Evaluation of a Rapid Antigen Test To Detect SARS-CoV-2 Infection and Identify Potentially Infectious Individuals

Abstract

The identification and isolation of highly infectious SARS-CoV-2-infected individuals is an important public health strategy. Rapid antigen detection tests (RADT) are promising tools for large-scale screenings due to timely results and feasibility for on-site testing. Nonetheless, the diagnostic performance of RADT in detecting infectious individuals is not yet fully determined. In this study, RT-qPCR and virus culture of RT-qPCR-positive samples were used to evaluate and compare the performance of the Standard Q COVID-19 Ag test in detecting SARS-CoV-2-infected and possibly infectious individuals. To this end, two combined oro- and nasopharyngeal swabs were collected at a routine SARS-CoV-2 diagnostic center. A total of 2,028 samples were tested, and 118 virus cultures were inoculated. SARS-CoV-2 infection was detected in 210 samples by RT-qPCR, representing a positive rate of 10.36%. The Standard Q COVID-19 Ag test yielded a positive result in 92 (4.54%) samples resulting in an overall sensitivity and specificity of 42.86 and 99.89%, respectively. For adjusted C_T values of <20 (n = 14), <25 (n = 57), and <30 (n = 88), the RADT reached sensitivities of 100, 98.25, and 88.64%, respectively. All 29 culture-positive samples were detected by the RADT. Although the overall sensitivity was low, the Standard Q COVID-19 Ag test reliably detected patients with high RNA loads. In addition, negative RADT results fully corresponded with the lack of viral cultivability in Vero E6 cells. These results indicate that RADT can be a valuable tool for the detection of individuals with high RNA loads that are likely to transmit SARS-CoV-2.

Keywords: RT-qPCR; SARS-CoV-2; antigen test; infectiousness; virus culture.

FIG 1

Study procedure and cohort description. (A) Flowchart including cohort sizes selected for RT-qPCR, RADT, and cell culture assays. Percentages in boxes 3 and 4 refer to all analyzed samples (n = 2,028). (B) Distribution and number of symptoms, as well as test frequencies, among individuals. (C) Age and gender distribution of the cohort. (D) Distribution of cycle threshold (C_T) values (adjusted to cobas 6800).

FIG 2

Head-to-head comparison of SARS-CoV-2 detection by RT-qPCR and RADT. (A) RT-qPCR and RADT results of all 2,028 specimens. (B) The 210 RT-qPCR-positive samples are plotted by adjusted C_T values and SARS-CoV-2 RNA load, respectively, and stratified by their RADT result (P < 0.0001, MWU). (C) The sensitivity of the RADT is stratified by adjusted C_T values and RNA load in log copies/ml. The top graph shows data points for two C_T units each. Due to low sample sizes, C_T values of >37 were combined.

FIG 3

RT-qPCR and RADT results in symptomatic and asymptomatic individuals. (A) The number of RT-qPCR-positive and -negative specimens from symptomatic individuals is plotted by the days since symptom onset. (B) The number of RADT-positive and -negative specimens within the RT-qPCR-positive samples of symptomatic participants (n = 130) is stratified by the days since symptom onset. (C) Symptomatic and asymptomatic RT-qPCR positives are plotted by adjusted C_T value and RNA load (P < 0.0001, MWU). (D) RT-qPCR-positive samples of symptomatic individuals are plotted by RNA load and stratified by the number of days since symptom onset. The RADT result is indicated in corresponding colors.

FIG 4

Virus culture analysis and RADT performance in the detection of replication-competent SARS-CoV-2. (A) Positive and negative viral cultures plotted by adjusted C_T and RNA load of the original swab medium (P < 0.0001, MWU). (B) Adjusted C_T values and RNA loads of cultured samples are stratified by culture and RADT result (P < 0.0001, MWU). (C) Culture-positive and -negative samples are plotted by RNA load of original swab medium and stratified by the days since symptom onset. (D) Probability of positive result for RADT and viral cultures in the context of adjusted C_T values and RNA load (Probit-Model, R-function GLM).