Peripheral eosinophil counts as a marker of disease activity in intrinsic and extrinsic asthma

Recently it has been suggested that the bronchospasm and hyperresponsiveness phenomena observed in asthma are secondary to the actions of the eosinophils; the purpose of this study was to evaluate the relationship between the peripheral number of eosinophils and various markers of disease activity in a group of asthmatics examined in childhood (mean age 10 years) and early adulthood (mean age 21 years).

The relationship between eosinophil count and pulmonary function (FEV1), respiratory symptoms, bronchial responsiveness to histamine and diurnal variation in peak expiratory flow rate (PEF) was studied in 70 subjects with bronchial asthma, of whom 24 had intrinsic and 46 extrinsic asthma. Self-reported symptoms of asthma were graded on a scale from 0 to 5, where 0 = no symptoms within the preceding 12 months and 5 = daily including nocturnal symptoms, and histamine responsiveness was analysed by means of the dose-response slope (DRS).

In both childhood and adulthood, a direct correlation was found between blood eosinophil count and symptom score (r = 0.69, P < 0.001 and r = 0.58, P < 0.001, respectively), whereas inverse correlations were observed between number of eosinophils and FEV1 % predicted (r = -0.75, P < 0.001 and r = -0.80, P < 0.001, respectively). Furthermore, in adulthood, eosinophil count was found to be significantly correlated to histamine responsiveness (logDRS) (r = 0.65, P < 0.001) and diurnal PEF variation (r = 0.81, P < 0.001); these correlations were also noted after dividing the subjects into intrinsic and extrinic asthmatics. In both groups of subjects a significant inverse correlation was also found between histamine responsiveness and prechallenge FEV1 % predicted. The eosinophil count in childhood was weakly correlated to the symptom score in adulthood (r = 0.29, P < 0.02).

This study showed a relationship between eosinophil count and severity of asthmatic symptoms, level of pulmonary function, histamine responsiveness and diurnal variation in PEF in both intrinsic and extrinsic asthma; suggesting that the peripheral eosinophil count reflects asthmatic activity, and possibly the degree of inflammation in the airways, in both children and adults. Furthermore, a low number of eosinophils in childhood might be related to a relatively favourable prognosis with regard to symptoms of asthma in early adulthood.