• The C. difficile bacteria produce two principal toxins – A and B – which cause diarrhoea and colitis. Toxin A & B are both pathogenic and strains produce both.
• Enzyme immuno assays (EIA) to detect C. Difficile toxin is the most common test used.
• Cytotoxin testing- detects C. difficile toxins, using an in-vitro cell sheet assay.
• Culture – this is relatively slow and requires supplementary testing to confirm that an isolate is a toxin producer.
• Detection of an antigen (e.g. the enzyme glutamate dehydrogenase) or a gene (e.g. the toxin B gene).

Discuss the limitation of EIA kits used to detect C. Difficile infection?
All currently available EIA kits have a high negative predictive value. However, caution is required (due to false positive rates) in the interpretation of toxin-positive results to ensure that these are consistent with the clinical presentation.
The use of a confirmatory or second test (like cytotoxin test) will increase the accuracy of toxin positive results. However, the need for it is unclear at present.
Should multiple repeat samples be submitted for testing?
A repeat test (so 2 tests in all) may be considered if there remains a high clinical suspicion of C. Difficile infection. However, multiple tests should be avoided as, with tests that have a sub-optimal positive predictive value, this increases the chance of obtaining false positive results.
What is the best single test for the detection of C. difficile toxins?

Well-performed cell-culture cytotoxicity assays are still regarded as the “gold standard” for diagnostic testing. However, these tests have a disadvantage in terms of speed.

Are further tests required to demonstrate clearance of the organism?
Tests of cure (i.e. clearance testing) are not clinically helpful as toxin excretion may continue even when symptoms have stopped. Thus

• Re testing is not recommended if the person is still symptomatic within a period of 28 days
• Only re-test to confirm recurrent C. difficile infection if the symptoms resolve and then recur.

Ref
http://www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1238055363795