Pleural Fluid B-Type Natriuretic Peptide Level as an Additional Indicator for the Fluid's Etiology.

Background: The identification of the etiology of a pleural effusion can be difficult. Measurement of serum B-type natriuretic peptide (BNP) levels is helpful in the diagnosis of congestive heart failure (CHF) as a cause of respiratory failure, but pleural fluid BNP measurement is still not part of the workup for pleural effusion.

Objectives: To identify the correlation between pleural fluid BNP levels and clinical diagnosis.

Methods: In this cross-sectional study, data from 107 patients admitted to the department of internal medicine between November 2009 and January 2015 were obtained from medical records. Patients underwent a diagnostic thoracocentesis as part of their evaluation. They were grouped according to final diagnosis at discharge and clinical judgment of the attending physician.

Results: Serum BNP levels were significantly higher in the CHF patients compared to patients with non-cardiac causes of pleural effusion (1519.2 and 314.1 respectively, P < 0.0001). Mean pleural fluid BNP was also significantly higher in the CHF patients (1063.2 vs. 208.3, P < 0.0001). Optional cutoff points to distinguish between cardiac and non-cardiac etiology of pleural effusion were 273.4 pg/ml (sensitivity 83.3%, specificity 72.3%, accuracy 76.7%) or 400 pg/ml (sensitivity 78.6%, specificity 86.2%, accuracy 83.0%). A strong correlation was found between serum BNP and pleural fluid BNP levels.

Conclusions: High levels of serum BNP in patients presenting with pleural effusion suggest CHF. In cases with doubt regarding the etiology of pleural effusion, high levels of pleural fluid BNP can support the diagnosis, but are not superior to serum BNP levels.