How to Collect a Stool Sample: Amount, Storage, Results

What a stool test actually is

Stool, faeces, poo, bowel motion, “number two” — the report may say any of these, and in the US you’ll usually see stool sample where the NHS says poo sample. They all mean the same thing: fresh bowel movement collected into a clean, sealed pot.

What varies enormously is what gets done with it. “Stool test” is not one test but a family of them running off a single specimen, and the preparation rules and the amount required change depending on which one your doctor ordered. That is worth checking before you start, because a sample collected correctly for one test can be useless for another.

When a stool test gets ordered

• Diarrhoea lasting more than three days, or coming back repeatedly
• Visible blood, mucus or pus in your poo
• Black tarry stools, or the opposite — pale, clay-coloured ones
• Cramping, bloating, excessive wind, persistent nausea
• Unexplained weight loss, fatigue, or anaemia found on a blood test
• Symptoms that began after foreign travel or a shared meal
• In children: night-time restlessness, an itchy bottom, poor weight gain
• Bowel cancer screening, even with no symptoms at all

The tests that run off one sample

Several of these can be run from one pot, but culture, parasitology and calprotectin want different storage conditions. That is why a lab sometimes hands you two containers. Check the label on each — they are not interchangeable.

How to collect a stool sample: step by step

1. Step 1
   Pee first. Urine mixed into the sample is the single most common reason a lab rejects one. It dilutes the specimen and interferes with both microscopy and occult-blood testing.
2. Step 2
   Put something in the toilet to catch it. The easiest method is cling film stretched loosely across the rim under the seat, dipping slightly in the middle. A sheet of newspaper, a clean disposable tub, or a cardboard "collection hat" from the pharmacy all work. The point is that the poo never touches the water.
3. Step 3
   Glove up and have the pot open. Most stool pots have a small spoon or spatula built into the lid — get it out before you need it.
4. Step 4
   Take from several places. Not one scoop from the middle. If any part looks bloody, slimy, watery or oddly coloured, include a piece of that too. Parasites and bacteria are not spread evenly through a stool — what the microscope needs may sit in the bit you skipped.
5. Step 5
   Fill it about a third. A walnut, roughly a teaspoon. For watery stool, 5-10 mL. Do not fill to the top: an overfilled pot leaks past the seal, and a leaking pot is biohazard waste, not a specimen.
6. Step 6
   Screw the lid on tightly and wipe the outside. Clean any smears off the pot with damp tissue, then bag it.
7. Step 7
   Label it. Full name, date of birth, and the date and time you collected it. The time matters: the lab uses it to decide which tests are still valid on that specimen.
8. Step 8
   Bin the rest properly. Cling film, paper and gloves go in a tied plastic bag in the bin — not down the toilet, where they block drains.
9. Step 9
   Wash your hands for at least 20 seconds. Most gut infections spread by the faecal-oral route; this step is what keeps the rest of the household out of it.

How much stool is needed, test by test

Don’t let the size of the pot mislead you. Standard stool containers hold 30-60 mL, and no test asks you to fill one.

The one exception to the one-third rule is the 72-hour fat collection: there you keep everything you pass over three days, refrigerated, in the large container the lab provides. Some US labs also ask for up to two tablespoons for certain panels — if your container has a fill line, that line wins over any rule of thumb.

FIT kits work differently

The bowel-screening kit that arrives in the post is not a pot. It is a slim tube of preservative liquid with a grooved stick attached to the cap:

1. Catch the stool on cling film or in a dry container; it must not touch the water.
2. Dip the stick into three to six different spots — filling the grooves is enough, don’t cake it on.
3. Push the stick back into the tube and click the cap shut; the excess is wiped off on the way in.
4. Invert the tube a few times to mix, and write the date on it.

The big advantage of this test is that no special diet is needed. Older guaiac kits (Hemoccult) required three days of avoiding red meat, horseradish, broccoli and vitamin C. The immunochemical test reacts only to human haemoglobin, so those restrictions are gone.

How long does a stool sample keep?

