Clostridioides difficile is a spore-forming bacterium residing in the human colon and a leading cause of infectious diarrhea worldwide. Infection occurs when the bacterium overgrows, typically following the use of broad-spectrum antibiotics that disrupt the normal gut microbiome. This overgrowth produces toxins that damage the intestinal lining, resulting in C. difficile infection (CDI). Identifying this toxigenic organism and confirming active disease requires specific laboratory testing.
Determining the Need for Testing
A physician orders a C. diff test only after evaluating a patient’s specific clinical signs and medical history, as testing asymptomatic individuals can lead to misleading results. The primary indicator for testing is the onset of unexplained diarrhea, defined as three or more unformed stools passed within a 24-hour period. Patients experiencing this symptom, along with new abdominal pain or a fever, are generally considered candidates for testing.
Testing is strongly recommended when these symptoms appear in conjunction with specific risk factors that make the infection more likely. Foremost among these is the recent use of antibiotics, typically within the preceding six to eight weeks, which is the most common trigger for the disease. Advanced age (over 65) and recent or prolonged stays in a healthcare facility, such as a hospital or nursing home, also elevate the risk.
To ensure accurate diagnosis and avoid unnecessary treatment, strict guidelines exist regarding the type of sample that can be tested. Only unformed, loose, or liquid stool specimens that conform to the shape of the collection container are acceptable for submission to the lab. Fully formed stool samples are generally not tested, as this typically indicates the absence of active disease.
Furthermore, testing is typically avoided for patients who are asymptomatic carriers, meaning they have the organism but no diarrhea. This is because many highly sensitive tests can detect the organism’s presence without indicating an active infection. Similarly, infants under 12 months of age are generally not tested due to a naturally high rate of asymptomatic colonization that rarely causes disease in this age group.
The Primary Diagnostic Methods
Diagnosing active C. diff infection often requires a multi-step laboratory algorithm rather than relying on a single test method alone. This approach combines the strengths of different tests to maximize both the detection of the organism and the confirmation of active toxin production. The three main types of assays used are those that screen for the organism, those that detect the toxin genes, and those that detect the actual toxins.
Glutamate Dehydrogenase (GDH) Test
The Glutamate Dehydrogenase (GDH) test is a common initial screening tool. This enzyme immunoassay detects the GDH enzyme, a protein produced by all C. diff bacteria. It is highly sensitive; a negative result reliably rules out the organism’s presence. However, GDH cannot differentiate between toxigenic and non-toxigenic strains, making it non-specific for active infection. A positive GDH result confirms the bacterium’s presence and usually triggers a second, more specific confirmatory test.
Nucleic Acid Amplification Tests (NAAT)
Nucleic Acid Amplification Tests (NAAT), often using PCR, detect the genes (tcdA and tcdB) that code for the toxins. NAATs are highly sensitive and specific for toxigenic strains, offering rapid results. While excellent for identifying disease potential, a positive NAAT result does not confirm active toxin production. It can be positive in both active infection and colonization, leading to potential over-diagnosis if not used within a multi-step algorithm.
Toxin Immunoassays (EIA)
Toxin Immunoassays (EIA) specifically look for the actual toxins A and B in the stool sample. Since the toxins cause the damage and symptoms, a positive Toxin EIA result correlates strongly with active disease and provides high specificity. However, Toxin EIA tests are significantly less sensitive than NAAT or GDH tests. Toxins are unstable and can degrade quickly, leading to false-negative results, especially with delayed sample processing. Therefore, EIA is rarely used alone and functions best when confirming a positive screening result.
Understanding Test Results and Implications
The interpretation of C. diff test results relies heavily on the specific testing algorithm used by the laboratory and must always be correlated with the patient’s clinical presentation. The fundamental challenge in diagnosis is distinguishing between C. diff infection and asymptomatic colonization. Colonization means the toxigenic bacteria are present in the gut but are not producing enough toxin to cause symptoms.
A fully positive result, often defined by a positive Toxin EIA (toxin detected), strongly indicates active infection. The toxin confirms that the organism is causing the disease, and such patients typically require immediate treatment. These cases are associated with more severe disease and an increased risk of complications.
The most common diagnostic challenge involves discordant results, such as a positive GDH or NAAT test combined with a negative Toxin EIA. This scenario confirms the presence of a toxigenic C. diff strain, but the lack of detectable toxin makes it difficult to determine if the patient has a true infection or is merely colonized. In these situations, the patient’s symptoms guide the clinical decision-making process.
If a patient with discordant results has severe symptoms, the physician may proceed with treatment, recognizing that the Toxin EIA’s low sensitivity may have produced a false negative. Conversely, a patient with mild or improving symptoms and a discordant result may be classified as colonized and monitored without immediate antibiotic intervention. This personalized approach helps to avoid unnecessary use of antibiotics, which can disrupt the microbiome further.
A negative result from a highly sensitive screening test, such as the GDH or NAAT, carries a high negative predictive value, reliably ruling out a C. diff infection. In these instances, the physician will typically look for other causes of the patient’s diarrhea, such as other bacterial or viral agents.
After a patient completes treatment and their symptoms resolve, retesting for cure is strongly discouraged by medical guidelines. The NAAT test, in particular, can remain positive for weeks to months because it continues to detect the gene, even long after the bacteria have stopped producing the active toxin. Retesting can lead to a positive result that does not represent active disease, prompting unnecessary and potentially harmful further courses of antibiotics.