Micro comments: Clostridium difficile testing

Here are our local Pathology North (NSW) comments together with their rationale:

Positive CDI test result comment (no test of clearance required!):


The duration of contact precautions following recovery are controversial. Patients will continue to excrete C. difficile for weeks following recovery and can represent a cross infection risk. However continent patients with formed stool who are compliant with hand washing represent an acceptable risk. In the setting of an outbreak, it is advisable to extend the post recovery isolation phase, perhaps until the patient is discharged.

Sample rejection (don’t test formed or soft stool!) :

CD reject

Many patients carry toxigenic strains in their stool and remain asymptomatic. Conversely , a recovered CDI patient will have formed or soft stool that remains toxin positive. As labs have moved to mulitplex faecal PCR detection strategies, there are increasing number of asymptomatic carriers identified, with the inherent danger that clinicians may then be tempted to treat them unnecessarily!

Don’t test infants!


The comment says it all really. Toxigenic C. difficile is frequently carried by infants and their gut mucosa is relatively resistant to C. difficile toxin-related damage. CDI in infants < 2 years is thought to be rare. Nearly all positive C. difficile results  represent false positives. See this interesting review for discussion: Clin Infect Dis. 2015 Mar 15;60(6): Lack of evidence for an unmet need to treat Clostridium difficile infection in infants aged <2 years: expert recommendations on how to address this issue.

Reflexive testing comment (add testing for loose or watery stools submitted from inpatients who stay longer than 72hrs)


Pathology North Microbiology will commence automatic addition of C. difficile testing for inpatients later this year.  We expect this will increase overall test numbers by 37%.

Negative test result comment (avoid pointless repeat tests!):


Current test methods are highly sensitive and performing repeat tests over a short time interval is wastful.  False negatives may occur with toxin A or B direct antigen assays due to lower sensitivity and this is not corrected by repeating the test!


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: