Sputum culture

Overview
A sputum culture is a test to detect and identify bacteria or fungi that are infecting the lungs or breathing passages. Sputum is a thick fluid produced in the lungs and in the airways leading to the lungs. A sample of sputum is placed in a container with substances that promote the growth of bacteria or fungi. If no bacteria or fungi grow, the culture is negative. If organisms that can cause infection (pathogenic organisms) grow, the culture is positive. The type of bacterium or fungus will be identified with a microscope or by chemical tests.

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

This test is done on a sample of sputum that is usually collected by coughing. For people who cannot cough deeply enough to produce a sample, a suction tube or needle may be inserted in the airway to collect the sputum.

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. The reasons for the discrepancy is that normal, healthy lungs have bacteria, and sputum cultures collect both normal bacteria and the bacteria causing disease. It is difficult to tell which bacteria is causing the disease in this setting.