JAMA Diagnostic Test Interpretation
August 21, 2023
Multistep Testing Algorithms for Clostridioides difficile Infection
Maribeth R. Nicholson, Curtis J. Donskey
JAMA. Published online August 21, 2023. doi:10.1001/jama.2023.15875
Case
A10-year-old female with spina bifida and neurogenic bowel was hospitalized for abdominal pain, dehydration, and more than 20 episodes of watery diarrhea per day. Her symptoms began 5 days after completing a course of amoxicillin for streptococcal pharyngitis. Results of a gastrointestinal pathogen panel that included 18 bacterial, viral, and parasitic pathogens were negative. A stool sample result was positive for Clostridioides difficile by polymerase chain reaction (PCR) testing and an enzyme immunoassay test for C difficile toxin A and B had a negative result.
How Do You Interpret These Test Results?
- Antibiotic treatment for C difficile infection is not indicated.
- Antibiotic treatment for C difficile infection is warranted.
- Stool testing for fecal leukocytes is necessary.
- The gastrointestinal pathogen panel is likely a false-negative result.
B. Antibiotic treatment for C difficile infection is warranted.
Few studies are available to guide best testing practices for the diagnosis of C difficile infection (CDI) in children.1Colonization with toxigenic and nontoxigenic C difficile affects 3% to 40% of children younger than 3 years and 3% to 26% of hospitalized children and adults.2 Therefore, diarrhea from causes other than CDI, such as laxatives, viral infections, and prior antibiotic use, should be considered.3
Commonly used diagnostic stool tests for detection of CDI are summarized in the Table. Nucleic acid amplification tests (NAAT), such as PCR, detect genes that encode for toxins produced by toxigenic C difficile. Enzyme immunoassay (EIA) for glutamate dehydrogenase (GDH) antigen detects GDH protein, which is present in both toxigenic and nontoxigenic C difficile. NAAT and GDH antigen tests have high sensitivity (96% and 94%, respectively) for detection of C difficile, but lower specificity (94% and 90%, respectively) because they do not detect C difficile toxin.2-4 EIAs for C difficile toxins A and B have a high specificity for CDI (99%), but should not be used alone due to their low sensitivity for CDI (83%).2-4
Multistep algorithms typically include a test with high sensitivity (NAAT or GDH antigen test) and a test with high specificity (toxin EIA).1-5 A potential benefit of multistep algorithm testing, compared with NAAT or GDH antigen testing alone, is an improved ability to distinguish CDI from C difficile colonization, which may facilitate avoiding unnecessary antibiotics.5 Negative screening test results have high negative predictive value (>98% at CDI prevalence of 5% to 10%) and reliably exclude CDI.3 Patients with a positive toxin EIA result are classified as likely to have CDI, although some may be asymptomatic carriers of toxigenic C difficile.6,7
Discordant results on multistep screening testing (ie, a positive NAAT or GDH antigen test result and a negative toxin EIA result) may be due to CDI or asymptomatic C difficile carriage. Individuals with test results positive for GDH antigen and negative for toxin EIA can undergo NAAT to determine whether they have a toxigenic strain of C difficile. Many patients with discordant results on multistep screening testing have risk factors for C difficile and a clinical presentation consistent with CDI, and approximately 70% are treated with antibiotics for CDI.8-10 Compared with patients who have positive results on both C difficile multistep screening tests, patients with discordant results have less severe illness, reduced recurrence rates, and a lower mortality rate,8,9 although fulminant CDI can occur.10 Therefore, treatment should not be withheld if there is high clinical suspicion of CDI.2-4,8
Hospitals and clinics that use multistep algorithms for CDI should inform clinicians about optimal interpretation of these test results. In 2023, Medicare reimbursements were $37.27 for NAAT, $13.15 for GDH antigen testing, and $16 for toxin EIA.
Application of Test Results to This Patient
This patient had a high pretest probability for CDI due to voluminous diarrhea that developed after a recent course of antibiotics. Therefore, her positive PCR test result and negative toxin EIA result were unlikely to be due to C difficile colonization, and treatment for CDI was warranted.
Alternative Diagnostic Testing Approaches
According to the Infectious Diseases Society of America 2017 CDI practice guidelines, NAAT can be used alone for CDI testing in hospitals with a diagnostic stewardship program that educates clinicians and laboratory personnel to perform NAAT only for unexplained new-onset diarrhea (≥3 unformed stools in 24 hours) in patients not taking laxatives.2 Cell cytotoxicity neutralization assay for toxin and toxigenic culture are the reference tests for C difficile. However, these tests are rarely used in clinical laboratories because results are not available until several days after testing.3
The patient was treated with 10 days of oral vancomycin. Her diarrhea and abdominal pain improved within 2 days and resolved after 10 days. However, 7 days after completing the vancomycin, she developed recurrent copious diarrhea and was hospitalized for dehydration. Results of stool test were positive for C difficile PCR and negative for toxin EIA. Due to the high suspicion of recurrent CDI, she was prescribed fidaxomicin, 200 mg, twice daily. Her diarrhea improved within 2 days and resolved after 10 days of fidaxomicin. At her most recent clinic visit, 2 months after initial presentation, the patient had no recurrent diarrhea.
Clinical Bottom Line
- Multistep CDI test algorithms include a sensitive stool screening test, such as a nucleic acid amplification test (NAAT) or glutamate dehydrogenase (GDH) antigen testing, and a more specific test, enzyme immunoassay (EIA), for C difficile toxin A and B.
- Multistep CDI testing may help distinguish CDI vs colonization, potentially avoiding unnecessary antibiotics compared with NAAT or GDH antigen testing alone.
- Patients with a positive NAAT or GDH antigen test result and a negative toxin EIA result may have symptomatic CDI or may be asymptomatic carriers of C difficile.
