Doctors perform a sputum culture to look for bacteria in the lungs or bronchial tubes. In some cases, a sputum culture can test for fungus in the lungs or air passages. Doctors use the sputum culture to determine the cause of a person's illness and the best method of treatment. Sputum cultures can help a doctor diagnose pneumonia, bronchitis, and tuberculosis.
Doctors collect sputum by having the patient cough deep enough to produce phlegm, or sputum. The patient should then spit the sputum into a cup. If a patient is too weak to cough deeply, he may need to inhale a mist that will help him to cough, or the doctor may pound on his back to help him produce mucus.
The doctor can also perform a bronchoscopy to collect the mucus. During a bronchoscopy, the doctor slides a thin scope into the mouth and down the throat. A tracheal suction, which removes phlegm through a catheter inserted into the airway, can also be used to collect the sputum culture.
Once collected, the sputum is dyed to make sure that an adequate amount of mucus was collected from the lungs and that the patient didn't simply spit saliva into the cup. A Gram stain indicates to a doctor that bacteria is present. In some instances, the doctor may be able to diagnose a patient based on the Gram stain.
After it is collected, the sputum is mixed with a material that encourages the growth of any bacteria or fungi present and is sent to a laboratory. The culture commonly used encourages the growth of Streptococcus pneumoniae, Staphylococcus aureus, and Klebsiella pneumoniae, bacteria that point to pneumonia or bronchitis. If tuberculosis, a fungal infection, or an infection with Chlamydophila pneumoniae bacteria is suspected, a special culture is needed for those organisms to grow. Usually, enough bacteria grows within three days to determine the type of infection. Fungus takes about a week to grow, while the organism that causes tuberculosis can take up to six weeks.
If a sputum culture produces a positive result, meaning bacteria or fungus grows, the type of bacteria is usually identified by examining the culture under a microscope. After the bacteria or fungus is identified, a doctor can perform a sensitivity test on the bacteria to determine what antibiotics will be effective on it. Sensitivity testing also tells the doctor how large of a dose a patient needs to kill all of the bacteria present.