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Changes in Laboratory Testing for the Screening of Clostridium Difficile
Since July 19, 2011, Hôpital Montfort has been simultaneously conducting two types of tests. The interpretation criteria include an infection risk factor (e.g. having taken antibiotics) and liquid stools.
1. Test for the bacteria's GDH enzyme (antigen) using a quick method specific to Clostridium difficile. A positive result only indicates that the C. difficile bacteria is present.
2. Test for the C. difficile cytotoxin.
Both tests are performed in our laboratory. For patients who meet the above criteria, the two tests are interpreted as per the table below.
| GDH | Toxin | Interpretation |
| +(pos) | +(pos) | · 95% sensitivity and 99% specificity (production of toxin associated with pseudomembranous colitis) |
| -(neg) | -(neg) | · 95% sensitivity and 99% specificity No toxins (**) see note 1 |
| +(pos) -(neg) |
-(neg) +(pos) |
· Indeterminate result (10% to 20% of cases) A PCR is performed to confirm the presence of toxin B. |
For indeterminate results (10% to 20% of cases), a confirmatory test is performed by a reference laboratory to definitively determine whether or not the toxin is present. Specimens in this category are sent to the regional laboratory for a PCR (polymerase chain reaction) to detect the gene responsible for producing toxin B. The response time is 18 to 24 hours from the time our microbiology laboratory identifies an indeterminate result. These results are interpreted in light of the clinical picture and risk factors.
(**) note 1: Given the excellent specificity of the GDH test (99%), specimens initially found to be negative are retested no sooner than seven days later.