Sputum culture

A sputum culture is a test to detect and identify bacteria or fungi that infect the lungs or breathing passages. Sputum is a thick fluid produced in the lungs and in the adjacent airways. A sample of sputum is placed in a sterile container and sent to the laboratory for testing. Sputum may be expectorated (produced by coughing), induced (sailine is srayed in the lungs to induce sputum productio), or taken via an endotracheal tube (commonly used on patiens on respirators) in an intensive care setting. If no bacteria or fungi grow, the culture is negative. If organisms that can cause the infection (pathogenic organisms) grow, the culture is positive. The type of bacterium or fungus will be identified by microscopy, colony morphology and biochemical tests of bacterial growth.

If bacteria or fungi that can cause infection grow in the culture, other tests may be done to determine which antimicrobial agent will be most effective in treating the infection. This is called susceptibility or sensitivity testing.

In a hospital setting, a sputum culture is most commonly ordered if a patient has a pneumonia. The Infectious Diseases Society of America recommends that sputum cultures be done in pneumonia requiring hospitalization, while the American College of Chest Physicians does not. One reason for such a discrepancy is that normal, healthy lungs have bacteria, and sputum cultures collect both normal bacteria and those which are pathogenic. However, pure cultures of common respiratory pathogens in the absence of upper respiratory flora combined with symptoms of respiratory distress provides strong evdience of the infectious agent, and its significance. Such pathogens include "Streptococcus pneumoniae", "Haemophillus influenzae" and the highly infectious "Mycobacterium tuberculosis" which are transmitted by inhailing aerosols. For this reason, laboratory processing of sputum for respiratory pathogens are perfomed with the aid of a biological safety cabinet.