Assessment of Clostridium difficile testing results

CMPT recognizes that there can be a “seemingly interminable delay” between submitting some test results and getting an assessment of performance back.  Part of that delay has to do with the planning and scheduling the clinical bacteriology committee for a meeting.  Our solution to speed up our reporting is to forgo the committee process for some results that should be straight forward.

Clostridium difficile testing has been selected as one assay acceptable for faster turnaround.

C. difficile testing has two elements, one of which is antigen testing and the other is toxin testing.  The toxin component may be tested directly looking specifically for toxin or its activity, or tested indirectly by looking for detection of the appropriate molecular marker.   Antigen detection is less specific but straightforward for testing, while toxin testing is more specific but testing may be more complex.

In order to provide faster turnaround on responses we have created the following grading structure.

Antigen testing is either “positive” or “negative” or “indeterminant” and there is little justification to pass the testing on to a reference laboratory.  Antigen test results will be taken as face value and either be assessed as “Acceptable” or “Unacceptable” based on the challenge’s CMPT quality control and reference laboratory group performance  as well as the total laboratory group performance.

Toxin testing will be assessed in the same way, unless the laboratory had sufficient concerns with their test result and reports to us that the sample would have been sent to a reference laboratory for further investigation.

If CMPT quality control results or the reference group results or the total group performance results indicate that a proportion of other laboratory had similar difficulty, CMPT will take that into consideration and designate the report as clinically appropriate and acceptable.
If not, the reported result will stand on its own merits.

Proficiency testing performance can be a valuable measure of a laboratory’s performance.  If the laboratory is having difficulty with CMPT samples, that can be an indicator of a systemic problem at some stage of the testing process.  Laboratories are encouraged to use CMPT results as a basis to trigger internal testing review.  Rapid reporting turnaround by CMPT can help the laboratory initiate review much more quickly.

We are interested in you feedback.  If you want to let us know of your thoughts on CMPT’s grading policy for C. difficile testing, please send us and email to

Posted in CDNews