Stool culture test: what it looks for, how long it takes and what results mean

A stool culture is a laboratory test on a sample of poo. Lab Tests Online UK describes it as a test that checks for harmful bacteria and some parasites that can cause diarrhoea and other symptoms such as abdominal cramps, nausea, vomiting, fever and dehydration. In the laboratory a small amount of the sample is spread on special growth media, which encourage certain bacteria to grow while suppressing others. The dishes are incubated and checked regularly over 24 to 48 hours, and colonies are identified by their shape, colour and chemical properties.

If a harmful bacterium is found, susceptibility testing may follow to find out which antibiotics would be effective. Lab Tests Online UK notes that many UK laboratories now use PCR to screen for STEC first, with culture used to confirm a positive result, which is one reason the exact method depends on where your sample is sent.

The test is usually grouped with other stool tests in clinical practice. A separate test is used for C. difficile, another for parasites, and another, the faecal immunochemical test, for hidden blood. A stool culture answers one specific question: is a bacterial pathogen from the usual list growing from this sample?

Most diarrhoea does not need a stool culture. The NHS says diarrhoea and vomiting are often caused by a stomach bug, usually stop in a few days, and can be managed at home with plenty of fluids. In adults and children it says diarrhoea usually stops within 5 to 7 days and vomiting within 1 or 2 days.

Lab Tests Online UK lists the situations in which a GP may ask for a culture: diarrhoea or vomiting for more than a few days, especially with blood, mucus or pus in the stools; suspected contaminated food or water, for example after undercooked meat, raw eggs or unpasteurised dairy; recent travel abroad; a suspected outbreak; and follow-up after treatment for infections such as typhoid, where people can continue to carry bacteria.

The NHS lists signs that should prompt a call to 111: bloody diarrhoea or bleeding from the bottom, diarrhoea for more than 7 days, vomiting for more than 2 days, being unable to keep fluid down, and concerns about a baby under 12 months. Those situations are also the ones where a stool sample is most likely to be requested.

A positive result, sometimes reported as "isolated", means a harmful bacterium was detected and named, for example Salmonella or Campylobacter. Lab Tests Online UK says this is most likely the cause of your illness and that most bacterial gut infections in otherwise healthy people clear up by themselves within a week with rest and fluids. Antibiotics are not always needed and, for some infections such as E. coli O157, can increase the risk of complications and are generally avoided.

A negative result, sometimes reported as "not isolated", means no harmful bacteria of the tested types were found in this sample. Lab Tests Online UK says this is reassuring but does not always rule out a bacterial infection, because bacteria can be present in the gut but not found in a single sample. If symptoms continue, your doctor may request another culture and consider other causes such as viral or parasitic infection, food intolerance or a bowel condition such as inflammatory bowel disease.

The NHS lists a number of other causes of diarrhoea: medicines, food intolerance or allergy, irritable bowel syndrome, anxiety, appendicitis, inflammatory bowel disease, coeliac disease, diverticular disease and radiation treatment in the pelvic area. A culture only tests one category of cause.

Several gut infections are notifiable. Lab Tests Online UK says a positive result must be reported to the local public health team as a legal requirement, and that the team may contact you to gather information about where you may have caught the infection to prevent it spreading in the community.

The NHS says to stay off school or work until you have not been sick or had diarrhoea for at least 2 days, and not to use a swimming pool until at least 48 hours after symptoms stop. Lab Tests Online UK adds that if you work in food handling, healthcare or with vulnerable people, or if your child is under 5, you should speak to your GP or the public health team about when it is safe to return.

Practical hygiene is the same whatever the result. The NHS recommends frequent hand washing with soap and water, washing contaminated clothing and bedding separately on a hot wash, cleaning toilet seats, flush handles, taps, surfaces and door handles daily, not preparing food for others if possible and not sharing towels or utensils.

Lab Tests Online UK says the freshness of the sample is the most important factor, because bacteria in stool continue to grow after collection and can change the balance of organisms present. A sample contaminated with urine, toilet water or the inside of a nappy can inhibit bacterial growth. Recent antibiotics can also reduce the chance of detecting infection, so tell your doctor if you have taken any.

If your doctor suspects parasites, the laboratory may test the same sample for ova, cysts and parasites. Lab Tests Online UK says that when this is done by microscopy rather than more sensitive molecular methods, three specimens should be sent at least two days apart because parasites are shed intermittently. If C. difficile is suspected, especially after antibiotics or a hospital stay, a separate toxin test is arranged because it needs different methods.

None of this means the test is unreliable. It means the culture answers a defined question and must be read with your history, travel, medicines and symptoms.