These recommendations are based on guidelines issued by the Department of Health and the Health Protection Agency (HPA) [HPA, 2007a; DH and HPA, 2008].

Laboratory confirmation of Clostridium difficile toxin

• Laboratory diagnosis of C. difficile infection is based on detection of C. difficile toxins (A and B) in the stool sample.
• Laboratory confirmation is required because there are no specific clinical features for C. difficile infection and a number of disorders may have similar clinical presentation (for example infections by other enteric pathogens, see the section on Causes in the CKS topic on Gastroenteritis) [Bartlett and Gerding, 2008].

Repeat testing

• Repeated stool testing is recommended if the first sample tests negative and there is a strong suspicion of C. difficile [DH and HPA, 2008].
• The HPA warns that results from studies indicate that currently-available kits may miss about 1 in 10, to 1 in 5, cases of C. difficile and generally have suboptimal positive predictive values (that is, positive results may sometimes be false positives) [Wilcox et al, 2009].
• This is based on findings of a study undertaken by the National Health Service Centre for Evidence-based Purchasing which found none of the C. difficile-toxin detection assays tested had a particularly high sensitivity [Wilcox and Eastwood, 2009]. In this study (of 600 faecal samples), the sensitivities of the nine commercial assays ranged between 67–92% when compared with the cytotoxin assay (the gold standard). Specificities ranged between 91–99%. The positive predictive values (PPVs) ranged between 49–87%, and the report warned that 'the poor PPVs of toxin detection kits, especially in the context of widespread testing, raises doubts about their appropriateness when used as single tests for the laboratory detection of C. difficile toxins'.
• Consequently, the HPA recommends that toxin detection results should be interpreted cautiously, particularly if the test result is not consistent with the clinical details [Wilcox et al, 2009].

28 day rule

• The Department of Health and the HPA do not recommend re-testing people with positive C. difficile infection if the person is still symptomatic within a period of 28 days [DH and HPA, 2008].
• Based on Department of Health guidance on mandatory healthcare associated infection surveillance system for C. difficile infection, positive results on the same individual within 28 days of the first specimen should be regarded as a single episode [Chief Medical Officer and Chief Nursing Officer, 2008].
• Separate episodes should be reported if positive results on the same individual was reported more than 28 days apart, irrespective of the number of specimens taken in the intervening period, or where they were taken. If a new episode is reported under the 28 day rule, this rule is reset for the new episode.
• The 28 day rule was recommended by the National Clostridium difficile Standards Group, with the intention to avoid bias caused by the difficulties in distinguishing relapse from re-infection and to permit consistency of interpretation [National Clostridium difficile Standards Group, 2003]. This recommendation was based on expert opinion rather than published evidence.

Children younger than 2 years of age

• In healthy newborn babies and infants, asymptomatic colonization of C. difficile is very common, ranging from 15–80%, and is rarely associated with overt disease (possibly due to a lack of toxin receptor expression in the immature gut) [Kelly, 2009; Williams and Spencer, 2009]. By around 2 years of age, the colonization rate drops to levels comparable with those of healthy adults (less than 4%).
• Consequently, the Department of Health and the HPA recommend that testing is generally not advisable in children less than 2 years of age as toxigenic strains of C. difficile, and toxins A and B, may be present in the absence of symptoms [DH and HPA, 2008].