Impact of the Reverse 2-Step Algorithm for Testing in the Microbiology Laboratory on Hospitalized Patients.

Background: Multistep laboratory testing is recommended for the diagnosis of infection (CDI). The aim of this study was to present the impact of multistep CDI diagnostic testing in an academic hospital system and evaluate the toxin B gene polymerase chain reaction (PCR) cycle threshold (Ct) values of PCR-positive tests.

Methods: In October 2022, our system began reflex testing all PCR-positive stool samples with the QUIK CHEK COMPLETE (Techlab), an enzyme immunoassay-based test with results for the glutamate dehydrogenase antigen (GDH) and toxin A/B. Hospital-onset (HO) CDI and CDI antibiotic use before and after testing were tracked. Ct values were obtained from the Infectious Diseases Diagnostic Laboratory. Receiver operating curve analysis was used to examine the sensitivity and specificity for identifying GDH/toxin and GDH/toxin at various Ct thresholds.

Results: The HO-CDI rate decreased from 0.352 cases per 1000 patient-days to 0.115 cases per 1000 patient-days post-reflex testing ( < .005). Anti-CDI antibiotics use decreased, but the decrease was not commensurate with CDI rates following reflex testing. PCR/GDH/toxin samples had a lower mean Ct value than PCR/GDH/toxin samples (23.3 vs 33.5, < .0001). A Ct value of 28.65 could distinguish between those 2 groups. Fifty-four percent of PCR/GDH/toxin samples had a Ct value below that cut-off, suggesting the possibility of CDI with a negative toxin test.

Conclusions: Reflex testing for a laboratory diagnosis of CDI results in rapid, systemwide decreases in the rate of HO-CDI. Additional research is needed to distinguish CDI from colonization in patients with discordant testing.