The rule is simple: fresher is better. A sample that reaches the lab within one to two hours is fine for any test. If it can’t, these are the limits.

In transit, bag the pot, keep it out of direct sun and never leave it in a parked car. Interior car temperatures pass 50 °C (120 °F) within an hour in summer heat — enough to shift the bacterial balance and to degrade a calprotectin measurement.

Preparing for the test: food, fasting, medicines

You do not need to fast. This is not a blood test; eating normally changes nothing. What does interfere:

• Antibiotics: wait 7-10 days after the last dose before a culture or parasite exam. Sampling while on antibiotics gives a clean-looking result and a problem that doesn’t go away.
• Antiparasitic drugs (metronidazole, albendazole and similar): same 7-10 days.
• Barium studies (barium enema, upper GI series): two weeks. Barium crystals blanket the slide and hide everything under them.
• Bismuth preparations, mineral oil, laxatives, antidiarrhoeals: 7-10 days.
• Antacids: 3-7 days.
• Enemas and bowel-prep solutions: don’t sample afterwards — the washed-out stool is unsuitable for both culture and parasitology.

Tell the doctor who ordered the test about every medicine and supplement you take. If stopping something isn’t safe for you, they will plan around it — don’t stop prescribed medication on your own.

Collecting a sample from a baby or child

For babies the sample comes straight out of the nappy, but three details decide whether it is usable:

• Keep urine out of it. Take from the middle of the stool, not from the wet area, and check the nappy often so you catch it fresh.
• Avoid the absorbent gel. Modern nappies wick liquid away; once watery stool is in the gel, parasitology and pH testing can’t be done. If you’re expecting loose stool, turn the nappy inside out so the plastic backing faces the skin, or lay a piece of cling film inside it.
• The cling film trick — a palm-sized square inside the nappy — keeps the sample clean and preserves the volume.

Potty-trained children follow the adult method. If a potty is used, wash and rinse it thoroughly first; leftover detergent invalidates the sample.

If threadworm (pinworm) is suspected, the test is not a stool pot at all but a tape test: clear tape pressed against the skin around the anus first thing in the morning, before washing or opening the bowels.

How many samples will you need?

One is usually enough — with three exceptions.

• Parasite exams: parasites are shed intermittently, so a single specimen misses them often. The standard is three separate samples, every other day, within a 10-day window. This lifts the detection rate substantially over one sample.
• Faecal fat: a full 72-hour collection while eating a normal diet of roughly 100 g fat per day. Eating low-fat during the collection makes an abnormal result look normal.
• Travel-related diarrhoea that persists: your doctor may ask for several samples spread over 7-10 days, since the organism can appear and disappear.

Where amoebic infection is suspected, the first sample being fresh and still warm genuinely matters — the motile trophozoite form breaks apart as the specimen cools, leaving only cysts to be seen. How that illness behaves and how it is treated is set out in our guide to recognising and treating amoebic dysentery.

When do results come back?

Culture is slow because the lab is genuinely waiting for growth: the sample is plated onto selective media, incubated at 35-37 °C for about 48 hours, and any colonies that appear are then identified and, if needed, tested against antibiotics. That is the answer to “why has my result taken a week?”

Reading the report: normal ranges

Treat the figures below as orientation. Every lab prints its own reference range beside your result and that is the one your value should be read against.

One caveat worth repeating: a single abnormal stool value is not a diagnosis. Calprotectin rises with regular NSAID use as well as with inflammatory bowel disease; occult blood can turn positive from swallowed blood after a nosebleed. Your result gets read alongside your symptoms, examination and other tests.

What colour and consistency tell you

Consistency is scored worldwide on the Bristol Stool Scale: types 1-2 are hard and lumpy and mean constipation, types 3-4 are the ideal, and types 5-7 are increasingly loose and mean rapid transit or diarrhoea. If your report says “type 6”, that is the scale it refers to.

