Many of the systemic manifestations of chronic obstructive pulmonary disease (COPD) are mediated through increased systemic levels of inflammatory proteins. We assessed the long term repeatability of Interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and C-reactive protein (CRP) over one year and examined the relationships between these systemic markers in COPD.
Fifty-eight stable COPD patients completed a baseline and one-year visit. Serum IL-6, plasma CRP, and plasma TNF-α were measured. Repeatability was expressed by intraclass correlation coefficient (R i) and the Bland–Altman method. Pearson correlations were used to determine the relationships between the systemic markers at both visits.
There was moderate repeatability with a very high degree of statistical significance (p ≤ 0.001) between the two visits for all the systemic biomarkers (IL-6, CRP, and TNF-α). CRP was significantly associated with IL-6 at both visits (r = 0.55, p = 0.0001, r = 0.51, p = 0.0002, respectively). There were no other significant associations between the systemic markers at either of the visits.