Stool Culture Test: Purpose, Procedure & Results

What Is a Stool Culture?

A stool culture is a laboratory test that examines a sample of your stool to look for harmful bacteria that can cause gastrointestinal infections. Doctors in the UK and US often order this test when a patient has severe, prolonged, or bloody diarrhea — especially after recent travel, a known foodborne outbreak, or exposure to someone with similar symptoms. In my clinical practice, I typically see two groups of patients who need this test: those with acute diarrheal illness that doesn’t settle in 2–3 days, and those with bloody stools, fever, or severe cramping.

Why Is a Stool Culture Ordered?

Common reasons for ordering a stool culture include:

• Diarrhea that lasts more than 2–3 days
• Blood or mucus in the stool
• High fever (above 38.5°C)
• Severe abdominal pain or cramping
• Suspected food poisoning (e.g., after eating undercooked poultry, eggs, or unpasteurized dairy)
• Recent travel to regions with poor sanitation
• Outbreak investigation (e.g., in a daycare, nursing home, or cruise ship)
• History of antibiotic use (to rule out Clostridioides difficile)

The test helps identify the specific bacterium causing your infection, which guides appropriate antibiotic treatment if needed.

Common Bacteria Detected by Stool Culture

The most frequently isolated pathogens include:

• Salmonella (from eggs, poultry, reptiles)
• Shigella (person-to-person spread, poor hygiene)
• Campylobacter (undercooked poultry, unpasteurized milk)
• Shiga toxin-producing E. coli (STEC, e.g., O157:H7) — from contaminated beef or leafy greens
• Yersinia enterocolitica (undercooked pork, contaminated water)
• Vibrio (raw or undercooked seafood, especially oysters)
• Clostridioides difficile (often after antibiotic use; requires a specific toxin test, but stool culture can help)

How Is the Stool Culture Test Performed?

You will be given a clean, dry container (often a sterile tube or wide-mouthed jar) and instructions to collect a small sample of stool. The sample should ideally be fresh — passed that same day. Avoid mixing urine or toilet paper with the stool. In my practice, I always tell patients: "Collect the sample first thing in the morning if possible, and bring it to the lab within an hour." If you can’t deliver it promptly, refrigerate it (do not freeze) for up to 24 hours.

In the lab, the sample is placed on various selective culture media that encourage bacterial growth while suppressing normal flora. After 24–48 hours of incubation, technologists identify any suspicious colonies using biochemical tests, gram staining, and sometimes mass spectrometry (MALDI-TOF). If a pathogen is found, antibiotic sensitivity testing (antibiogram) is performed to determine which antibiotics will work best.

Stool Culture Reference Ranges

Interpreting Stool Culture Results

Negative (Normal) Result

A negative result means no harmful bacteria were found. However, it does not completely rule out infection — viruses (like norovirus), parasites (like Giardia), or non-infectious causes (like inflammatory bowel disease) could still be responsible for your symptoms.

Positive Result

If a pathogenic bacterium (e.g., Salmonella, Shigella, Campylobacter) is isolated, the test is considered positive. Your doctor will decide whether antibiotics are necessary based on the bacteria identified, your immune status, and the severity of illness. Many bacterial diarrheas resolve without antibiotics, but for some infections (like Shigella or invasive Salmonella) treatment is recommended.

Is a Positive Stool Culture Dangerous?

Most healthy people recover from bacterial gastroenteritis within a week without lasting harm. However, certain infections (e.g., E. coli O157:H7 or Shigella) can cause serious complications like hemolytic uremic syndrome (HUS) or severe dehydration. In my practice, I pay close attention to children under 5, elderly patients, and those with weakened immune systems — these groups are at higher risk for complications.

Stool Culture During Pregnancy

Stool culture is safe during pregnancy and is sometimes necessary if you develop severe diarrhea, bloody stools, or signs of dehydration. The test itself poses no risk to the fetus because it only requires a stool sample. If a bacterial infection is found, your obstetrician can select pregnancy-safe antibiotics — avoid drugs like tetracyclines or fluoroquinolones if possible. Untreated severe gastroenteritis can harm the baby through dehydration or maternal fever, so it's important to get tested if symptoms are concerning.

Limitations of Stool Culture

• It only detects bacteria; viruses and parasites are missed. Additional tests (e.g., ova and parasite exam, viral PCR) may be needed.
• False negatives can occur if you've already taken antibiotics, if the sample is not fresh, or if the pathogen is fastidious (e.g., Campylobacter requires special culture conditions).
• Results take 2–4 days; in emergencies, rapid molecular tests (PCR) may be used for quicker answers.
• Normal gut flora is not reported, except to confirm that the sample was adequate for culture.

How to Prepare for a Stool Culture

No special preparation is needed — you do not need to fast or change your diet. However, if you are taking antibiotics, inform your doctor, as this can affect the result. It may be necessary to stop antibiotics for 2–3 days before collecting the sample (if safe to do so).

When to Call Your Doctor

Contact your healthcare provider if you have:

• Diarrhea that lasts more than 3 days without improvement
• Blood, mucus, or pus in your stool
• A fever above 38.5°C (101°F)
• Signs of dehydration: dry mouth, dark urine, dizziness, or decreased urination
• Severe abdominal pain that does not go away
• Recent travel to a high-risk area or contact with someone who has similar symptoms