Ultrasensitive Detection of Clostridium difficile Toxins Reveals Suboptimal Accuracy of Toxin Gene Cycle Thresholds for Toxin Predictions

Pediatric Clostridioides difficile Infection: Diagnosis and Diagnostic Stewardship

Although the pathogenesis of Clostridioides difficile infection (CDI) is complex and incompletely understood, it is believed that the elaboration of C. difficile toxins is necessary for disease. There are a variety of tests available for the detection of both the C. difficile organism and its toxins; however, each has limitations and the best application of these tests to the diagnosis of CDI in children remains uncertain. Nucleic acid amplification tests are unable to reliably discriminate between CDI and C. difficile colonization, while commercially available enzyme immunoassays for toxin detection lack sensitivity. An understanding of preanalytic factors, relevant patient features, and test performance characteristics is essential to the accurate diagnosis of CDI in children. Specific diagnostic stewardship strategies can also increase the likelihood that positive tests reflect disease rather than colonization. Ultimately, CDI remains a clinical diagnosis and clinical judgment is essential when interpreting test results, regardless of the methods used.

Latent class analysis for the diagnosis of Clostridioides difficile infection

BACKGROUND

Clostridioides difficile infection (CDI) is an opportunistic disease that lacks a gold standard test. Nucleic acid amplification tests (NAATs) such as real-time PCR demonstrate excellent an limit of detection (LOD) whereas antigenic methods are able to detect free toxin. Latent class analysis (LCA) provides an unbiased statistical approach to resolving true disease.

METHODS

A cross-sectional study was conducted with suspected CDI patients (n=96). Four commercial real-time PCR tests, toxin antigen detection by enzyme immunoassay (EIA), toxigenic culture, and fecal calprotectin were performed. CDI clinical diagnosis was determined by consensus majority of three experts. LCA was performed using laboratory and clinical variables independent of any gold standard.

RESULTS

Six LCA models were generated to determine CDI probability using four variables including toxin EIA, toxigenic culture, clinical diagnosis, and fecal calprotectin levels. Three defined zones as a function of real-time PCR cycle threshold (Ct) were identified using LCA: CDI likely (>90% probability), equivocal (<90% and >10%), CDI unlikely (<10%). A single model comprising toxigenic culture, clinical diagnosis, and toxin EIA showed the best fitness. The following Ct cut-offs for four commercial test platforms were obtained using this model to delineate three CDI probability zones: [GeneXpert ® : 24.00, 33.61], [Simplexa ® 28.97, 36.85], [Elite MGB ® 30.18, 37.43], and [BD Max ™ 27.60, 34.26].

CONCLUSION

The clinical implication of applying LCA to CDI is to report Ct values assigned to probability zones based on the commercial real-time PCR platform. A broad range of equivocation suggests clinical judgement is essential to the confirmation of CDI.

Discordant Clostridioides difficile diagnostic assay and treatment practice: a cross-sectional study in a tertiary care hospital, Geneva, Switzerland

OBJECTIVES

To determine the proportion of patients who received a treatment for Clostridioides difficile infection (CDI) among those presenting a discordant C. difficile diagnostic assay and to identify patient characteristics associated with the decision to treat CDI.

DESIGN

Cross-sectional study.

SETTING

Monocentric study in a tertiary care hospital, Geneva, Switzerland.

PARTICIPANTS

Among 4562 adult patients tested for C. difficile between March 2017 and March 2019, 208 patients with discordant tests' results (positive nucleic acid amplification test (NAAT+)/negative enzyme immunoassay (EIA-)) were included.

MAIN OUTCOME MEASURES

Treatment for CDI.

RESULTS

CDI treatment was administered in 147 (71%) cases. In multivariate analysis, an abdominal CT scan with signs of colitis (OR 14.7; 95% CI 1.96 to 110.8) was the only factor associated with CDI treatment.

CONCLUSIONS

The proportion of NAAT+/EIA- patients who received treatment questions the contribution of the EIA for the detection of toxin A/B after NAAT to limit overtreatment. Additional studies are needed to investigate if other factors are associated with the decision to treat.

