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

Early Economic Modeling to Inform a Target Product Profile: A Case Study of a Novel Rapid Test for Clostridioides difficile Infection

Background. Target product profiles (TPPs) specify the essential properties tests must have to be able to address an unmet clinical need. Aim. To explore how early economic modeling can help to define TPP specifications based on cost-effectiveness considerations using the example of a new rapid diagnostic for Clostridioides difficile infection (CDI), a contagious health care-associated infection causing potentially fatal diarrhea. Methods. A resource-constrained simulation model was developed to compare a hypothetical test for CDI with current practice (i.e., test with glutamate dehydrogenase enzyme immunoassay first; if positive, test with polymerase chain reaction and cytotoxicity assay) for adult individuals with suspected CDI at the Leeds Teaching Hospital National Health System (NHS) Trust in the United Kingdom. Parameters are taken from UK-based observational data collected between 2018 and 2021, published literature, and expert opinion. A methodological framework was developed 1) to derive minimum diagnostic sensitivity and specificity and maximum price for different test turnaround-time values based on cost-effectiveness considerations from the health care perspective using the National Institute of Health Care Excellence willingness-to-pay threshold of £20,000 per quality-adjusted life-years and 2) to test their robustness using a series of sensitivity analyses. Results. A new rapid test for CDI with a 15-min turnaround time would require a minimum diagnostic sensitivity and specificity both equal to 96% and a maximum price of £44 to maintain cost-effectiveness compared with standard of care. Conclusions. This study provides a framework to inform the essential test properties based on cost-effectiveness considerations and to isolate the most influential model parameters and scenarios via a series of sensitivity analyses. These specifications, in turn, could be used to inform future TPPs for tests.

Highlights

Target product profiles (TPPs) for new medical tests provide test developers with performance benchmarks and technical requirements for new tests. Early economic evaluation has already been used to identify acceptable ranges for certain performance requirements for new tests. Currently, however, early economic evaluation methods are yet to be used in the context of TPP development, and there is no guidance as to how this could and should be done.A de novo approach was developed to identify the minimum performance requirements and maximum costs for new tests, based on cost-effectiveness considerations, while also isolating most influential parameters. The added value of this framework lies in structuring early economic evaluation methods as a means of informing transparent, evidence-based minimum TPP performance specifications while also accounting as much as possible for the (inevitable) uncertainty surrounding the minimum performance requirements.This study represents the first application of early economic modeling as a means of deriving the minimum performance specifications for a novel point-of-care test for Clostridioides difficile infection as set out in a future TPP.

The future of Clostridioides difficile diagnostics

PURPOSE OF REVIEW

Although the epidemiology of Clostridioides difficile has changed, this organism continues to cause significant morbidity and mortality. This review addresses current and future approaches to the diagnosis of C. difficile disease.

RECENT FINDINGS

Over the last several years, large prospective studies have confirmed that there is no single optimal test for the diagnosis of C. difficile disease. The pendulum has swung from a focus on rapid molecular diagnosis during the years of the ribotype 027 epidemic, to a call for use of algorithmic approaches that include a test for toxin detection. In addition, diagnostic stewardship has been shown to improve test utilization, especially with molecular methods. Advances in testing include development of ultrasensitive toxin tests and an expansion of biomarkers that may be more C. difficile specific. Microbiome research may be leveraged to inform novel diagnostic approaches based on measurements of volatile and nonvolatile organic compounds in stool.

SUMMARY

As rates of C. difficile infection decline, emphasis is now on improving test utilization and a quest for improved diagnostic approaches. These approaches may involve implementation of technologies that improve toxin testing, predict patients likely to have disease and/or a severe outcome, and harnessing research on changes in the microbiome to advance metabolomics.

Advances and required improvements in methods to diagnosing Clostridioides difficile infections in the healthcare setting

INTRODUCTION

Clostrididioides difficile is associated with adverse clinical outcomes and increased morbidity, mortality, length of hospital stay, and health-care costs.Areas Covered: We searched relevant papers in PubMed for the last 10 years. In major papers, we scanned the bibliographies to ensure that important articles were included. This review addresses the evolving epidemiology of Clostridioides difficile infection (CDI) and discusses novel methods/approaches for improving the diagnosis of this important disease.

EXPERT OPINION

No single diagnostic test to date has demonstrated optimum sensitivity and specificity for detection of CDI. Many institutions have developed multi-step algorithms consistent with guidelines established by various professional societies. Some institutions have successfully tried to improve the pretest probability of molecular assays by implementing appropriate sample rejection criteria and establishing best practice alerts at the time of electronic order entry. Others have established PCR cycle threshold cutoffs to attempt to differentiate symptomatic patients from asymptomatic carriers or to make predictions about severity of disease with variable success. As research advances our understanding of C. difficile pathogenesis and pathophysiology, more information on CDI specific biomarkers is emerging. Finally, assessments of the microbiome and metabolome may expand the diagnostic armamentarium with advances in mass spectrometry and sequencing technologies.

