The Clostridium difficile toxin test is used to diagnose antibiotic-associated diarrhea and pseudomembranous colitis that is caused by C. difficile. It may also be ordered to detect recurrent disease.

If the patient has a positive toxin test, the doctor will typically discontinue any antibiotics that the patient may be taking and prescribe an appropriate treatment of oral antibiotic, such as metronidazole or vancomycin, to eliminate the C. difficile bacteria.

A C. difficile toxin test may be ordered when a hospitalized patient has frequent loose stools, abdominal pain, fever, and/or nausea during or following a course of antibiotics or following a recent gastrointestinal surgery. It may be ordered when an outpatient develops these symptoms within 6-8 weeks after taking antibiotics, several days after chemotherapy, or when a patient has a chronic gastrointestinal disorder that the doctor suspects is being worsened by a C. difficile infection. The C. difficile toxin test may be ordered to help diagnose the cause of frequent diarrhea when no other discernable cause (such as parasites or pathogenic bacteria) has been detected.

If a patient treated for antibiotic-associated diarrhea or colitis relapses and symptoms re-emerge, C. difficile toxin testing may be ordered to confirm the presence of the toxin.

If the C. difficile toxin test is positive, it is likely that the patient’s diarrhea and related symptoms are due to an overgrowth of toxin-producing C. difficile. Occasionally, false positives may be seen with grossly (visibly) bloody stool samples.

If the test is negative but the diarrhea continues, another sample needs to be tested. The rapid C. difficile toxin tests detect less than 85% of cases, so the toxin may have been missed the first time. Since the toxin breaks down at room temperature, a negative result may also indicate that the sample was not transported, stored, or processed promptly. A negative test result may also mean that the diarrhea and other symptoms are being caused by something other than C. difficile.

There is a rapid test to detect a common antigen expressed by all strains of C. difficile, but it does not tell the doctor if the bacteria are producing the harmful toxins. An additional test to detect toxin A and B must be performed to confirm C. difficile-associated disease. The usual method to detect C. difficile toxin A and B is by a rapid enzyme immunoassay. Results are available after 1 to 4 hours, depending on the test. A cytotoxin test that looks for the toxic effects of stool on human cells grown in culture is a more sensitive method to detect toxin, but it requires 24 to 48 hours to get the result.

Clostridium difficile can be grown and isolated on a stool culture, but its presence does not indicate whether the strain present is a toxin producer. It also does not distinguish between C. difficile colonization and overgrowth/infection.

An endoscopic procedure can be used to diagnose C. difficile colitis. A specialist (gastroenterologist) can observe and biopsy any characteristic pseudomembranous lesions that may be present.

Molecular testing methods are being investigated for use in the detection of the presence of C. difficile toxin A and B. However, these tests are not able to distinguish between an active infection and a prior infection since the toxin can be detected long after the patient has recovered.