Comparing serum and pleural fluid pro-brain natriuretic peptide (NT-proBNP) levels with pleural-to-serum albumin gradient for the identification of cardiac effusions misclassified by Light's criteria - Porcel,J.M.; Chorda,J.; Cao,G.; Esquerda,A.; Ruiz-González,A.; Vives,M. (Respirology, 2007)