Improving the Rapid Diagnosis of Clostridioides difficile Infection: A Look at the Sofia® 2 Fluorescent Immunoassay

 

Clostridioides difficile infection (CDI) remains one of the most significant healthcare-associated infections worldwide, particularly affecting hospitalized patients, older adults, and individuals who have recently received antibiotic therapy. The infection can cause symptoms ranging from mild diarrhea to severe complications such as pseudomembranous colitis, toxic megacolon, and sepsis. Because CDI can spread easily within healthcare settings and significantly increase patient morbidity and healthcare costs, rapid and accurate diagnosis is essential for both patient management and infection control. The Sofia® 2 C. difficile Fluorescent Immunoassay (FIA), a novel diagnostic tool designed to detect C. difficile glutamate dehydrogenase (GDH) and toxins A and B.

Diagnosing CDI is often challenging because the bacterium can colonize the intestines without causing disease. As a result, detecting the organism alone is not always sufficient to confirm an active infection. Current diagnostic approaches commonly involve identifying GDH, a highly abundant antigen produced by both toxigenic and non-toxigenic strains of C. difficile, as well as detecting toxins A and B, which are shows the colonisation of bacteria. While GDH detection offers high sensitivity, it cannot distinguish infection from asymptomatic colonization. Toxin detection is more specific for active disease but tends to have lower sensitivity. Therefore, many clinical laboratories employ a combination of tests to improve diagnostic accuracy.

The Sofia® 2 system introduces a fluorescence-based immunoassay that simultaneously detects GDH and toxins A/B using a single cartridge. Unlike conventional enzyme immunoassays that rely on visual interpretation of test lines, the Sofia 2 platform uses an automated fluorescence reader to objectively analyze results. This automation reduces the risk of human error and subjective interpretation while providing results within approximately fifteen minutes. Such rapid turnaround times can be particularly valuable in clinical settings where timely treatment decisions and infection control measures are critical.

To evaluate the assay, researchers collected 262 stool specimens from patients suspected of having CDI at Penn State Hershey Medical Center between December 2022 and March 2023. The samples were tested using the Sofia 2 assay and compared with established commercial enzyme immunoassays, including the C. diff Quik Chek Complete assay and Immunocard assays for discrepancy resolution. After excluding duplicate and invalid samples, 250 specimens were included in the final performance analysis.

The results demonstrated excellent analytical performance of the Sofia 2 assay. For GDH detection, the assay achieved a positive percentage agreement of 100%, a negative percentage agreement of 94%, and an overall agreement of 95% when compared with the consensus results from comparator methods. Interestingly, Sofia 2 identified thirteen additional GDH-positive samples that were not detected by the other assays. The researchers suggested that this finding may indicate improved sensitivity due to fluorescence detection, although the possibility of false-positive results could not be completely ruled out.

Performance was even stronger for toxin detection. The Sofia 2 assay achieved a positive percentage agreement of 100%, a negative percentage agreement of 99%, and an overall agreement of 99% for toxins A and B. Only three samples showed discordant results, all of which were positive by Sofia 2 but negative by the comparator immunoassays. These findings indicate that the assay performs exceptionally well in detecting the toxins that are directly associated with active disease.

The study also compared Sofia 2 results with the hospital's standard-of-care testing algorithm, which involved PCR detection of the toxin B gene followed by toxin antigen testing. In this comparison, Sofia 2 demonstrated a positive percentage agreement of 100% and a negative percentage agreement of 98%. The assay also achieved a negative predictive value of 100%, indicating a very strong ability to rule out CDI when results were negative. However, five specimens were identified as positive by Sofia 2 but not by the standard testing method. Even though, researchers were unable to determine whether these represented true infections that were missed by standard testing or false-positive results generated by the assay.

Beyond its analytical performance, the Sofia 2 platform offers several practical advantages. The automated fluorescence reader eliminates the uncertainty associated with interpreting faint test lines, a common challenge with visually read immunoassays. Laboratory technologists involved in the study reported that the system was easy to use and simplified result interpretation. The instrument also supports laboratory information system connectivity and requires minimal hands-on time, making it a convenient option for busy clinical laboratories.

Reference:

Garrett EM, Pu M, Bobenchik AM. Evaluation of a Fluorescence Immunoassay for Detection of Clostridioides difficile Glutamate Dehydrogenase and Toxin Antigens. J Appl Lab Med. 2025 May 2;10(3):671-678. doi: 10.1093/jalm/jfaf010. PMID: 40105901.

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