Intended for healthcare professionals

Rapid response to:

Practice Lesson of the Week

A diarrhoeal illness with a difference?

BMJ 2009; 339 doi: https://doi.org/10.1136/bmj.b2648 (Published 20 July 2009) Cite this as: BMJ 2009;339:b2648

Rapid Response:

Warning lab users about false-negative result with C. difficile EIA tests

The report highlights an important issue i.e. that EIA tests for C.
difficile toxin can give false-negative results. It would be interesting
to know which EIA kit was used for testing this patient, as different kits
vary in sensitivity: in our laboratory we use the Meridian Premier toxin
A+B EIA as this seems to be the toxin detection EIA with the highest
sensitivity (91.7% sensitivity when compared with cell culture cytotoxin
testing).1

When we recently analysed the data from our laboratory over a three
month period during which we did 1067 tests for C. difficile, we estimated
that the prevalence of specimens from patients with a genuine infection
was 5.1%. We also estimated that, on the basis of a sensitivity of 91.7%,
as described for this EIA test,1 we may possibly have missed five cases
of C. difficile infection during this 3-month period as a consequence of
false-negative EIA results.

We have now decided that all our EIA negative results will be issued
with the following warning added to the report: “False-negative results
may occasionally occur. If this patient has persistent diarrhoea which you
think might be caused by C. difficile, please send a REPEAT specimen and
CONTACT ONE OF THE MEDICAL MICROBIOLOGISTS. (alternative C. difficile
tests may also be arranged)”. If we were contacted as a result of this
warning, we would immediate arrange further tests with different detection
modalities including testing for C. difficile glutamate dehydrogenase
(this gives an immediate result but does not differentiate C. difficile
carriage from infection) and cell culture cytotoxin (which takes up to two
days but is more specific for infection).

1. Eastwood K, Else P, Charlett A, Wilcox M. Comparison of nine
commercially available Clostridium difficile toxin detection assays, a
real-time PCR assay for C. difficile tcdB, and a glutamate dehydrogenase
detection assay to cytotoxin testing and cytotoxigenic culture methods. J
Clin Microbiol 2009; 47: 3211-3217.

Competing interests:
None declared

Competing interests: No competing interests

24 October 2009
Giuseppe E Bignardi
Consultant, Clinical Microbiology
Microbiology Department, Sunderland Royal Hospital, Kayll Road, Sunderland SR4 7TP, UK