Reproducibility of biomarkers in induced sputum and in serum from chronic smokers

RationaleSoluble inflammatory markers obtained from non-invasive airway sampling such as induced sputum may be useful biomarkers for targeted pharmaceutical interventions. However, before these soluble markers can be used as potential targets, their variability and reproducibility need to be established in distinct study populations.ObjectiveThis study aimed to assess the reproducibility of biomarkers obtained from induced sputum and serum in chronic smokers and non-smokers.MethodSputum and serum samples were obtained from 16 healthy non-smokers and 16 asymptomatic chronic smokers (for both groups: 8M/8F, 30–52 years, FEV1 ≥80% pred.; ≥10 pack years for the smokers) on 2 separate visits 4–10 days apart. Soluble markers in serum and sputum were analysed by ELISA. The differences between smokers vs non-smokers were analysed with a t-test and variability was assessed on log-transformed data by a mixed model ANOVA.ResultsAnalysable sputum samples could be obtained from all 32 subjects. In both study populations neutrophils and macrophages were the predominant cell types. Serum Pulmonary Surfactant Associated Protein D had favourable reproducibility criteria for reliability ratio (0.99), intra-subject coefficient of variation (11.2%) and the Bland Altman limits of agreement. Furthermore, chronic smokers, compared to non-smokers, had significantly higher sputum concentrations of IL-8 (1094.6 pg/mL vs 460.8 pg/mL, p = 0.006)), and higher serum concentrations of Pulmonary Surfactant Associated Protein D (110.9 pg/mL vs 64.7 pg/mL, p = 0.019), and lower concentrations of Serum Amyloid A (1352.4 pg/mL vs 2297.5 pg/mL, p = 0.022).ConclusionSerum Pulmonary Surfactant Associated Protein D proved to be a biomarker that fulfilled the criteria for reproducibility in both study groups.