Ultrasensitive Clostridioides difficile Toxin Testing for Higher Diagnostic Accuracy

Currently available diagnostic tests for Clostridioides difficile infection (CDI) lack specificity or sensitivity, which has led to guideline recommendations for multistep testing algorithms. Ultrasensitive assays for detection of C. difficile toxins provide measurements of disease-specific markers at very low concentrations. These assays may show improved accuracy compared to that of current testing methods and offer a potential standalone solution for CDI diagnosis, although large studies of clinical performance and accuracy are lacking.

The Role of Diagnostic Stewardship in Clostridioides difficile Testing: Challenges and Opportunities

PURPOSE OF REVIEW

Accurate and timely diagnosis of Clostridioides difficile infection (CDI) is imperative to prevent C. difficile transmission and reduce morbidity and mortality due to CDI, but CDI laboratory diagnostics are complex. The purpose of this article is to review the role of laboratory tests in the diagnosis of CDI, and the role of diagnostic stewardship in optimization of C. difficile testing.

RECENT FINDINGS

Results from C. difficile diagnostic tests should be interpreted with an understanding of the strengths and limitations inherent in each testing approach. Use of highly sensitive molecular diagnostic tests without accounting for clinical signs and symptoms may lead to over-diagnosis of CDI and increased facility CDI rates. Current guidelines recommend a two-step, algorithmic approach for testing. Diagnostic stewardship interventions, such as education, order sets, order search menus, reflex orders, hard and soft stop alerts, electronic references, feedback and benchmarking, decision algorithms, and predictive analytics may help improve use of C. difficile laboratory tests and CDI diagnosis. The diagnostic stewardship approaches with the highest reported success rates include computerized clinical decision support (CCDS) interventions, face-to-face feedback, and real-time evaluations.

SUMMARY

CDI is a clinical diagnosis supported by laboratory findings. Together, clinical evaluation combined with diagnostic stewardship can optimize the accurate diagnosis of CDI.

High Agreement Between an Ultrasensitive Clostridioides difficile Toxin Assay and a C. difficile Laboratory Algorithm Utilizing GDH-and-Toxin Enzyme Immunoassays and Cytotoxin Testing

The Singulex Clarity C. diff toxins A/B (Clarity) assay is an automated, ultrasensitive immunoassay for the detection of Clostridioides difficile toxins in stool. In this study, the performance of the Clarity assay was compared to that of a multistep algorithm using an enzyme immunoassay (EIA) for detection of glutamate dehydrogenase (GDH) and toxins A and B arbitrated by a semiquantitative cell cytotoxicity neutralization assay (CCNA). The performance of the assay was evaluated using 211 residual deidentified stool samples tested with a GDH-and-toxin EIA (C. Diff Quik Chek Complete; Techlab), with GDH-and-toxin discordant samples tested with CCNA. The stool samples were stored at -80°C before being tested with the Clarity assay. For samples discordant between Clarity and the standard-of-care algorithm, the samples were tested with PCR (Xpert C. difficile; Cepheid), and chart review was performed. The testing algorithm resulted in 34 GDH⁺/toxin⁺, 53 GDH^-/toxin^-, and 124 GDH⁺/toxin^- samples, of which 39 were CCNA⁺ and 85 were CCNA^- Clarity had 96.2% negative agreement with GDH^-/toxin^- samples, 100% positive agreement with GDH⁺/toxin⁺ samples, and 95.3% agreement with GDH⁺/toxin^-/CCNA^- samples. The Clarity result was invalid for one sample. Clarity agreed with 61.5% of GDH⁺/toxin^-/CCNA⁺ samples, 90.0% of GDH⁺/toxin^-/CCNA⁺ (high-positive) samples, and 31.6% of GDH⁺/toxin^-/CCNA⁺ (low-positive) samples. The Singulex Clarity C. diff toxins A/B assay demonstrated high agreement with a testing algorithm utilizing a GDH-and-toxin EIA and CCNA. This novel automated assay may offer an accurate, stand-alone solution for C. difficile infection (CDI) diagnostics, and further prospective clinical studies are merited.

