Sputum analysis and culture.

Sputum analysis and culture is the most common method of specific diagnosis of lower respiratory tract infections. Culture of a properly screened, expectorated sputum sample will identify the pathogen in most cases of bacterial pneumonia. Expectorated sputa, as well as those obtained by aspiration from the upper airway, are subject to oropharyngeal bacterial contamination. Other techniques, such as transtracheal aspiration, are more "sterile," but have a much higher incidence of morbidity. Microscopic screening of expectorated or aspirated sputum samples will reduce the number and increase the diagnostic accuracy of cultures, resulting in considerable cost savings. Mucopurulent material is selected by gross inspection, and microscopic examination of a wet mount and Gram stain yields information about cell type and predominant organism. More important, however, the presence of fewer than 25 squamous epithelial cells per low-power field indicates that true lower respiratory tract secretions have been collected. Culture results must be correlated carefully with semiquantitative grading, initial microscopic screening, clinical presentation, and response to initial therapy. When properly performed and interpreted, sputum analysis and culture are valuable tools in the diagnosis and treatment of lower respiratory tract infection.