Pleural fluid C-reactive protein contributes to the diagnosis and assessment of severity of parapneumonic effusions

Background and aimsPrompt identification of parapneumonic effusions has immediate therapeutic benefits. We aimed to assess whether C-reactive protein (CRP) and routine biochemistries in pleural fluid are accurate markers of parapneumonic effusions, and to evaluate their properties as indicators for drainage (complicated parapneumonic effusion).MethodsA retrospective review of 340 non-purulent parapneumonic effusions and 1,659 non-parapneumonic exudates from a single center was performed and the discriminative properties of pleural fluid routine biochemistries and, when available, CRP were evaluated. CRP, along with classical fluid parameters, was also applied to classify patients as having complicated or uncomplicated parapneumonic effusions. ROC analysis established the threshold of CRP for discriminating between groups.ResultsPleural fluids with neutrophilic predominance and CRP levels > 45 mg/dL were most likely to be parapneumonic in origin (likelihood ratio = 7.7). When attempting to differentiate non-purulent complicated from uncomplicated effusions, a CRP > 100 mg/L had the same performance characteristics (area under the curve = 0.81) as the widely accepted biochemistries pH and glucose. Combinations of CRP with pH or glucose resulted in incrementally discriminating values, pertaining to either sensitivity (75–80%) or specificity (97%), for complicated effusions.ConclusionPleural fluid CRP may be a useful adjunctive test in pleural effusions, both as a marker of parapneumonics and, particularly, as a differentiator between complicated and uncomplicated effusions.