Differences in outcome according to Clostridium difficile testing method: a prospective multicentre diagnostic validation study of C difficile infection

Abstract

SummaryBackgroundDiagnosis of Clostridium difficile infection is controversial because of many laboratory methods, compounded by two reference methods. Cytotoxigenic culture detects toxigenic C difficile and gives a positive result more frequently (eg, because of colonisation, which means that individuals can have the bacterium but no free toxin) than does the cytotoxin assay, which detects preformed toxin in faeces. We aimed to validate the reference methods according to clinical outcomes and to derive an optimum laboratory diagnostic algorithm for C difficile infection.MethodsIn this prospective, multicentre study, we did cytotoxigenic culture and cytotoxin assays on 12 420 faecal samples in four UK laboratories. We also performed tests that represent the three main targets for C difficile detection: bacterium (glutamate dehydrogenase), toxins, or toxin genes. We used routine blood test results, length of hospital stay, and 30-day mortality to clinically validate the reference methods. Data were categorised by reference method result: group 1, cytotoxin assay positive; group 2, cytotoxigenic culture positive and cytotoxin assay negative; and group 3, both reference methods negative.FindingsClinical and reference assay data were available for 6522 inpatient episodes. On univariate analysis, mortality was significantly higher in group 1 than in group 2 (72/435 [16·6%] vs 20/207 [9·7%], p=0·044) and in group 3 (503/5880 [8·6%], p<0·001), but not in group 2 compared with group 3 (p=0·4). A multivariate analysis accounting for potential confounders confirmed the mortality differences between groups 1 and 3 (OR 1·61, 95% CI 1·12–2·31). Multistage algorithms performed better than did standalone assays.InterpretationWe noted no increase in mortality when toxigenic C difficile alone was present. Toxin (cytotoxin assay) positivity correlated with clinical outcome, and so this reference method best defines true cases of C difficile infection. A new diagnostic category of potential C difficile excretor (cytotoxigenic culture positive but cytotoxin assay negative) could be used to characterise patients with diarrhoea that is probably not due to C difficile infection, but who can cause cross-infection.FundingDepartment of Health and Health Protection Agency, UK.

Underdiagnosis of Clostridium difficile across Europe: the European, multicentre, prospective, biannual, point-prevalence study of Clostridium difficile infection in hospitalised patients with diarrhoea (EUCLID)

“…20 The diagnostic method used in our study (QUIK CHEK COMPLETE, Techlab, US) was chosen as toxin detection in faecal samples correlates with clinical outcome, whilst detection of a toxigenic strain does not. 12,13 Toxin detection is therefore a better indicator of clinically relevant CDI. Our false-negative rate was much lower (1.6% vs. 19.0%) compared with a previous study, but this used toxigenic culture to determine CDI status, which will over-estimate clinically relevant CDI.…”

“…7 Sub-optimal case ascertainment, either due to inadequate laboratory diagnosis or lack of clinical suspicion means that the true burden of CDI is unclear. [9][10][11][12] A recent point prevalence study in Spain found that 66% of CDI patients on a single day were undiagnosed or misdiagnosed, due either to lack of clinical suspicion (47%) or inadequate laboratory testing (19%). 9 Optimal laboratory diagnosis of CDI depends on testing the right patients, at the right time with the right tests.…”

“…Importantly, detection of C. difficile toxin in patient faecal samples, as opposed to toxigenic C. difficile (strains that produce toxin in vitro, or have toxin genes present), correlates with disease severity and mortality. 12,13 To improve the sub-optimal sensitivity of commercially available toxin detection assays, two-stage laboratory diagnosis, involving a sensitive C. difficile screening test followed by a C. difficile toxin assay is recommended. 1,12 Nevertheless, a questionnaire-based study in 125 European laboratories in 2010 showed wide variation in use of CDI diagnostic methods, with a quarter still using a single assay.…”

“…12,13 To improve the sub-optimal sensitivity of commercially available toxin detection assays, two-stage laboratory diagnosis, involving a sensitive C. difficile screening test followed by a C. difficile toxin assay is recommended. 1,12 Nevertheless, a questionnaire-based study in 125 European laboratories in 2010 showed wide variation in use of CDI diagnostic methods, with a quarter still using a single assay. 14 Notably, however, such data do not ascertain the true extent of missed CDI diagnoses.…”

Strategies for Optimizing the Diagnostic Predictive Value of Clostridium difficile Molecular Diagnostics

“…Thus, like toxigenic culture, NAATs identify a strain in stool that has the potential to produce toxin in vivo. Two large clinical research studies in the United States (10) and the United Kingdom (11) questioned the clinical significance of a positive PCR test (10) (or toxigenic culture [11]) when the patient also tests negative by toxin EIA because these patients have benign clinical outcomes that are similar to those seen with patients who do not have CDI. Furthermore, several studies have demonstrated the high rate of NAAT (or toxigenic culture) positivity in patients without diarrhea, including hospitalized children (12) and adults (9) as well as children with cancer (13).…”