Sputum cultures for the evaluation of bacterial pneumonia
●Lower respiratory tract infections are common in the general population, occurring with increased frequency in older individuals and those with chronic diseases or compromised immune function. A diagnosis is made by culture of respiratory tract secretions, by isolation of a compatible organism from blood or pleural fluid cultures, or by molecular methods.
●While a positive blood or pleural fluid culture may definitively identify the pathogen, an organism growing from a respiratory specimen or detected by a molecular amplification assay is not definitive proof that it is the etiologic agent. Many bacterial species are normal flora or colonizers of the respiratory tract and, although present in respiratory secretions, they may not be responsible for the clinical illness in an individual patient with pneumonia due to another cause.
●Expectorated sputum is the most common lower respiratory tract specimen received by the microbiology laboratory. Expectorated sputum is often difficult to assess because many specimens consist mostly of upper airway specimens and not sputum. The first and most important step in evaluation is the Gram stain. Gram stains are useful to (see ‘Expectorated sputum’ above):
•Assess the suitability of the sputum specimen for further processing and interpretation
•Predict a likely etiologic agent by identification of a predominant bacterial morphology in an adequate (purulent) specimen
●Several guidelines have been proposed to evaluate the quality of sputum samples. These guidelines have proposed different combinations and cutoffs of the minimum number of squamous epithelial cells (SECs) and/or polymorphonuclear leukocytes (PMNs) per low-power field (LPF; 10x objective), but none of these parameters can be considered to be clearly superior. Our laboratory rejects all specimens with more than 10 SECs/LPF without considering the number of PMNs.
●Sputum Gram stain and culture are indicated for all patients with hospital-acquired pneumonia and for certain patients with community-acquired pneumonia. Sputum Gram stain and culture have no role in the evaluation of acute bronchitis in otherwise healthy individuals. Similarly, they are not indicated in the initial evaluation of patients with acute exacerbations of chronic obstructive pulmonary disease.
●Culture results are reported in a semiquantitative manner (1+ to 4+ in some laboratories, rare-few-moderate-abundant in others). Most true pathogens are present in at least 3+ (moderate) amounts.
●Any positive sputum culture result must be interpreted in the context of the clinical setting, since organisms can colonize the upper and/or lower respiratory tract. Some organisms are virtually never pulmonary pathogens, such as Candida spp, coagulase-negative staphylococci, and enterococci.
●Bronchoscopy can be used to acquire samples of lower respiratory tract secretions in patients with suspected pneumonia in whom respiratory samples cannot be obtained by expectoration or in whom such samples have been nondiagnostic. Bronchoscopy for specimen collection is most useful for the diagnosis of infection due to Mycobacterium tuberculosis in patients with negative sputum studies, for Pneumocystis jirovecii (formerly Pneumocystis carinii) or other fungal or viral pathogens, or for establishing a noninfectious etiology such as malignancy.
