Clostridium difficile toxin lab tests

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Clostridium difficile toxin tests are used to diagnose antibiotic-associated diarrhea caused by toxin-producing C. difficile.

There are a number of tests available to detect the infection and to determine if the strain that is present produces toxin. Some tests are very sensitive and can take days to receive results. Other tests are rapid (several hours) and are not considered to be very sensitive.  Therefore, utilizing a combination of tests may be used to help make a diagnosis.

Commonly used tests include:

  • C. difficile toxin B, or toxins A and B, by enzyme immunoassay (EIA) tests are some of the most common tests used by laboratories. Results are typically available within 1 to 4 hours. Though these tests are rapid and widely available, they are not sensitive enough to detect many infections; they miss up to 30% of cases. Therefore, they are not recommended for use by some professional organizations.
  • A glutamate dehydrogenase (GDH) test detects an antigen that is produced in high amounts by C. difficile, both toxin and non-toxin producing strains. It may be used as a first step to rule out an infection with C. difficile, but it should not be used alone. Since it is not very specific for toxin-producing C. difficile, it is often used in combination with a test for toxin by EIA or a cytotoxicity culture.
  • Tissue culture to detect the C. difficile toxin is a test that looks for the effects of the cytotoxin on human cells grown in culture. It is a more sensitive testing method to detect toxin, but it requires 24 to 48 hours to get the test result.
  • A relatively new molecular PCR (polymerase chain reaction) lab test can rapidly detect the     C. difficile toxin B gene (tcdB) in a stool sample. This test is sensitive but is not widely available.
  • Toxigenic stool culture, which requires growing the bacteria in a culture and detecting the presence of the toxins, is the most sensitive test for C. difficile, and it is still considered to be the gold standard. This test does take 2 to 3 days for results.  * A culture will not distinguish between C. difficile  colonization and overgrowth/infection.

* Currently, there is not one test that is rapid, widely available, and sufficiently sensitive and specific.

Until the development of such a test, the Society for Healthcare Epidemiology of America (SHEA) and the Infectious Diseases Society of America (IDSA) recommend a two-step testing process:

  • Perform an initial screen on stool samples using a test for glutamate dehydrogenase (GDH). This test is considered to be very sensitive.
  • Follow up positive screening results with a specific test for the toxin or the gene that codes for the toxin.

When are lab tests ordered?

Tests for C. difficile toxin may be ordered when an individual has symptoms such as: frequent watery stools (diarrhea) , abdominal pain, fever, and/or nausea during or has been treated with a course of antibiotics recently or over the past 6-8 weeks, or following a recent gastrointestinal surgery, several days after chemotherapy, or when a person has a chronic gastrointestinal disorder that the Physician./Healthcare professional suspects is being worsened by a C. difficile infection.

Tests for C. difficile toxin may be ordered to help diagnose the cause of diarrhea when no other discernible cause, such as parasites or pathogenic bacteria, has been detected from other lab tests.

When an individual is treated for antibiotic-associated diarrhea or colitis relapses and symptoms return, C. difficile toxin testing may be ordered to confirm the presence of the toxin.

Testing should not be ordered to monitor the effectiveness of treatment or on those who are symptomatic.      *  A reduction of symptoms associated with C. difficile, such as if diarrhea ceases and stools are formed, indicates a cure from infection.

**  Physicians and healthcare professionals may order lab testing at their own discretion **


Test results:

If tests for C. difficile toxin are positive, it is likely that the person’s diarrhea and related symptoms are due to an overgrowth of toxin-producing C. difficile.

Negative test results, with symptoms present, may mean that the diarrhea and other symptoms are being caused by something other than C. difficile  (Physicians may advise further testing).

Since the C. difficile toxin breaks down at room temperature within 2 hours, a negative result may also indicate that the sample was not transported, stored, or processed promptly.

* *  If there is a concern that a stool specimen has not been collected and processed properly, a repeat test may be ordered and performed.