Multilevel and Continuous Pleural Fluid pH Likelihood Ratios for Draining Parapneumonic Effusions

Abstract

Background: Although clinical practice guidelines endorse the use of pleural fluid pH to select patients with parapneumonic effusions for pleural drainage, no studies have reported likelihood ratios for pleural fluid pH. Objectives: We derived and tested the value of continuous likelihood ratios for selecting pneumonia patients for pleural drainage. Methods: Patient level pleural fluid pH results were obtained from a registry of primary studies that assessed the discriminative properties of pH. Multilevel likelihood ratios were calculated for four pH intervals. Continuous likelihood ratios were derived from logistic regression using discrete pH values. Binary, multilevel and continuous likelihood ratios were compared to evaluate the statistical (χ²) and clinical advantages of continuous likelihood ratios. Results: Hundred and ninety-seven pleural fluid pH results were retrieved from published reports and categorized into four pH ordinal intervals. Multilevel likelihood ratios ranged from a low of 0.13 (95% CI, 0.04–0.41) for pH values >7.40 to a high of 15.80 (95% CI, 7.04–35.45) for pH values ≤7.00. Logistic regression derived the following equation for continuous likelihood ratios: exp[–7.168(measured pH – 7.207)]. Continuous likelihood ratios offered more diagnostic information both statistically (p < 0.005) and clinically compared with binary and multilevel likelihood ratios. Conclusions: Analysis of a patient registry allows the derivation of an exponential equation that calculates continuous likelihood ratios for discrete pleural fluid pH values. Continuous likelihood ratios provide more clinically and statistically significant information compared with binary and multilevel likelihood ratios for calculating posttest probabilities of the need to drain parapneumonic effusions.