A Clinical Usefulness of Quantitative Antigen Test for COVID-19 Diagnosis

Background：The reverse-transcription polymerase chain reaction (RT-PCR) test is currently the gold standard for the detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, until date, few studies have reported on the performance of the quantitative antigen test. In this study, we attempted to validate the performance of a quantitative antigen test at a medium-sized hospital. Materials and Methods：A total of 149 nasopharyngeal swabs from 115 hospitalized patients were tested in parallel by-SARS-CoV-2 RT-PCR and the quantitative antigen test, and the results were compared. We assessed：(1) the detection rates with the quantitative antigen test and RT-PCR；(2) the factors associated with discordant results；(3) correlation of antigen concentration over the course of the clinical illness. Results：(1) The positive concordance, negative concordance, and overall concordance of the results of the quantitative antigen test with the results of RT-PCR were 100%, 88.4%, and 90.4% respectively. (2) There were no obvious differences in the performance of the test according to the age, sex, period from onset, body temperature, or presence/absence of pneumonia. The antigen concentration was significantly higher in the positively concordant samples (5,000pg/mL；P = 0.002) than in the negatively concordant samples (74.88 pg/mL). (3) The antigen titer decreased with time from the onset of infection (r = ­0.65595). Conclusion：The sensitivity and specificity of the SARS-CoV-2 quantitative antigen test was high and comparable to those of the RT-PCR test. The results indicate that the SARS-CoV-2 quantitative antigen test could be useful not only for screening, but also in clinical practice. The SARS-CoV-2 quantitative antigen test showed sufficient sensitivity throughout the clinical course, from the first screening to discharge from the hospital. The test may be useful for infection control and evaluation of the clinical course of COVID-19 in medium-sized hospitals.