Virus variant-specific clinical performance of a SARS-CoV-2 rapid antigen test with focus on Omicron variants of concern

OBJECTIVES: Antigen rapid diagnostic tests (Ag-RDTs) play an important role in the diagnosis of SARS-CoV-2. They are easier, quicker, and less expensive than the ‘reference standard’ RT-PCR and therefore widely in use. Reliable clinical data with respect to Ag-RDT performance in SARS-CoV-2 Omicron variants of concern (VOCs) are limited. Consequently, the objective of this study was to determine the impact different VOCs—especially Omicron—have on the clinical performance of an Ag-RDT. METHODS: We compared the clinical performance of the Sofia SARS-CoV-2 Ag-RDT to RT-PCR in a real-world, single-centre study in a clinical point-of-care setting in patients admitted to a large hospital via the emergency department from 2 November 2020 to 4 September 2022. RESULTS: Among 38 434 Ag-RDT/RT-PCR tandems taken, 1528 yielded a SARS-CoV-2 positive RT-PCR test result, with a prevalence of 4.0% (95% CI, 3.8–4.2). Overall sensitivity of the Ag-RDT was 63.7% (95% CI, 61.3–66.1) and overall specificity was 99.6% (95% CI, 99.5–99.6). Ag-RDT sensitivity was dependent on viral load (VL), because the sensitivity increased to 93.2% (95% CI, 91.5–94.6) in samples with a VL > 10(6) SARS-CoV-2 copies/mL. Furthermore, the Ag-RDT was more sensitive in men, and older patients. Variant-dependent sensitivity assessment showed that the sensitivity was significantly lower in Omicron-VOC (64.1%; 95% CI, 60.5–67.6) compared with SARS-CoV-2 wild-type samples (70.0%; 95% CI, 59,8–78,6) (binomial test; p value < 0.001). Analysing the limits of detection showed a 27 times higher 95% limit of detection for the Omicron-VOC BA.5 compared with the SARS-CoV-2 wild-type. DISCUSSION: Ag-RDT sensitivity for detection of patients with lower VLs and with Omicron-VOC is reduced, limiting the effectiveness of Ag-RDTs. However, Ag-RDTs are still an unreplaceable tool for widely available, quick, and inexpensive point-of-care SARS-CoV-2 diagnostics.