Clostridium difficile-associated infection (CDI) causes up to 25 percent of cases of antibiotic-associated diarrhea among inpatients,¹ and is also responsible for serious presentations such as colitis, toxic megacolon, colon perforation, sepsis and death.¹ The presence of toxin B from C. difficile is essential for disease.² Risk factors for CDI include antibiotic treatment, longer hospital stay, and age over sixty-five years.¹ Incidence, severity, and mortality from CDI have been rising for at least a decade.^1,2 The Association for Professionals in Infection Control and Epidemiology (APIC) estimates the incidence of CDI to be at least 13 per 1,000 inpatients, with 109,000 deaths per year in the U.S.¹ APIC calculated that a diagnosis of CDI adds between $2,454 and $7,179 in non-reimbursable costs per affected patient, and between 3.6 to 7 days to their hospital stays. U.S. expenditures for CDI treatment and prevention have been estimated at $1 billion per year,¹ a figure that is likely quite conservative based on APIC's findings. The recent emergence of one strain of C. difficile, BI/NAP1/027, has recently emphasized the need for rapid and accurate CDI diagnostic testing.

C. difficile, which exists in vegetative and spore forms, is spread through the fecal-oral route. The persistence of spores, which can survive on fomites for months, complicates isolation and containment strategies. Soap and water are necessary for hand hygiene while environmental disinfection requires dilute bleach.

Traditional Diagnostics

Diagnosis of CDI requires that the patient have diarrhea, with either a positive stool test for toxigenic C. difficile, or typical pseudomembranes on colonoscopy or typical pathology from the colon.¹ Since diarrhea is a hallmark of CDI, stool testing at our institution is restricted to samples that take the shape of their container.

Conventional CDI diagnostics lack acceptable sensitivity, specificity and timeliness. Stool culture for C. difficile is labor-intensive, takes 48 to 96 hours and does not differentiate between toxigenic and non-toxigenic strains. Cytotoxicity assays, the gold standard,¹ are sensitive and specific but take up to one week. The glutamate dehydrogenase assay is rapid but fails to distinguish toxigenic from non-toxigenic strains¹ so it must be combined with an additional test for toxigenicity. Rapid enzyme immunoassays (EIAs) for C. difficile toxin have become the default test for CDI despite their poor sensitivity (50-99 percent) and specificity (70-100 percent).¹ Physician mistrust of negative EIAs leads to excessive re-testing, inappropriate treatment and isolation of an unacceptable number of patients.¹

Move to Molecular

Real-time polymerase chain reaction (PCR) has the potential to improve CDI diagnosis because of increased accuracy relative to cytotoxicity testing, along with rapid results determination similar in speed to that of EIAs. The BD GeneOhm^TM C. diff assay amplifies and detects tcdB, the gene coding for C. difficile toxin B. The test takes approximately one hour with reported sensitivity and specificity greater than 95 percent^1,2,3 and negative predictive values (NPVs) of 99 percent.^1,2

Cepheid and Prodesse also have recently FDA-approved real-time PCR tests for C. difficile. Both tests detect tcdB; the Cepheid Xpert^® C. difficile test runs on the company's modular GeneXpert^® automated sytem. Prodesse's ProGastro™ Cd Assay consists of a three-step process including stool clarification, nucleic acid extraction on the bioMérieux NucliSENS™ easyMAG, and real-time PCR using Cepheid's SmartCycler® II.

Superior test performance should provide confidence in interpreting and reacting to both positive and negative test results. For negative test results, which have been suspect with lower sensitivity tests, clinicians can now spare patients from unnecessary empiric antibiotic treatment, and free physicians to investigate alternative diagnoses, along with preventing the excessive isolation of patients.

A comparison study between EIA and real time PCR at our hospital suggested that PCR could cut false negative and false positive tests to nearly zero, thus practically eliminating unnecessary re-testing, and inappropriate isolation and treatment, while providing a higher standard of care. Our research indicates that the accrued savings will easily offset the higher cost of real time PCR testing.

Dr. Currie is vice president and medical director for Research at Montefiore Medical Center and Assistant Dean for Clinical Research at the Albert Einstein College of Medicine.

References

1. Blossom et al. 2007. Clin Inf.Dis.45:222

2. Centers for Disease Control and Prevention. (2005, July, 22). Information for healthcare providers. Retrieved June 27, 2008 from http://www.cdc.gov/ncidod/dhqp/id_CdiffFAQ_HCP.html