Long-term repeatability of induced sputum cells and inflammatory markers in stable, moderately severe COPD

Study objectives: Neutrophilic inflammation is a major feature of COPD. Induced sputum is increasingly used to monitor inflammatory airway diseases. Although short-term repeatability of selected sputum markers has been extensively studied in several populations, data on the long-term repeatability of induced sputum markers in stable COPD are scant. Design: Sputum supernatant of 12 patients with stable COPD was analyzed on three separate occasions with 4-weekly intervals. Sputum cells and inflammatory markers interleukin (IL)-8 and soluble intercellular adhesion molecule (sICAM)-1 were measured in supernatant using enzyme-linked immunosorbent assay. Repeatability of sputum markers was expressed by intraclass correlation coefficients (Ri). Measurements and results: Sputum induction was safe in all patients. None of the sputum parameters analyzed changed significantly throughout the study. The repeatability for cell differential counts instable COPD was as follows: total cells, Ri=0.07; neutrophils, Ri=0.66; macrophages, Ri=0.47; eosinophils, Ri=0.49; and lymphocytes, Ri=0.58. The repeatability of soluble markers was as follows: IL-8, Ri=0.50; and sICAM, Ri=0.58. Sputum neutrophils were negatively correlated with lung function on each separate occasion, whereas soluble markers were not correlated with sputum cells (p>0.16, all correlations) or lung function (p>0.24, all correlations). Conclusions: Clinically stable, moderate COPD is associated with equally stable sputum inflammatory markers. Repeatability of induced-sputum markers of neutrophilic inflammation in stable COPD is satisfactory, even over extended periods of time. These data support the usefulness of serial monitoring of induced-sputum inflammatory markers in COPD.