Parapneumonic effusions (PPEs) occur in approximately 40% of patients who require hospital admission for bacterial pneumonia, and represent the second most common cause of exudates, exceeded only by malignant effusions.2 Although most PPEs will resolve with antibiotic treatment alone (uncomplicated parapneumonic effusion [UPPE]), it is recommended that all patients with more than a minimal PPE undergo a thoracentesis to determine the gross appearance as well as the biochemical and microbiological characteristics of the pleural fluid. When pus is present (empyema), pleural space drainage is mandatory. However, some but not all patients found to have a clear pleural fluid may eventually need drainage for resolution of pleural sepsis (complicated parapneumonic effusion [CPPE]), Identification of this latter group represents a major challenge to clinicians. Traditionally, determination of pleural pH, glucose, and lactate dehydrogenase (LDH) has been claimed to assist in the decision to drain PPE; however, these biochemical parameters lack both enough sensitivity and reliable discriminating cutoff values.
As a PPE progresses from the exudative to the fibropurulent stage, polymorphonuclear leukocytes and soluble factors (eg, complement activation products, proinflammatory cytokines)- become progressively higher within the pleural space.
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