What it is: A stool culture detects and identifies bacteria causing lower digestive tract infections, distinguishing disease-causing (pathogenic) bacteria from the normal flora that naturally lives in the gut.
Why it's ordered: Providers typically order it for prolonged diarrhea (>3 days), bloody diarrhea, mucus in stool, abdominal pain/cramping, or nausea — especially with recent travel or a shared illness pattern with someone close to the patient. It's often paired with a broader PCR-based GI pathogens panel or a separate ova-and-parasite exam.
What it detects: Bacteria like Campylobacter, Salmonella, Shigella, and Vibrio cholerae, plus certain parasites (Giardia, Cryptosporidium). C. difficile is usually tested separately via toxin/antigen assay rather than standard culture, since it often overgrows after antibiotics suppress competing normal flora.
Collection & prep: Fresh sample collected in a clean container, free of urine/water contamination; must reach the lab within ~2 hours or be placed in a preservative vial, since delayed/unpreserved samples distort bacterial proportions. Patients should avoid barium or bismuth for 7–10 days before collection; otherwise no special prep is needed.
Negative – no pathogens found; symptoms may be non-infectious
Inconclusive – may require repeat testing
Possible complications of bacterial GI infection: Dehydration/electrolyte imbalance from prolonged diarrhea (risk highest in children/elderly); rarely, hemolytic uremic syndrome (red blood cell destruction and kidney failure) from certain toxin-producing strains.
What it is: A fecal occult blood test (FOBT) checks a stool sample for blood that's invisible to the eye, using either a chemical (guaiac) reaction or an immunochemical (FIT) method that targets human hemoglobin directly.
Why it's ordered: Most often used for routine colorectal cancer screening in average-risk adults ages 45–75, and to evaluate unexplained anemia, suspected GI bleeding, or to help distinguish IBS from inflammatory bowel disease (which is more likely to bleed).
What it detects: Hidden blood that may point to colon polyps, colorectal cancer, hemorrhoids, anal fissures, or ulcers. FIT detects human hemoglobin specifically and doesn't pick up upper-GI bleeding, since hemoglobin breaks down during digestion before reaching the lower tract.
Collection & prep: An at-home kit collects 2–3 stool samples smeared on test cards. The older guaiac-based FOBT usually requires avoiding red meat, certain vegetables, and NSAIDs for 48–72 hours beforehand to prevent false results; FIT doesn't require dietary changes. Testing should be avoided during active menstruation or bleeding hemorrhoids.
Results:
Positive – blood detected; warrants follow-up, typically a colonoscopy, to find the source
Negative – no blood detected in the sample(s) tested
Possible complications of colorectal bleeding: An FOBT is a screening tool, not a diagnosis — it can't say where blood is coming from, so a positive result leads to further workup. Guaiac tests are also prone to false positives/negatives from diet and medications, which is part of why FIT has largely replaced them for population screening.
What it is: A fecal fat test measures the amount of fat in stool to detect steatorrhea (excess fat excretion), using either a quick qualitative stain (Oil Red O) or a quantitative 72-hour timed collection.
Why it's ordered: Ordered for symptoms of malabsorption — persistent diarrhea, greasy or foul-smelling floating stools, unexplained weight loss, or fat-soluble vitamin deficiencies (A, D, E, K) — and to work up suspected pancreatic exocrine insufficiency, celiac disease, Crohn's disease, short bowel syndrome, or bacterial overgrowth.
What it detects: Excess fecal fat, which signals a breakdown somewhere in fat digestion — pancreatic enzyme production, bile acid emulsification, or intestinal absorption — without identifying which stage failed.
Collection & prep: The quantitative version requires a 72-hour timed stool collection while eating a standardized ~100g/day fat diet for 2–3 days before and during collection; a dietary log is used to calculate the coefficient of fat absorption. The qualitative version uses a single random sample.
Results:
Elevated – typically >7g fat/24 hours; suggestive of malabsorption but doesn't identify the cause
Normal – doesn't fully exclude malabsorption, especially if diet wasn't followed correctly during collection
Possible complications of fat malabsorption: Untreated malabsorption can lead to fat-soluble vitamin deficiencies and unintended weight loss. The test itself is burdensome — the multi-day collection and dietary requirements are cumbersome, which can affect patient compliance and result accuracy, and abnormal results usually prompt further testing (e.g., pancreatic elastase, imaging) to pinpoint the cause.
What it is: An ova and parasite (O&P) exam is a microscopic examination of stool, in which a lab professional looks for parasites and their eggs (ova) or cysts on stained slides.
Why it's ordered: Typically ordered for persistent or recurrent diarrhea lasting more than 7–10 days, especially after travel to endemic areas, camping, or exposure to untreated water; also used for immunocompromised patients or during daycare/household outbreaks with known parasitic exposure. Often paired with a GI pathogens panel, stool culture, or targeted antigen tests.
What it detects: Helminths (eggs and larvae of roundworms, whipworms, hookworms, tapeworms) and protozoa (cysts/trophozoites of organisms like Giardia and Entamoeba). Some parasites — Cryptosporidium, Cyclospora, Microsporidia — are shed in forms that need separate antigen tests, acid-fast stains, or PCR.
Collection & prep: Usually 2–3 separate stool samples collected on different days, since parasites are shed intermittently and a single sample only catches roughly 60% of infections versus over 95% with three. Samples must avoid contamination with urine or water, and recent barium, antidiarrheals, or antibiotics can interfere with results.
Results:
Positive – parasite(s) identified, with genus/species and life-cycle stage noted
Negative – no parasites seen; doesn't fully rule out infection, particularly if only one sample was tested
Possible complications of parasitic GI infection: Untreated infections can cause chronic diarrhea, malabsorption, and nutrient deficiencies. Because single-sample testing can miss intermittent shedders like Giardia, persistent symptoms after a negative result often warrant repeat O&P testing or a targeted PCR/antigen panel.
What it is: A stool virus test detects viruses that cause gastroenteritis, most commonly via a rapid antigen test for rotavirus or norovirus, or a broader PCR-based panel that can also cover adenovirus, sapovirus, and astrovirus.
Why it's ordered: Ordered to confirm the cause of acute diarrhea and vomiting, especially in young children, hospitalized patients, or during suspected outbreaks (daycares, cruise ships, nursing homes); often used alongside stool culture or O&P testing to rule out bacterial or parasitic causes.
What it detects: Rotavirus and norovirus are the two most common targets of rapid stool tests; PCR panels can additionally identify adenovirus, sapovirus, and astrovirus, which don't have quick standalone tests.
Collection & prep: A fresh stool sample is collected in a clean container per the lab's kit instructions; no dietary or medication restrictions are typically required beforehand.
Results:
Positive – virus identified (e.g., rotavirus particles seen or a PCR match), confirming viral gastroenteritis
Negative – doesn't rule out a viral cause entirely, since rapid tests don't cover every gastroenteritis virus
Possible complications of viral gastroenteritis: Antibiotics aren't effective against viruses, so treatment is supportive; the main risk is dehydration from fluid loss, which can be serious in young children, older adults, and the immunocompromised and may require rehydration therapy.
Statements made in the Clinical Manual have not been evaluated by the FDA. The products mentioned are not intended to diagnose, treat, cure or prevent any disease.
