Interleukin 6 activity in pleural effusion. Its diagnostic value and thrombopoietic activity

We measured interleukin 6 (IL-6) concentrations in the pleural fluid of various patients to determine its role in pathophysiology and diagnosis by using specific functional bioassay. IL-6 levels were significantly higher in exudate than in transudate (79.3 +/- 176.2 U/ml [n = 55] vs 1.7 +/- 1.8 U/ml [n = 12]; p < 0.01). Tuberculous effusion contained a significantly higher amount of IL-6 than malignant effusion (181.3 +/- 176.2 U/ml [n = 13] vs 29.4 +/- 71.5 U/ml [n = 29]; p < 0.005). Pleural IL-6 levels were invariably higher than serum IL-6 levels, and both were significantly correlated (n = 21, r = 0.632; p < 0.02). Pleural IL-6 levels were significantly correlated with lactate dehydrogenase (LDH) in pleural fluid (r = 0.392; p < 0.01), ratio of pleural/serum LDH (r = 0.571; p < 0.01), pleural adenosine deaminase activity (r = 0.599; p < 0.01), and serum C-reactive protein (r = 0.494; p < 0.01). Furthermore, IL-6 levels were significantly correlated with peripheral blood platelet counts (r = 0.447; p < 0.001). These results suggest that (1) IL-6 is produced locally in pleural space, (2) pleural IL-6 level is helpful for differential diagnosis, and (3) locally produced IL-6 could leak to circulation and cause systemic effects such as the induction of C-reactive protein and thrombocytosis.