The objective of this study was to compare the diagnostic accuracy of pleural fluid brain natriuretic peptide (BNP), N-terminal pro-BNP (NT-pro-BNP) and ST2, which are biomarkers of myocyte stress, for diagnosing pleural effusions due to heart failure (HF). BNP and ST2 have not been previously evaluated in pleural fluid.
The three biomarkers were measured in the pleural fluid of 90 cardiac effusions and 91 noncardiac effusions by commercially available methodologies. The area under the curve (AUC) quantified the overall diagnostic accuracy of the tests.
Pleural fluid NT-pro-BNP, BNP, and ST2 demonstrated AUCs of 0.96, 0.90 and 0.59, respectively, for diagnosing effusions due to HF. The cutoff values of 1,300 and 115 pg/mL, respectively, for NT-pro-BNP and BNP had the best discriminating properties. The reference level for BNP was particularly accurate in men > 75 years of age (AUC, 0.98), but age, gender, and serum creatinine level did not influence the NT-pro-BNP levels. Of the 20 patients whose cardiac effusions were misclassified as exudates by the criteria of Light et al, 18 patients (90%) and 14 patients (70%), respectively, would have been correctly categorized by NT-pro-BNP and BNP, whereas only 10 patients (50%) would have been appropriately classified by the serum-pleural protein gradient.
The pleural fluid NT-pro-BNP level is very useful in establishing the diagnosis of HF-associated effusions, and it confirms this diagnosis better than pleural BNP levels. The measurement of NT-pro-BNP rather than the serum-to-pleural protein gradient is recommended for identifying mislabeled cardiac transudates. The pleural fluid ST2 level is not helpful in diagnosing HF.