Diagnosis of pneumonia
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For the microbiologic diagnosis of pneumonia, by definition, an infiltrate must be present on a chest x-ray. A deeply expectorated sputum sample will yield the causitive organism from 20-80% of the time, depending on the study. Sputum specimens will be rejected by the laboratory if they have >10 epithelial cells per high power field. The reason is that epithelial cells are usually from the upper airway, and have many adherent bacteria (see photomicrograph, left). These bacteria constitute the normal flora of the upper airway, and confuse interpretation of the culture.