INFECTION CONTROL
ileus, and it is suggested that a rectal swab be used for (toxigenic) culture, NAAT or GDH EIA in patients suspected of ileus.
A key recommendation to come from the study findings is that, despite the development of new tests for CDI, the routine use of any single test for diagnosis is not recommended, irrespective on the technology on which it is based. The easiest way to diagnose CDI would be to use a single rapid laboratory test that is able to reliably predict disease status. According to Barbutt et al2
a
rapid CDI diagnosis is associated with more prompt CDI treatment and less unnecessarily treated patients. However, two problems
(a)
arise if the rapid assays are used as stand- alone test for diagnosing CDI. Firstly, the positive predictive value (PPV) of even the most specific tests are inadequate at low disease prevalence. If toxin EIAs were to be used in an endemic situation an unacceptably high percentage of patients (19%) with a positive test result would not actually have CDI. Secondly, as the targets identified by the index tests are different from each other, a positive index test does not necessarily indicate a real CDI case. Using NAAT as a stand-alone test and relying on clinical symptoms to discern patients with CDI from asymptomatic carriers
Step 1: Highly sensitive test: NAAT or GDH EIA Positive test result Step 2:
Highly specific test: Toxin A/B EIA
Positive test result Negative test result Negative test result
No further testing required: CDI is unlikely to be present
CDI is likely to be present
Clinical evaluation: CDI of carriage of (toxigenic) C.difficile is possible
Step 3 (optional):
Perform TC or NAT (in case first test was a GDH EIA)
is not an optimal approach. Patients colonised by a toxigenic C. difficile strain may very well develop diarrhoea due to other causes, and no specific clinical symptoms exist to differentiate CDI from other causes of diarrhoea. The study authors concluded that neither glutamate dehydrogenase (GDH) EIA nor toxin A/B EIA or NAAT can reliably be used as a stand-alone test to diagnose CDI. Because no single test is suitable to be used as a stand-alone test, it is advised that two tests be combined in an algorithm in order to optimise the diagnosis of CDI. The advantage of an algorithm is that tests can be combined in such a way that the percentage of false-positive results can be decreased. This can be done by testing all samples with a first test, then performing reflex testing on samples with a positive first test result only. The first test should be a test that reliably classifies samples with a negative test result as non-CDI – either NAAT or GDH EIA. Samples with a negative test result should not be tested further, but samples with a positive first test result should then be tested with a toxin A/B EIA. Samples with a positive second test results can be reported as CDI-positive. An alternative algorithm is to screen samples with both a GDH and toxin A/B EIA. Samples with concordant positive or negative results can be reported as such. Samples with a negative GDH result but positive for toxin need to be retested as this is an invalid result. Samples with a positive first test result and negative second test result and samples with a GDH-positive test result but negative toxin A/B test result may represent samples with CDI or C. difficile carriage and may optionally be tested with toxigenic culture (TC) or NAAT if this has not yet been performed. It is also recommended that TC and molecular typing of recovered isolates should be performed in case of outbreak situations.
(b) Step 1: Highly sensitive test: GDH and Tox A/B EIA Both negative
No further testing required: CDI is unlikely to be present
GDH postitive, Tox A/B negative
Step 2 (optional): NAAT or TC
Negative test result Both positive
No further testing required: CDI is unlikely to be present
Positive test result Repeated testing
CDI is unlikely to be present
Clinical evaluation: CDI of carriage of (toxigenic) C.difficile is possible
Recommended algorithms for CDI testing. (a) GDH or NAAT–Tox A/B algorithm. (b) GDH and Tox A/B–NAAT/TC algorithm. CDI, Clostridium difficileinfection; GDH, glutamate dehydrogenase; NAAT, nucleic acid amplification test; TC, toxigenic culture; Tox A/B, toxin A/B; EIA, enzyme immunoassay.
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In endemic situations repeated testing after a first positive sample during the same diarrhoeal episode is not recommended although repeated testing after a first negative sample during the same diarrhoeal episode may be useful in selected cases with ongoing clinical suspicion during an epidemic situation or in cases with high clinical suspicion during endemic situations. A test of cure is also not recommended. According to the guidelines, although the use of an algorithm for CDI testing based on two rapid assays is recommended, every laboratory should also be able to isolate C. difficile, ideally via TC from selected samples. This is because TC offers the ability
The decision to treat CDI will, ultimately, be a clinical one, guided by laboratory results.
OCTOBER 2016
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