Dual Reporting of Clostridioides difficile PCR and Predicted Toxin Result Based on PCR Cycle Threshold Reduces Treatment of Toxin-Negative Patients without Increases in Adverse Outcomes

Nucleic acid amplification tests are commonly used to diagnose Clostridioides difficile infection (CDI). Two-step testing with a toxin enzyme immunoassay is recommended to discriminate between infection and colonization but requires additional resources. Prior studies showed that PCR cycle threshold (CT ) can predict toxin positivity with high negative predictive value. Starting in October 2016, the predicted toxin result (CT-toxin) based on a validated cutoff was routinely reported at our facility. To evaluate the clinical efficacy of this reporting, all adult patients with positive GeneXpert PCR results from October 2016 through October 2017 underwent a chart review to measure the recurrence of or conversion to a CT-toxin+ result and 30-day all-cause mortality. There were 482 positive PCR tests in 430 unique patients, 282 CT-toxin+ and 200 CT-toxin- Patient characteristics were similar at testing, though CT-toxin+ patients had higher white blood cell (WBC) counts (12.5 × 103 versus 9.3 × 103 cells/μl; P = 0.001). All cases (n = 21) of fulminant CDI had a CT-toxin+ result. Index CT-toxin+ patients were significantly more likely to have a CT-toxin+ result within 90 days than CT-toxin- patients (17.4% [n = 49] versus 8.0% [n = 16], respectively; P = 0.003). Thirty-day all-cause mortality was higher in CT-toxin- patients (11.1% versus 6.8%; P = 0.1), though no deaths in CT-toxin- patients were directly attributable to CDI. Of the 200 CT-toxin- patients, 51.5% (n = 103) were treated for CDI. The rates of conversion to a CT-toxin+ result (8.8% versus 7.2%; P = 0.8) and all-cause mortality (8.8% versus 13.4%; P = 0.3) were similar between treated and untreated CT-toxin- patients, respectively. CT -based toxin prediction may identify patients at higher risk for CDI-related complications and reduce treatment among CT-toxin- patients.

Ultrasensitive Detection of Clostridioides difficile Toxins in Stool by Use of Single-Molecule Counting Technology: Comparison with Detection of Free Toxin by Cell Culture Cytotoxicity Neutralization Assay

Laboratory tests for Clostridioides difficile infection (CDI) rely on the detection of free toxin or molecular detection of toxin genes. The Singulex Clarity C. diff toxins A/B assay is a rapid, automated, and ultrasensitive assay that detects C. difficile toxins A and B in stool. We compared CDI assays across two prospective multicenter studies to set a cutoff for the Clarity assay and to independently validate the performance compared with that of a cell culture cytotoxicity neutralization assay (CCCNA). The cutoff was set by two sites testing fresh samples from 897 subjects with suspected CDI and then validated at four sites testing fresh samples from 1,005 subjects with suspected CDI. CCCNA testing was performed at a centralized laboratory. Samples with discrepant results between the Clarity assay and CCCNA were retested with CCCNA when the Clarity result agreed with that of at least one comparator method; toxin enzyme immunoassays (EIA), glutamate dehydrogenase (GDH) detection, and PCR were performed on all samples. The cutoff for the Clarity assay was set at 12.0 pg/ml. Compared to results with CCCNA, the Clarity assay initially had 85.2% positive agreement and 92.4% negative agreement. However, when samples with discrepant results between the Clarity assay and CCCNA in the validation study were retested by CCCNA, 13/17 (76.5%) Clarity-negative but CCCNA-positive samples (Clarity⁺/CCCNA^-) became CCCNA^-, and 5/26 (19.2%) Clarity⁺/CCCNA^- samples became CCCNA⁺, resulting in a 96.3% positive agreement and 93.0% negative agreement between Clarity and CCCNA results. The toxin EIA had 59.8% positive agreement with CCCNA. The Clarity assay was the most sensitive free-toxin immunoassay, capable of providing CDI diagnosis in a single-step solution. A different CCCNA result was reported for 42% of retested samples, increasing the positive agreement between Clarity and CCCNA from 85.2% to 96.3% and indicating the challenges of comparing free-toxin results to CCCNA results as a reference standard.