A C. diff test needs far less stool than most people expect, often less than a teaspoon.1Seattle Children’s Hospital / Mayo Clinic Laboratories. Clostridium difficile Antigen and Toxin by EIA, plus Reflex PCR The lab cares more about whether the stool is loose than about how much you send.2Peggy Lillis Foundation. Guest Blog: Q & A With Dr. Gary Hoffman – Peggy Lillis Foundation A small, runny, fresh sample usually beats a large, firm one.
The Amount Labs Ask For
- Preferred amount: about 0.5 grams or 0.5 mL of loose stool at many labs.3Seattle Children’s Hospital / Mayo Clinic Laboratories. Clostridium difficile Antigen and Toxin by EIA, plus Reflex PCR
- Minimum some labs accept: about 0.2 grams or 0.2 mL.4Seattle Children’s Hospital / Mayo Clinic Laboratories. Clostridium difficile Antigen and Toxin by EIA, plus Reflex PCR
- Labcorp: lists 0.5 g or 0.5 mL of liquid stool as its minimum and 5 g as its standard amount.5Labcorp. 086207: Clostridioides difficile Toxins A and B, EIA Information
- Some hospital labs: ask for at least 5 mL of fresh liquid stool.6University of Michigan Medical Laboratories. Clostridioides difficile Detection by PCR and EIA
The test itself uses only a tiny portion, about 25 microliters in some kits, a drop or two.7PubMed Central. Effectiveness of a Two-Step Testing Algorithm for Reliable and Cost-Effective Detection of Clostridium difficile Infection in a Tertiary Care Hospital in Saudi Arabia Larger requested volumes give the lab room to rerun the sample or add tests, not because the test needs that much. Follow the amount your lab or container specifies. If you’re unsure, aim for about a teaspoon to a tablespoon.
Many cups have a fill line, or a scoop built into the lid, sized for the amount that lab wants. Overfilling isn’t needed and just makes the cup harder to seal.
People tend to get this wrong in both directions. A smear on a stick may fall below the 0.2 mL minimum, and too little can mean the lab can’t finish all its steps and asks for a new sample.
If the cup and paperwork give no amount, check the lab’s printed instructions or the label on the cup first, since the container is often designed around what that lab wants. If you can’t find anything, call the lab or the ordering office. It takes a minute and can prevent a recollection.
Why the Stool Has to Be Loose
Labs usually test only watery or soft stool, which lab instructions call unformed stool.8Peggy Lillis Foundation. Guest Blog: Q & A With Dr. Gary Hoffman – Peggy Lillis Foundation Semi-formed stool that takes the shape of the container is often accepted.9Peggy Lillis Foundation. Guest Blog: Q & A With Dr. Gary Hoffman – Peggy Lillis Foundation10Seattle Children’s Hospital / Mayo Clinic Laboratories. Clostridium difficile Antigen and Toxin by EIA, plus Reflex PCR Formed or hard stool is usually refused when sent for C. diff testing.11Peggy Lillis Foundation. Guest Blog: Q & A With Dr. Gary Hoffman – Peggy Lillis Foundation12University of Michigan Medical Laboratories. Clostridioides difficile Detection by PCR and EIA Expert guidance is to test only loose stool in people with real diarrhea.13PubMed. Laboratory diagnosis of Clostridium difficile infections: there is light at the end of the colon
A simple check: if it pours or spreads to fill the bottom of the cup, it qualifies. If it holds a shape like a log, the lab will likely reject it. Some hospital labs make exceptions for specific patients, such as bone marrow transplant patients, but only when the doctor says so.14University of Michigan Medical Laboratories. Clostridioides difficile Detection by PCR and EIA
The people best suited for testing have 3 or more new, unexplained loose stools in 24 hours.15Clinical Infectious Diseases. Clinical Practice Guidelines for Clostridium difficile Infection in Adults and Children Many healthy people carry C. diff without being sick, so a positive result on firm stool would not mean infection.16CDC. Clinical Testing and Diagnosis for C. diff Infection If your stool has firmed up, tell your doctor before collecting rather than sending it.
If diarrhea stops before you collect, testing may no longer be needed. Call the office instead of waiting for a later loose stool days away.
Soft stool that slumps and takes the shape of the cup is the borderline case, and many labs accept it. If you’re unsure, collect it anyway and note its consistency on the form, or ask the lab at drop-off, rather than throwing away a possibly acceptable sample. Never mix water or anything else into a firm stool to make it look looser; that spoils the sample.
How to Collect the Sample Cleanly at Home
The sample goes in a clean, watertight, screw-capped container with no preservative.17University of Michigan Medical Laboratories. Clostridioides difficile Detection by PCR and EIA18Minnesota Department of Health. Clostridioides difficile Toolkit for Long-term Care Facilities: Lab Tests Added transport liquid isn’t needed and may raise the chance of a false positive.19Minnesota Department of Health. Clostridioides difficile Toolkit for Long-term Care Facilities: Lab Tests Some labs do use special preserved swab kits that give results equal to plain stool, so if your lab gives you one, use it as directed.
