Diagnostic Plan
Frontal and lateral chest radiographs are a mainstay of diagnosis and are often the only imaging needed when diagnosing effusions of varying severity. Typically, pleural fluid is seen as a homogenous density that obscures the underlying lung. Large effusions may lead to a shift of the mediastinum to the contralateral side and small effusions may simply blunt the costophrenic angle. Providers may choose to order a CT scan or ultrasonography to obtain a more detailed view of the pleural cavity. Ultrasonography will show small volumes of fluid and information about viscosity. CT will, however, provide the most information, displaying fluid, loculation, and thickening of the pleural membranes. Unfortunately, neither technique can reliably identify the stage of pleural infection or predict which patients will most likely benefit from surgical intervention. A thoracentesis or blood cultures may be drawn to confirm the causative agent. Reported diagnostic yield from pleural and blood cultures ranges from 60 to 70%. Blood cultures are only positive in about 10% of cases of complicated pneumonia but should be drawn before initiating antibiotic therapy in children who are hospitalized for pneumonia in order to better inform the choice of therapy. The yield from pleural cultures is also often very low due to the fact that most children have already started antibiotic therapy. Tuberculin tests are also appropriate for any child with extensive lung disease, regardless of risk factors.
Patients often present with elevated white blood cell count with a left shift, meaning that there is an increase in immature white cells suggestive of an acute infection. Blood cultures may be positive and thoracentesis will display findings consistent with an exudate. Pleural fluid commonly contains neutrophils with bacterial disease and lymphocytes in tuberculosis effusions. Pleural fluid pH and glucose are often low in bacterial disease and a pH of less than 7.2 suggests active bacterial infection. Gram stain, cultures, and counterimmunoelectrophoresis often indicate the offending organism.