Stool Culture

Overview :

Stool culture may also be called fecal culture. To obtain a specimen for culture, the patient is asked to collect a stool sample into a special sterile container. In some cases, the container may contain a transport solution. Specimens may need to be collected on three consecutive days. It is important to return the specimen to the doctor's office or the laboratory in the time specified by the physician or nurse. Laboratories do not accept stool specimens contaminated with water, urine, or other materials.

The culture test involves placing a sample of the stool on a special substance, called a medium, that provides nutrients for certain organisms to grow and reproduce. The medium is usually a thick gel-like substance. The culture is done in a test tube—or on a flat round culture plate—which is incubated at the proper temperature for growth of the bacteria. After a colony of bacteria grows in the medium, the type of bacteria is identified by observing the colony's growth, its physical characteristics, and its microscopic features. The bacteria may be dyed with special stains that make it easier to identify features specific to particular bacteria.

The length of time needed to perform a stool culture depends on the laboratory where it is done and the culture methods used. Stool culture usually takes 72 hours or longer to complete. Some organisms may take several weeks to grow in a culture.

An antibiotic sensitivity test may be done after a specific bacterium is identified. This test shows which antibiotics will be most effective for treating the infection.

Although most intestinal infections are caused by bacteria, in some cases a fungal or viral culture may be necessary. The most common bacterial infections of the digestive tract are caused by Shigella, Salmonella, Campylobacter, and Yersinia. Patients taking certain antibiotics may be susceptible to infection with Clostridium difficile. In some cases, as with Clostridium difficile, the stool culture is used to detect the toxin (poison or harmful chemical) produced by the bacteria.

Patients with AIDS, or other immune system diseases, may also have gastrointestinal infections caused by such fungi as Candida, or viral organisms including cytomegalovirus.

Several intestinal parasites may cause gastrointestinal infection and diarrhea. Parasites are not cultured, but are identified microscopically in a test called "Stool Ova and Parasites."

Insurance coverage for stool culture may vary among different insurance plans. This common test usually is covered if ordered by a physician approved by the patient's insurance plan, and if it is done at an approved laboratory.

Alternative methods

Newer methods of testing stool samples for specific disease organisms include various forms of polymerase chain reaction (PCR) assays. One type that has been used to test for several different types of intestinal viruses at the same time is the RT-PCR, which stands for reverse transcriptase polymerase chain reaction. This assay measures changes in an organism's messenger RNA. RT-PCR assays have several advantages over standard stool cultures: they require only very small samples of material; they can be performed much more rapidly; and they can be used to test environmental water for virus contamination as well as human stool samples.