The white blood cell (WBC) count with viral pneumonias is often normal or mildly elevated, with a predominance of lymphocytes, whereas with bacterial pneumonias the WBC count is elevated (>20,000/mm3) with a predominance of neutrophils. Mild eosinophilia is characteristic of infant C. trachomatis pneumonia. Blood cultures should be performed on hospitalized children to attempt to diagnose a bacterial cause of pneumonia. Blood cultures are positive in 10% to 20% of bacterial pneumonia and are considered to be confirmatory of the cause of pneumonia if positive for a recognized respiratory pathogen. Urinary antigen tests are especially useful for L. pneumophila (legionnaires’ disease).
Viral respiratory pathogens can be diagnosed using polymerase chain reaction (PCR) or rapid viral antigen detection, but neither rules out concomitant bacterial pneumonia. M. pneumoniae should be suspected if cold agglutinins are present in peripheral blood samples and can be confirmed by Mycoplasma PCR. CMV and enterovirus can be cultured from the nasopharynx, urine, or bronchoalveolar lavage fluid. The diagnosis of M. tuberculosis is established by tuberculin skin test, serum interferon-gamma release assay, or analysis of sputum or gastric aspirates by culture, antigen detection, or PCR.
The need to establish an etiologic diagnosis of pneumonia is greater for patients who are ill enough to require hospitalization, immunocompromised patients, patients with recurrent pneumonia, or patients with pneumonia unresponsive to empirical therapy. For these patients, bronchoscopy with bronchoalveolar lavage and brush mucosal biopsy, needle aspiration of the lung, and open lung biopsy are methods of obtaining material for microbiologic diagnosis.
When there is a pleural effusion or empyema, a thoracentesis to obtain pleural fluid can be diagnostic and therapeutic. Evaluation differentiates between empyema and a sterile parapneumonic effusion caused by irritation of the pleura contiguous with the pneumonia. Gram stain, culture, or broad-range bacterial PCR may lead to microbiologic diagnosis. The pleural fluid should be cultured for bacteria, mycobacteria, fungi, and viruses. If the fluid is grossly purulent, removal reduces the patient’s toxicity and associated discomfort and may facilitate more rapid recovery. If the accumulation is large and impairs the ability of the lung to expand, removal of the fluid improves pulmonary mechanics and gas exchange.
Frontal and lateral radiographs are required to localize disease and adequately visualize retrocardiac infiltrates and are recommended for diagnosis among hospitalized children, but they are not necessary to confirm the diagnosis in well-appearing outpatients. Although there are characteristic radiographic findings of pneumonia, radiography alone cannot provide a definitive microbiologic diagnosis. Bacterial pneumonia characteristically shows lobar consolidation, or a round pneumonia, with pleural effusion in 10% to 30% of cases (Fig. 110-1).Viral pneumonia characteristically shows diffuse, streaky infiltrates of bronchopneumonia (Fig. 110-2) and hyperinflation. Atypical pneumonia, such as with M. pneumoniae and
Figure 110-1 Acute lobar pneumonia of the lingula in a 6-year-old child with high fever, cough, and chest pain. Frontal chest radiograph shows airspace consolidation, which obliterates the silhouette of the heart border on the left. The left hemidiaphragm is mildly elevated as a result of splinting. (From Markowitz RI: Diagnostic imaging. In Jen-son HB, Baltimore RS, editors: Pediatric Infectious Diseases: Principles and Practice, ed 2, Philadelphia, Saunders, 2002, p 133.)