Clostridioides difficile (C. diff) Testing: Understanding Your Report

Section Editor: Rodney E. Rohde PhD
June 4, 2026

Clostridioides difficile, usually shortened to C. diff, is a bacterium that can cause diarrhea and inflammation of the colon (the large intestine). A bacterium is a microorganism, a living thing too small to see without a microscope. C. diff illness often develops after a course of antibiotics, because antibiotics can disturb the normal balance of bacteria in the gut and allow C. diff to grow and release toxins, which are the harmful substances that cause the symptoms.

This article explains the different tests used to check for C. diff, what the wording on your report means, and why a positive test must be read together with your symptoms, so you can better understand the report you have received.

What is C. difficile, and why is testing done?

C. diff is found in the environment and in the bowel of some healthy people. It causes illness mainly when it grows out of balance and produces toxins, leading to diarrhea and sometimes a more serious inflammation of the colon called colitis. This often happens during or after a course of antibiotics. C. diff is usually tested for separately from a routine stool culture.

One point is central to understanding any C. diff result: the bacterium can live in the gut without causing illness, a state called colonization or carriage. Because of this, testing is done only in people who have diarrhea (loose or watery stool). Testing someone without diarrhea, or testing a formed stool sample, tends to find harmless carriage rather than a true infection, which can lead to unnecessary treatment.

The different C. diff tests

Several tests exist, each looking for different things, which is why your report may mention more than one.

Toxin test — Looks for the toxins that C. diff produces. Because toxins cause illness, a positive toxin test supports an active infection. This test can, however, miss some true infections.
GDH test — Looks for a substance called glutamate dehydrogenase (GDH) that C. diff produces. It is very good at detecting whether C. diff is present, but it cannot tell whether the bacterium is making toxin, so it is used mainly as a first screen.
Molecular (NAAT or PCR) test — Looks for the gene that allows C. diff to produce toxin, using the methods described in the article on PCR, antigen, and antibody tests. It is very good at detecting toxin-producing C. diff, but because it detects the gene rather than the toxin itself, it can be positive even when the bacterium is only being carried and is not causing illness.

Why more than one test is often used

No single test is perfect, so laboratories often combine tests in a series. A common approach starts with the GDH test, which reliably shows whether C. diff is present at all:

If GDH is negative — C. diff is unlikely, and the result is reported as negative.
If GDH is positive, a toxin test is done — If the toxin test is also positive, an active infection is likely.
If GDH is positive but the toxin test is negative — The result is unclear, and a molecular (NAAT) test is often used to settle it.

This stepwise approach helps separate a true, toxin-producing infection from simple carriage.

How to read your result

The combinations below cover the most common patterns. In every case, the result is interpreted in light of whether you have diarrhea.

Negative (GDH or molecular test negative) — C. diff was not found, so it is unlikely to be the cause of your symptoms.
GDH positive and toxin positive — Toxin-producing C. diff is present and active, which supports a C. diff infection in someone with diarrhea.
GDH positive, toxin negative, molecular test positive — Toxin-producing C. diff is present, but the toxin itself was not detected. Whether this is a true infection or carriage depends on your symptoms.
Molecular (NAAT) test positive on its own — Toxin-producing C. diff is present. Because this test can be positive in carriage as well as in infection, the result is interpreted alongside your symptoms.

A positive test does not always mean infection

This is worth stating on its own. A positive C. diff test in a person without diarrhea, or after symptoms have already resolved, often reflects carriage rather than an active infection. This is why C. diff testing is meaningful only when you have diarrhea, and why a positive result is always read alongside how you feel rather than on its own.

What happens after these tests

A C. diff test describes what was found and informs the decisions you and your healthcare team make together, rather than dictating them on its own.

If you have a C. diff infection, an important early step is often to stop the antibiotic that triggered it when possible. Specific antibiotics are then used to treat C. diff itself, guided by your provider. C. diff can return after treatment, as recurrence is common, and additional treatments are available for repeated infections. A test is generally not repeated just to confirm that you are cured, because C. diff tests can stay positive for weeks after you feel better; recovery is judged by your symptoms improving instead. Because C. diff spreads through spores that are not killed by alcohol-based hand sanitizers, washing your hands with soap and water is important to prevent its spread to others. Severe symptoms, such as severe abdominal pain, a high fever, or many episodes of diarrhea, should prompt urgent medical care.

Questions to ask your doctor

Which C. diff test or tests did I have, and what did they show?
Does my result mean I have an active infection, or could I be carrying the bacterium without it causing illness?
Do my symptoms fit with a C. diff infection?
Could a recent antibiotic have led to this?
If I need treatment, what does it involve?
Should the antibiotic I was taking be stopped or changed?
How likely is C. diff to come back, and what would we do if it does?
Do I need a repeat test to confirm I am better, and if not, why not?
How can I avoid spreading C. diff to people at home?
What symptoms should prompt me to seek urgent care?