Clostridioides difficile Infection: The Challenge, Tests, and Guidelines

Clostridioides difficile is a dangerous human pathogen because it can grow to high numbers in the intestine, cause colitis with its potent toxins, and persist as spores. C. difficile infection (CDI) is the primary hospital-acquired infection in North America and Europe, and it now is a global disease. Even with newer laboratory tests, there still is confusion on accurately diagnosing this disease. Three guidelines from three different healthcare-affiliated societies have recently been published. Consensus consolidated recommendations from these guidelines should be recognized by healthcare professionals, who need to understand why this disease continues to be difficult to diagnose and need a clear understanding of the advantages and limitations of current tests. Hopefully, these combined efforts will lead to an improvement in the recognition of this pathogen and a reduction in the suffering and economic loss caused by CDI.

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.

Evaluation of Cycle Threshold, Toxin Concentration, and Clinical Characteristics of Clostridioides difficile Infection in Patients with Discordant Diagnostic Test Results

Clostridioides difficile infection (CDI) is one of the most common health care-associated infections that can cause significant morbidity and mortality. CDI diagnosis involves laboratory testing in conjunction with clinical assessment. The objective of this study was to assess the performance of various C. difficile tests and to compare clinical characteristics, Xpert C. difficile/Epi (PCR) cycle threshold (C_T ), and Singulex Clarity C. diff toxins A/B (Clarity) concentrations between groups with discordant test results. Unformed stool specimens from 200 hospitalized adults (100 PCR positive and 100 negative) were tested by cell cytotoxicity neutralization assay (CCNA), C. diff Quik Chek Complete (Quik Chek), Premier Toxins A and B, and Clarity. Clinical data, including CDI severity and CDI risk factors, were compared between discordant test results. Compared to CCNA, PCR had the highest sensitivity at 100% and Quik Chek had the highest specificity at 100%. Among clinical and laboratory data studied, prevalences of leukocytosis, prior antibiotic use, and hospitalizations were consistently higher across all subgroups in comparisons of toxin-positive to toxin-negative patients. Among PCR-positive samples, the median C_T was lower in toxin-positive samples than in toxin-negative samples; however, C_T ranges overlapped. Among Clarity-positive samples, the quantitative toxin concentration was significantly higher in toxin-positive samples than in toxin-negative samples as determined by CCNA and Quik Chek Toxin A and B. Laboratory tests for CDI vary in sensitivity and specificity. The quantitative toxin concentration may offer value in guiding CDI diagnosis and treatment. The presence of leukocytosis, prior antibiotic use, and previous hospitalizations may assist with CDI diagnosis, while other clinical parameters may not be consistently reliable.

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.

Increased Clinical Specificity with Ultrasensitive Detection of Clostridioides difficile Toxins: Reduction of Overdiagnosis Compared to Nucleic Acid Amplification Tests

Clostridioides difficile infection (CDI) is one of the most common health care-associated infections, resulting in significant morbidity, mortality, and economic burden. Diagnosis of CDI relies on the assessment of clinical presentation and laboratory tests. We evaluated the clinical performance of ultrasensitive single-molecule counting technology for detection of C. difficile toxins A and B. Stool specimens from 298 patients with suspected CDI were tested with the nucleic acid amplification test (NAAT; BD MAX Cdiff assay or Xpert C. difficile assay) and Singulex Clarity C. diff toxins A/B assay. Specimens with discordant results were tested with the cell cytotoxicity neutralization assay (CCNA), and the results were correlated with disease severity and outcome. There were 64 NAAT-positive and 234 NAAT-negative samples. Of the 32 NAAT⁺/Clarity^- and 4 NAAT^-/Clarity⁺ samples, there were 26 CCNA^- and 4 CCNA^- samples, respectively. CDI relapse was more common in NAAT⁺/toxin⁺ patients than in NAAT⁺/toxin^- and NAAT^-/toxin^- patients. The clinical specificity of Clarity and NAAT was 97.4% and 89.0%, respectively, and overdiagnosis was more than three times more common in NAAT⁺/toxin^- than in NAAT⁺/toxin⁺ patients. The Clarity assay was superior to NAATs for the diagnosis of CDI, by reducing overdiagnosis and thereby increasing clinical specificity, and the presence of toxins was associated with negative patient outcomes.