/L.

Conclusions The upper limit of the normal range for blood eosinophil count is lower in a population who have no clinical history of allergic disease.