Pleural fluid parameters identifying complicated parapneumonic effusions

Controversy exists regarding the clinical utility of pleural fluid parameters as prognosticators of complicated parapneumonic effusions that require drainage.

The purpose of this prospective study is to further assess the utility of these parameters in the management of a larger series of parapneumonic effusions and to determine appropriate binary decision thresholds.

We studied 238 consecutive patients with parapneumonic effusions who underwent diagnostic thoracentesis.

We found that pleural fluid pH had the highest diagnostic accuracy (area under the curve, AUC: 0.928; 95% confidence interval, CI: 0.894-0.963) compared with pleural fluid glucose (AUC: 0.835; 95% CI: 0.773-0.897), LDH (AUC: 0.824; 95% CI: 0.761-0.887) or pleural fluid volume (AUC: 0.706; 95% CI: 0.634-0.777). The optimal binary decision threshold for pleural fluid pH identifying complicated effusions requiring drainage was 7.15. Binary, multilevel and continuous likelihood ratios (LRs) for pH were calculated to estimate the likelihood of complication of the pleural effusion. Values for the LRs were compared for each of the three strategies, and relative clinical and statistical significances were assessed. Binary LRs provided significantly less information than continuous strategies.

The pH has the highest diagnostic accuracy for identifying complicated parapneumonic pleural effusions. The binary decision threshold determining the need for chest drainage is 7.15 in our patient series. We recommend continuous LRs to estimate the post-test probability of the complication as they provide the most information compared with binary LRs. Our results do not support the use of pleural fluid LDH as independent predictor of complicated parapneumonic effusions.