On colour:

• Black and tarry: possible bleeding from the stomach or small bowel — get this assessed promptly. Iron tablets, bismuth medicines and large amounts of blueberries also darken stool, so mention what you are taking.
• Bright red: bleeding from the colon, rectum or haemorrhoids. Beetroot and tomato juice can mimic it.
• Pale, clay-coloured: bile is not reaching the bowel — think bile duct obstruction.
• Yellow, greasy, floating, strongly smelling: fat is not being absorbed — coeliac disease, pancreatic insufficiency, giardia.
• Green: usually harmless; fast transit or a lot of leafy greens.
• Mucus-covered: irritation of the bowel lining, from inflammatory bowel disease or infection.

Bowel cancer screening: who gets a kit

In England, everyone aged 50 to 74 registered with a GP is sent an NHS FIT kit by post every two years; the positivity threshold is 120 µg Hb/g in England and Northern Ireland and 80 µg Hb/g in Scotland and Wales. In the United States, screening now starts at 45 and runs to 75, with an annual FIT being one of the accepted options for average-risk adults.

Why this matters: bowel cancer can spend years as a polyp without producing a single symptom. What a FIT often catches is not the cancer at all but a polyp that gets removed before it ever becomes one.

Elsewhere the thresholds differ slightly — Germany offers a free iFOBT every two years from 50, France posts kits to 50-74 year-olds and also hands them out free in pharmacies, and Turkey screens 50-70 year-olds every two years at family health centres.

Why results come back wrong

When a lab asks for a repeat specimen, or treatment doesn’t fix the symptoms, the cause is usually one of these eight:

1. Urine mixed into the sample
2. Specimen taken out of the toilet bowl
3. Pot filled to the brim and leaking
4. Sample left at room temperature for hours
5. Culture collected while on antibiotics
6. Parasite sample taken soon after a barium study
7. Only one sample submitted for a parasite exam
8. Occult-blood test done during a period or with bleeding piles

Every one of those happens at the collection stage — which is to say, in your hands. Reading this list once, on the day you bring the pot home, is what saves you from doing the test twice.

Frequently asked questions

Do I need to fast before a stool test? No. Eat normally. The only dietary rule is in the 72-hour fat test, where you keep to the fat intake your doctor specifies.

How much poo is needed for a stool sample? A walnut-sized piece, roughly a teaspoon (5 g), or 5-10 mL if it is liquid. Fill no more than a third of the pot.

Should the container be filled to the top? No — the opposite. An overfilled pot leaks past the seal, contaminates the outside and may be rejected by the lab.

Can I collect a sample the night before? For culture and calprotectin, usually yes if it is refrigerated at 2-8 °C. For parasite testing, no — bring a fresh morning sample.

Can I do a stool test during my period? Microscopy and culture are fine. An occult-blood or FIT test should wait until three days after bleeding stops.

What if toilet paper gets into the sample? Paper fibres obstruct microscopy and some papers can interfere with occult-blood chemistry, so the sample needs collecting again.

Are home occult-blood kits reliable? Immunochemical kits are reliable when used correctly, but they only show that bleeding is happening. A positive result always goes to a doctor and is investigated by colonoscopy.

How long do stool test results take? Microscopy and FIT come back same day, calprotectin in 1-3 days, culture in 48-72 hours, plus another day if sensitivity testing is needed.

Can a stool sample be frozen? Not if a culture is wanted — freezing kills the bacteria and produces a false negative. Freeze only when the lab has specifically instructed it.

Sources

1. NHS — How to collect a sample of poo (stool sample)
2. Memorial Sloan Kettering Cancer Center — How to Collect a Stool Sample Using a Stool Collection Kit
3. Nationwide Children’s Hospital — Stool Collection Guidelines
4. Medscape — Stool Ova and Parasite Test: Reference Range, Interpretation, Collection
5. Cancer Research UK — Bowel cancer screening
6. Labcorp — Ova and Parasites Examination: collection information

This article is for information only and does not replace a medical consultation. Visible blood in your stool, diarrhoea lasting more than three days, abdominal pain with fever and unexplained weight loss all need prompt medical assessment.