Bacteria or a fungus may be found by using special dyes on the sputum sample. This is called a Gram stain. A Gram stain can help a doctor:
Find out if the sputum sample is good enough for culture. For example, was the sputum collected in the right way? Is there enough sputum in the sample? Does it contain other bacteria that aren't infecting the lungs? If the sample is not good enough for culture, another sample can be obtained.
Make a diagnosis before the culture results come back from the lab. In some cases, Gram stain results can be available within 30 minutes. But other culture results may not be available for 1 to several days. Information received from a Gram stain can help your doctor treat the infection sooner, rather than waiting for the culture results.
Sputum obtained by coughing most often contains bacteria from your mouth. So the doctor will consider your symptoms, condition, and other test results (such as a chest X-ray) along with your culture results.
Even if your culture does not grow any bacteria or fungi, you may still have an infection.
Sensitivity testing helps a doctor choose the best medicine to kill certain types of bacteria or fungi infecting a person. Differences in the genetic material (DNA) of bacteria or fungi may make them resistant to certain antibiotics. In such cases, those antibiotics can't kill all of the bacteria. When an antibiotic is chosen, you must take all of the medicine to lower the chance that the bacteria will become resistant to the antibiotic. Stopping antibiotic treatment early kills only the weakest bacteria. Then those that are stronger and more resistant can multiply and prolong the infection. Later infections may also be harder to treat if they are caused by resistant bacteria.
Bronchoscopy is often done if a serious or recurring lung infection is present. This is especially true when other tests have not definitely found the cause. To learn more, see the topic Bronchoscopy.
Other Works Consulted
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.