Gilligan 13/5/08 3:51 pm Page 53
The Role of Rapid Diagnostic Tests in the Diagnosis of Clostridium difficile-associated Disease
week and cytotoxicity assays take a minimum of 24 hours for positives Lateral flow immunochromatogenic assays were a potential useful
and 48 hours for negatives. approach for the diagnosis of CDAD: the technology was rapid,
technically simple to perform, and easily interpreted. One of the earliest
Both of the above approaches are labor-intensive and the cytoxicity assay, applications of lateral flow to CDAD diagnosis was an assay that
which requires filtration of stool supernatants, has the added disincentive of detected a combination of GDH with toxin A (Triage Micro C. difficile
burst filtering devices resulting in fecal supernatant showers for individuals Panel, BioSite, San Diego, CA). The rationale for this approach was that
performing the assay. For both diagnostic and infection control purposes, a GDH is produced in significantly higher quantities than C. difficile toxin
more rapid, less labor-intensive technique is needed. and should therefore yield a more sensitive assay than solid-phase
Over the last 20 years, C. difficile diagnostic testing has evolved, but all
methods have been dependent on the detection of either toxin A and/or B
It is interesting that the ImmunoCard
or the so called ‘common antigen,’ glutamate dehydrogenase (GDH), using
various immunoassay approaches.
4
Initially counterimmuno-electrophoresis
toxin A/B assay has found a relatively
and then latex agglutination were used to attempt to rapidly detect
large market share despite no
C. difficile toxin A.
5,6
However, it became clear that the preparation used to
produce C. difficile toxin A antibodies had a second, highly antigenic protein
independent verification of its
present—glutamate dehydrogenase—and that these antibody preparations
performance in the literature.
detected both antigens.
7
Since GDH or common antigen was produced in
significantly higher amounts than toxin A, false-positives due to toxin-
negative strains as well as other organisms were a distinct possibility.
4,7
As a toxin A/B EIAs. Toxin A detection was included to improve the specificity
result, these two promising rapid techniques were generally abandoned. of the assay. The test proved to be more sensitive that the two most
widely used toxin A/B EIAs;
12
however, the test was not as specific
The next technological advance in the rapid detection of CDAD because the toxin A portion of the assay was less sensitive than the solid-
was the introduction of solid-phase enzyme immunoassays (EIAs) for the phase EIA. Because of the high sensitivity of the Triage assay, the
detection of toxin A. The rationale for EIA detection of toxin A was negative predictive value (NPV) of the antigen test (98.7%) compared
improved protein separation resulting in highly specific antibodies and with that of tissue culture cytotoxicity was similar to the two assays that
improvements in the understanding of C. difficile pathogenesis indicating are most widely used in the US: the Clostridium difficile Tox A/B (TechLab
that toxin A played a key role in the pathogenesis of disease.
8
Commercial Inc Blacksburg, VA), with an NPV of 97.5%, and the Premier Cytoclone
EIAs produced by several manufacturers generally showed good to excellent A/B (Meridian Diagnostics, Cincinnati, OH), with an NPV of 97.1%. The
specificity (95–99%), but sensitivity in the 60–90% range.
9,10
The NPV of the combination of the GDH and toxin A assay was 95%.
12
comparatively low sensitivity suggests that multiple specimens need to be
tested, but how well that increases the detection of positives remains an Despite these impressive results and its ease of use, in a 2006 College of
unresolved question. American Pathologists (CAP) survey only 5% of laboratories in the US
were using this test for CDAD diagnosis, while 19% of laboratories were
using a lateral flow device that detected toxin A/B (ImmunoCard A/B,
Over the last 20 years, Clostridium
Meridian Diagnostics, Cincinnati, OH) and 37% of laboratories were
difficile diagnostic testing has evolved,
using one of the two solid-phase toxin A/B EIAs that have been
discussed.
15
It is interesting that the ImmunoCard toxin A/B assay has
but all methods have been dependent on
found a relatively large market share despite no independent verification
the detection of either toxin A and/or B
of its performance in the literature.
or glutamate dehydrogenase, using
A potential explanation for the GDH/toxin A assay not being more widely
various immunoassay approaches.
used is that GDH-positive/toxin A-negative results need to be verified,
since both non-toxigenic and toxin B-only C. difficile strains may be
GDH-positive. One study suggested that as many as 12% of specimens
The observation that certain strains of C. difficile that produce toxin B but would be GDH-positive/toxin A-negative, but >90% would be confirmed
not toxin A could cause CDAD resulted in a re-thinking of approaches to by tissue culture cytotoxicity as positives.
16
These data need to be viewed
CDAD diagnosis, as these strains would not be detected by a toxin A cautiously since the specimen number in this study is small and from a
EIA.
11
Performance characteristics (sensitivity/specificity/predictive values) single institution that may not have large numbers of patients harboring
of solid-phase toxin A + B EIAs were similar to those of toxin A EIAs.
10,12–14
non-toxigenic C. difficile strains.
Solid-phase EIAs have the disadvantage of taking approximately Another novel approach is to use GDH as a screening test. The rationale
two hours to complete and, for economy, really should be batched. here is that GDH detection is significantly more sensitive than toxin A/B
More rapid and less labor-intensive alternatives were needed. Early EIA detection, but much less specific. Because of the high sensitivity of
studies made it clear that latex agglutination tests lacked both sensitivity the assay, the negative specimens could be screened out and reported as
and specificity.
6
negative while a highly sensitive confirmatory test—tissue culture
US INFECTIOUS DISEASE 53
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