Pleural fluid/serum total protein ratio (TPR) (corresponding to ) plotted versus protein reflection coefficient. Pleural fluid/serum total protein ratio (TPR) (corresponding to ) plotted versus protein reflection coefficient. The original data were taken from the two regressions presented in [20] referring to exudates and transudates. The figure intends to show that since TPR is a unique function of protein reflection coefficient, moving from transudates (·········) to exudates (–––) just corresponds to progressively increasing damage of the mesothelium. G. Miserocchi Eur Respir Rev 2009;18:244-252 ©2009 by European Respiratory Society