A small amount of urine mixed in is acceptable.20infectioncontrol.ucsfmedicalcenter.org. CDI Frequently Asked Questions A lot of urine can dilute the sample too much to detect C. diff.21infectioncontrol.ucsfmedicalcenter.org. CDI Frequently Asked Questions Try to pass urine before collecting stool, and keep toilet water out.
Start by placing a collection hat, or plastic wrap, across the toilet seat, with the wrap sagging slightly under the back. Pass urine first, then have the bowel movement into the catcher. Use the lid scoop or a disposable spoon to move loose stool into the cup. Close it tightly, wash your hands well, and label it with name, date, and time.
A food jar or an old specimen cup can carry residue that interferes with the test. Use the sterile cup supplied by the lab or clinic, and keep your fingers off the inside of the cup and lid.
If the stool lands in the bowl with no catcher, don’t scoop it from the water. Wait for the next bowel movement and use a catcher. Stool that has soaked in toilet water or cleaner is diluted and may be rejected, so keep toilet-bowl cleaner and disinfectant tablets out of the toilet until you’ve collected.
C. diff spores survive ordinary hand sanitizer, so wash with soap and water. Clean the toilet area afterward with a bleach-based cleaner, and bag the used catcher and spoon in the trash.
Collecting From a Baby or Toddler in Diapers
Line the diaper with plastic wrap placed so urine runs into the diaper, not the wrap.22Alberta Health Services. Patient Instructions Enteric Bacterial Panel and C. difficile Stool Collection Pull the wrap loosely across the diaper rather than tight, so it doesn’t push stool out to the sides. For runny stool, turning the diaper so its plastic side faces the skin keeps the stool from soaking in.23Nationwide Children’s Hospital. Stool Collection Guidelines Then scoop the stool from the plastic into the container with a disposable spoon or stick.24Alberta Health Services. Patient Instructions Enteric Bacterial Panel and C. difficile Stool Collection Most labs do not accept a stool-soiled diaper as the sample.25Alberta Health Services. Patient Instructions Enteric Bacterial Panel and C. difficile Stool Collection
Leave the diaper on long enough to collect enough stool,26Nationwide Children’s Hospital. Stool Collection Guidelines and check every few minutes so it isn’t absorbed. A few drops is often enough, since minimums can be 0.2 mL. It’s also worth asking the doctor whether testing is useful at all, since young babies often carry C. diff without illness.
To move the stool, use the disposable spoon or stick from the kit, or a clean plastic spoon, and scrape it from the plastic into the cup without touching the cup’s sides. Choose the runniest portion, and avoid scooping up diaper cream or gel, which can interfere with the sample.
Getting the Sample to the Lab in Time
Store the sample in the refrigerator, about 2 to 8 °C, until it’s dropped off.27Minnesota Department of Health. Clostridioides difficile Toolkit for Long-term Care Facilities: Lab Tests Some labs require the sample within 24 hours of collection.28University of Michigan Medical Laboratories. Clostridioides difficile Detection by PCR and EIA Refrigerator storage barely affects the toxin, and does better than freezing.29PubMed Central. Pragmatic Strategy for Fecal Specimen Storage and the Corresponding Test Methods for Clostridioides difficile Diagnosis30PubMed. Stability of Clostridioides difficile toxins in stool samples
Leaving it out does matter. In one hospital, samples left unrefrigerated over 13 hours tested positive less than half as often, 5% versus 13%.31PubMed. Does the handling time of unrefrigerated human fecal specimens impact the detection of Clostridium difficile toxins in a hospital setting? A sample left warm overnight could therefore give a falsely reassuring negative. If you can’t deliver it the same day, refrigerate it in a sealed bag away from food and call the lab.
At home, put the sealed cup in a zip bag, then in a second bag. Don’t freeze it unless the lab tells you to.
Labs check the 24-hour limit, so make sure the label shows the collection date and time, and bring the requisition form along. Unlabelled samples are commonly rejected.
Plan the drop-off before you collect: check the lab’s hours, whether it takes samples on weekends, and whether your clinic collects them. If the lab is closed when you finish, refrigerate right away and deliver first thing the next morning instead of leaving it on the counter.
A short trip at room temperature is fine. For a long trip, a small cooler with a cold pack keeps it cool without freezing it. Keep the bag upright so the lid doesn’t leak.
