Evaluation of the Analytical Sensitivity of a Molecular Point-of-Care Assay as Compared to 3 Lateral Flow Assays for Group A Streptococcus.

Background: An accurate and timely diagnosis of pharyngitis caused by group A streptococcus (GAS) is essential for ensuring appropriate antibiotic therapy. Early detection through point-of-care (POC) testing is valuable in initiating effective treatment. This study aimed at evaluating the analytical sensitivity of the molecular ID NOW™ Strep A 2 POC test as compared to 3 POC lateral flow assays: BD Veritor™ Plus System, Sofia® Strep A+ Fluorescent Immunoassay, and Sekisui Diagnostic OSOM® Strep A test for GAS detection.

Methods: Two Streptococcus pyogenes bacterial American Type Culture Collection (ATCC) isolates and one clinical isolate from a throat swab sample were used to evaluate the limit of detection (LoD) of each assay. Ten-fold serial dilutions of the isolates starting from 107 colony factor units (CFU)/mL were prepared, and each dilution was tested in triplicate for all 4 assays simultaneously. All tests were performed as per manufacturers' instructions. The LoD was defined as the last dilution that yielded positive results for all 3 replicates. CFU per swab was also calculated.

Results: For the 3 isolates evaluated, LoDs of ID NOW Strep A 2 ranged from 3.125 × 103 to 2.5 × 104 CFU/mL; for Quidel Sofia the range was 1 × 106 to 1 × 107 CFU/mL; for BD Veritor the range was 1 × 107 to 1.5 × 107 CFU/mL; and for Sekisui OSOM the LoD was 1 × 107 CFU/mL for all 3 isolates.

Conclusions: Compared to antigen-based lateral flow assays (LFAs), the molecular ID Now Strep A 2 assay demonstrated a lower LoD, which translates into higher sensitivity. In a clinical setting, this could enable detection of samples with a lower bacterial load that could be missed by low-sensitivity LFAs.