Evaluation and clinical implications of the time to a positive results of antigen testing for SARS-CoV-2

ABSTRACT

Antigen tests for severe acute respiratory coronavirus 2 sometimes show positive lines earlier than their specified read time, although the implication of getting the results at earlier time is not well understood. This study aimed to evaluate the clinical utility of an antigen test by evaluating the time period to get positive results and by comparing the test sensitivity with that of a digital immunoassay (DIA) test. We prospectively collected additional nasopharyngeal samples from patients who had already tested positive for SARS-CoV-2 by reverse transcription PCR. The additional swab was used for an antigen test, QuickNavi-COVID19 Ag, and the time periods to get positive results were measured. The sensitivity of QuickNavi-COVID19 Ag was also compared with that of a DIA. In 84 of 96 (87.5%) analyzed cases, the results of QuickNavi-COVID19 Ag were positive. The time to obtain positive results was 15.0 seconds in median (inter quartile range 12.0-33.3, range 11-736), and was extended in samples with higher cycle thresholds (Ct) (p<0.001). Positive lines appeared within a minute in 85.7% of cases and within 5 minutes in 96.4%. The sensitivities of QuickNavi-COVID19 Ag and the DIA were 87.5% (95% confident interval [CI] 79.2%-93.4%) and 88.6% (95%CI 75.4%- 96.2%), respectively. Their results were concordant in 90.9% of cases, with discrepancies present only in cases with Ct values >32. QuickNavi-COVID19 Ag immediately showed positive results in most cases, and the time to a positive reaction may have indicated the viral load. In addition, the sensitivity of the test was comparable to the DIA.