What the Lab Does With Your Sample
Many labs first run a test for the germ alongside a toxin test, listed on reports as EIA, which looks for the toxins C. diff makes.32Seattle Children’s Hospital / Mayo Clinic Laboratories. Clostridium difficile Antigen and Toxin by EIA, plus Reflex PCR33Practical Gastro. Ileostomy and C. difficile Infection If the germ is found but toxin isn’t, the sample goes to PCR, a DNA test that’s more sensitive.34Seattle Children’s Hospital / Mayo Clinic Laboratories. Clostridium difficile Antigen and Toxin by EIA, plus Reflex PCR Some labs use the DNA test first, or a 2- to 3-step sequence.35PubMed. Laboratory diagnosis of Clostridium difficile infections: there is light at the end of the colon
Labs combine tests because each has a weakness. The toxin test can miss small amounts, so a negative doesn’t always settle the question. The DNA test rarely misses the germ, but it can’t tell whether the germ is making toxin or just present.
Because of this, someone with another cause of diarrhea who carries C. diff can be over-diagnosed and treated.36CDC. Clinical Testing and Diagnosis for C. diff Infection37CDC. Clinical Testing and Diagnosis for C. diff Infection A report showing DNA positive but toxin negative is worth discussing with your doctor.
That result means the germ’s genes were found but no toxin was detected. You may carry C. diff rather than have an infection, and your doctor weighs your symptoms and recent antibiotics to decide. It isn’t a lab mistake, or a reason to resubmit.
Both tests are fairly quick once the lab runs them, and results often come back the same day or within a day or two. It can take longer when the lab batches samples or runs extra steps, so ask when and how you’ll be told.
Things That Can Make a Test Misleading
Many people tested for C. diff had recently taken laxatives.38PubMed. Laxative use and testing for Clostridium difficile in hospitalized adults: An opportunity to improve diagnostic stewardship Among people on laxatives, those testing positive looked no sicker than those testing negative, which suggests many false positives.39PubMed. Evaluation of a best practice alert to reduce unnecessary Clostridium difficile testing following receipt of a laxative Some hospitals now warn doctors when a laxative was given within the last 48 hours.40PubMed. SEE the DIFFerence: Reducing unnecessary C. difficile orders through clinical decision support in a large, urban safety-net system Before collecting, tell your doctor about any laxative, stool softener, or bowel prep in the past two days. A laxative produces loose stool that meets the lab’s rule even without infection, and if the person carries C. diff, the DNA test may look like an infection.
Beyond obvious laxatives, mention stool softeners, fiber powders taken for constipation, magnesium products, and colonoscopy bowel preparations. Say when you took them and for how long, since that helps the doctor decide whether to wait.
Other everyday things loosen stool too. Antibiotics themselves often cause mild diarrhea without C. diff, and tube feeds or new medicines can do the same. Telling the doctor helps decide whether testing makes sense.
Should You Send a Second Sample
About 91% of people were correctly diagnosed from one stool sample.41PubMed. Repeat stool testing to diagnose Clostridium difficile infection using enzyme immunoassay does not increase diagnostic yield Repeats added only about 5% to 9% more positives.42PubMed Central. Diagnostic yield of repeat sampling with immunoassay, real-time PCR, and toxigenic culture for the detection of toxigenic Clostridium difficile in an epidemic and a non-epidemic setting Repeats within 2 days of a negative were positive only 1% to 4% of the time.43PubMed Central. Evaluation of Repeat Clostridium difficile Enzyme Immunoassay Testing That’s partly why many labs accept no more than one sample per 7 days.44University of Michigan Medical Laboratories. Clostridioides difficile Detection by PCR and EIA
If your first result is negative and your symptoms are mild or improving, a quick repeat is unlikely to find anything new and may be rejected under the one-per-week rule. If you still feel unwell, tell your doctor the diarrhea is continuing, because other causes may need to be looked into, such as other infections, medicines, or conditions. A second sample does make sense with new or worsening diarrhea a week or more later, with symptoms returning after treatment, or when your doctor asks because the first sample was rejected.
After a positive, tests often stay positive for days even as you recover, still 14% at 7 to 10 days.45PubMed Central. Evaluation of Repeat Clostridium difficile Enzyme Immunoassay Testing Retesting to prove a cure isn’t useful, so watch your symptoms instead.
When a Rectal Swab Can Replace a Stool Sample
For DNA testing, a dry rectal swab can substitute for stool.46PubMed Central. Evaluation of the Use of Rectal Swabs for Laboratory Diagnosis of Clostridium difficile Infection Swabs and stool found C. diff in nearly the same people, about 95% versus 94% of true infections.47PubMed Central. Comparison of rectal swabs and fecal samples for the detection of Clostridioides difficile infections with a new in-house PCR assay If you can’t produce a sample, ask whether your clinic offers a swab.
A nurse gently inserts a cotton-tipped swab just inside the rectum. It takes seconds and is mildly uncomfortable, not painful. It’s usually done in a clinic or hospital, not at home, and not every lab accepts swabs.
It helps people who can’t produce stool on request, such as those who are constipated, dehydrated, or very unwell. It may also get the doctor a result sooner when waiting for a loose stool would delay a decision.
A swab is run through the DNA test, so it carries the same limitation: it may show carrying rather than infection. And because it isn’t a stool sample, it may not fit a lab’s step-by-step process that begins with a toxin test.