Community-Acquired Pneumonia: Evaluation and Diagnosis
Imaging Evaluation
The presence of a pulmonary consolidation or an infiltrate on chest radiograph is required to diagnose CAP, and a chest radiograph should be obtained when CAP is suspected.16 However, there is no pattern of radiographic abnormalities reliable enough to differentiate infectious pneumonia from noninfectious causes.17
There are case reports and case series demonstrating false-negative plain chest radiographs in dehydrated patients18 or in patients in a neutropenic state. However, animal studies have shown that dogs challenged with pneumococcus showed abnormal pulmonary shadow, suggestive of pneumonia, regardless of hydration status.19 There is also no reliable scientific evidence to support the notion that severe neutropenia can cause false-negative radiographs because of the inability to develop an acute inflammatory reaction in the lungs.20
A chest computed tomography (CT) scan is more sensitive than a plain chest radiograph in detecting pneumonia. Therefore, a chest CT should be performed in a patient with negative plain chest radiograph when pneumonia is still highly suspected.21 A chest CT scan is also more sensitive in detecting cavitation, adenopathy, interstitial disease, and empyema. It also has the advantage of better defining anatomical changes than plain films.22
Because improvement of pulmonary opacities in patients with CAP lags behind clinical improvement, repeating chest imaging studies is not recommended in patients who demonstrate clinical improvement. Clearing of pulmonary infiltrate or consolidation sometimes can take 6 weeks or longer.23
Laboratory Evaluation
Generally, the etiologic agent of CAP cannot be determined solely on the basis of clinical signs and symptoms or imaging studies. Although routine microbiological testing for patients suspicious for CAP is not necessary for empirical treatment, determining the etiologic agent of the pneumonia allows the clinician to narrow the antibiotics from a broad-spectrum empirical regimen to specific pathogen-directed therapy. Determination of certain etiologic agents causing the pneumonia can have important public health implications (eg, Mycobacterium tuberculosis and influenza virus).24
Sputum Gram Stain and Culture
Sputum Gram stain is an inexpensive test that may identify pathogens that cause CAP (eg, Streptococcus pneumoniae and Haemophilus influenzae). A quality specimen is required. A sputum sample must contain more than 25 neutrophils and less than 10 squamous epithelial cells/low power field on Gram stain to be considered suitable for culture. The sensitivity and specificity of sputum Gram stain and culture are highly variable in different clinical settings (eg, outpatient setting, nursing home, ICU). Reed et al’s meta-analysis of patients diagnosed with CAP in the United States showed the sensitivity and specificity of sputum Gram stain (compared with sputum culture) ranged from 15% to 100% and 11% to 100%, respectively.24 In cases of proven bacteremic pneumococcal pneumonia, positive cultures from sputum samples were positive less than 50% of the time.25
For patients who cannot provide sputum samples or are intubated, deep-suction aspirate or bronchoalveolar lavage through a bronchoscopic procedure may be necessary to obtain pulmonary secretion for Gram stain and culture. Besides bacterial culture, sputum samples can also be sent for fungal and mycobacterial cultures and acid-fast stain, if deemed clinically necessary.
The 2019 ATS/IDSA guidelines for diagnosis and treatment of adults with CAP recommend sputum culture in patients with severe disease and in all inpatients empirically treated for MRSA or Pseudomonas aeruginosa.26
