How to solve your C. Difficile diagnosis challenges

It's among the most challenging problems in modern healthcare: Clostridium difficile (C. difficile, C. diff), the spore-forming bacterium, infects thousands of patients in hospitals every year. According to a study by the New England Journal of Medicine,¹ toward the end of the last century the incidence of C. difficile infection (CDI) in hospitals in the United States “remained stable" at less than 40 cases per 100,000 patients. By 2001, however, these numbers started to climb, and by 2005 the infection rate had more than doubled.

In 2011, according to the U.S. Centers for Disease Control and Prevention (CDC), nearly 500,000 patients were infected with the diarrhea-causing bacterium, including 29,000 who died within 30 days of their C. diff diagnosis.² “C. difficile has become the most common microbial cause of healthcare-associated infections in U.S. hospitals," the agency announced when it released its study in 2015. Estimates today peg the cost of C. difficile infection (CDI) at somewhere in the neighborhood of $5 billion annually.³

Below, find background on this perennial health hazard and an overview of the tools microbiologists can use to fight it.

Given how often C. diff  is found in U.S. hospitals, it may help to understand how this bacterium causes disease. Most strains of C. diff produce two toxins: TcdA (toxin A), an enterotoxin that attracts neutrophils and stimulates their release of cytokines; and TcdB (toxin B), a cytotoxin that increases permeability of the intestinal wall. While both toxins are inflammatory, toxin B is the one that causes diarrhea and is therefore the focus of CDI tests. The emergence, in 2000, of an epidemic C. difficile strain (BI/NAP1/027) that includes a binary toxin in addition to toxins A and B has been blamed for much of the uptick in CDI rates and severity between 2000 and 2011.

Because C. diff is shed in human feces, any contaminated surface (food, hands, countertops, etc.) within a hospital can potentially harbor its spores. Most people are able to fight off infection even if they come into contact with C. diff, but certain patients with ware more susceptible to its effects. Among the major risk factors for CDI, according to the CDC, are antibiotic exposure, long length of stay in a healthcare setting, and a weakened immune system. Those patients most likely to have CDI, the agency notes, are “older adults, who take antibiotics and also get medical care." Symptoms of CDI, which tend to cause complications like colitis and dehydration far more often than a fatality, include watery diarrhea, fever, and abdominal pain.³

While the stats surrounding C. diff can be severe, it's the trouble clinicians have diagnosing CDI that really alarms healthcare executives. The issue, as one recent study in the Journal of Clinical Microbiology (JCM) points out,⁴ involves the high prevalence of C. diff colonization. Anywhere from 4.4% to 21% of patients in U.S. hospitals are asymptomatically colonized with toxigenic C. diff, the researchers note. This can lead to over-diagnosis of CDI if hospitals test for C. diff in patients who don't have “clinically significant diarrhea" (having diarrhea three or more times over 24 hours, without the use of laxitives).

Furthermore, testing itself can be problematic, depending on the method a facility decides to use. For healthcare organizations focused on value-based purchasing and the impact—through safety ratings—their CDI rates may have on reimbursement, the inability to identify positive cases of CDI efficiently and effectively may have real repercussions for the bottom line.

So how should institutions go about the work of differentiating CDI from C. difficile colonization? First of all, because of the potential for false-positive results, testing for C. diff is only recommended when patients have frequent diarrhea or watery stools. From there, the various tests clinicians have at their disposal include toxin targeted toxigenic stool cultures, cytotoxicity assays, and enzyme immunoassays (EIA) and combination GDH and Toxin immunoassays. They also include approaches like real-time polymerase chain reaction (PCR) that employ nucleic acid amplification technology. The gold standard tests for CDI diagnosis are Culture and Cell Culture Cytotoxicity Neutralization Assay (CCNA), however due to their need for highly trained staff and long turn around times, these tests are no longer used in regular clinical practice.

Here's a look at the most commonly used diagnostic tests—and the possible benefits and drawbacks associated with each:

Toxigenic stool cultures: Stool culture has long been considered a “gold standard" for CDI diagnosis, but it can also lead to false-positive results in the event of C. difficile colonization. In addition, stool cultures tend to be labor intensive and time-consuming, and so are relatively more expensive than other testing methods.

Cytotoxicity assays (CYTAs): CYTAs detect toxin B only, and are therefore capable of distinguishing CDI from mere C. difficile colonization. Like toxigenic stool cultures, however, CYTAs are expensive and difficult to perform, and require multiple days to get results.

Enzyme immunoassays (EIAs): EIAs can detect both C. diff toxins, and can do so rapidly and relatively inexpensively. The problem with EIAs is their relative lack of sensitivity, which can lead to false-negative results. Due to this, labs that used this technology were typically doing testing from three separate patient samples before being confident that negative results were truly negative.

NAATs and GDH/EIA/PCR algorithms: The American College of Gastroenterology (ACG), in its 2013 “Guidelines for Diagnosis, Treatment, and Prevention of Clostridium difficile infections,"⁵ offers three initial recommendations around laboratory testing for the bacterium: First, the ACG notes, echoing other authorities on the matter, it's important to only test the stools of patients with diarrhea. Beyond that, two different testing procedures are equally appropriate: A) nucleic acid amplification tests (NAATs); or B) glutamate dehydrogenase (GDH) screening tests used in two- or three-step algorithms with subsequent EIA testing for toxins A and B. In the latter case, GDH-positive samples “must undergo additional testing for C. difficile either by NAAT or by EIA testing followed by NAAT if the EIA results are discordant," the guidelines recommend.

In its discussion around NAATs, the ACG guidelines note that “evidence suggests" they are “good stand-alone tests for toxigenic C. diff." Indeed, research shows that real-time PCR permits direct toxin gene detection from fecal samples; is relatively rapid and easy to implement; simplifies reporting, because it's a single test without algorithms; and has both high analytical sensitivity and specificity and high clinical sensitivity. The only real drawback associated with NAATs occurs when labs test samples from patients who don't exhibit clinically significant diarrhea. “Because [NAATs] do not distinguish [CDI] as asymptomatic C. diff carriage, the diagnostic value of NAATs is limited when used in patients with low probability of CDI," explains Larry Kociolek, MD, MSCI, in a 2017 JCM article.⁶ In the updated "Clinical Practice Guidelines for C. diff Infection in Adults and Children" published in 2018, use of a NAAT alone or a multi-step algorithm approach is recommended when there are preagreed institutional criteria for patient stool submission.⁷

The take-home message for anyone with a role in the ongoing fight against C. difficile? If you take a holistic approach to combatting C. diff—combining clinician education, departmental collaboration, and sensitive and accurate testing with PCR—there's a very good chance you're going to win.