Background: During the coronavirus disease 2019 (COVID-2019) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) rapid antigen tests (RATs) have been widely used with reverse transcription polymerase chain reaction (RT-PCR). RATs are simple, fast, and cost-effective. However, its diagnostic performance characteristics remain unclear. In this study, we compared RAT and RT-PCR to evaluate the diagnostic usefulness of these tests in routine clinical practice.
Methods: A total of 515 nasopharyngeal swab samples from patients in the emergency department were included in this study. We compared the Standard Q COVID-19 Ag test and RT-PCR using the Real-Q direct SARS-CoV-2 detection kit. We analyzed the sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), accuracy, and kappa value of the RAT using RT-PCR as a reference. Furthermore, the detailed sensitivity of RAT was calculated using cycle threshold (Ct) values, with stratified cut-offs.
Results: Of the 515 samples, 42 samples (8.2%) tested positive using RT-PCR and 26 samples (5.0%) tested positive using RAT. When compared to RT-PCR, the sensitivity, specificity, PPV, and NPV of the RAT were 59.5%, 99.8%, 96.2%, and 96.5%, respectively. The diagnostic accuracy was 96.5% and the kappa value was 0.718, thus demonstrating substantial agreement with RT-PCR. Additionally, the sensitivity of the RAT showed a decreasing trend with rising Ct values.
Conclusions: The RAT has many advantages and is a useful diagnostic method. However, care should be taken when considering it as an independent confirmatory test. It is reasonable to use both RAT and RT-PCR for SARS-CoV-2 detection. Importantly, it is necessary to consider appropriate standards when applying RATs to certain situations.

Keywords: SARS-CoV-2 antigen test, Reverse transcription polymerase chain reaction, COVID-19