TNF and chronic obstructive pulmonary disease: The aim of this study was to compare the levels of the six common inflammatory markers interleukin 6, 8 & 18 (IL-6, IL-8 IL-18), interferon gamma-inducible protein-10 (IP-10), tumor necrosis factor-alpha (TNF-α) and monokine induced by gamma interferon (MIG) in paired induced and spontaneous sputum samples collected from COPD patients in the stable state and/or during acute exacerbations.