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Enzyme immunoassay for Clostridium Difficile Infection: Once, Twice or Thrice? Is the Falling Incidence of Clostridium Difficile an Artefact?

Hettiarachchi IT, Williams OM, Greenwood R, Evans N, Strachan A and Probert CS

Many clinical laboratories rely on stool enzyme immunoassays (EIAs) to diagnose Clostridium difficile infection (CDI). The test is limited by its modest sensitivity which has led to repeat testing. This study examines the impact of repeat stool testing in suspected CDI and describes clinicians’ patterns of ordering tests.

In this retrospective study, analysis was performed on a database of toxin EIAs submitted to our laboratory for suspected CDI. For each patient, results of (up to) the first three stool samples, submitted in the first episode of diarrhoea, was analysed. Patterns of ordering tests were examined.

4,987 patients submitted 8,408 stool samples during the study period. Overall, 13.8% of EIAs were positive for C.difficile. Of these, 68% were diagnosed on the first test, 22% on the second and 10% on the third. 9.4% of first test samples were positive, rising to 14.8% for samples tested twice and 17.7% for samples tested thrice. Repeat testing increased CDI prevalence by 50%. Those testing negative for CDI on the first test were more likely to get re-tested compared to those testing positive (25% vs. 16.8%, [χ2= 15.8, p<0.0001]).

The prevalence of CDI increased under the three test policy using EIA justifying the need for repeat sample testing. Clinicians were more likely to request repeat tests if the initial result was negative. The implementation of single test rules in some trust will result in a fall in the CDI rate by a third. More work is needed to assess the impact of new two-stage testing policies.