Performance of a rapid antigen test in the diagnosis of SARS-CoV-2 infection

Diagnostics is crucial for a prompt identification of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infected patients, their isolation and treatment. Real-time PCR is the reference method for the diagnosis of SARS-CoV-2 infection; however, the unprecedented increase in the number of infections worldwide calls for faster and easy methods that do not require skilled personnel and special equipment. Rapid antigen tests have been developed and used as first line screening. Here, we assessed the performance of a rapid antigen test in comparison to a real-time qualitative PCR as gold standard. Fifty nasopharyngeal swabs from suspected cases of SARS-CoV-2 infection have been tested by Coris coronavirus disease 2019 Ag Respi-Strip test and Allplex 2019n-CoV assay. Of the 50 nasopharyngeal swabs tested, 11 were negative by both tests, 27 were negative by Ag test but positive by real-time PCR, and 12 were positive by both methods. PCR detected the 39 positive samples at a median cycle threshold (Ct) value of 22.78 (mean: 24.51; range: 13.59-39.6). In the 12 concordant samples, the median Ct value was 17.37. The sensitivity of the Ag test was 30.77% (95% confidence interval [CI]: 17.02%-47.57%), specificity 100% (95% CI: 71.51%-100.00%), positive predictive value 100%, negative predictive value 85.25% (95% CI: 82.42%-87.69%), and accuracy 86.15% (95% CI: 73.45%-94.28%). The level of agreement between the two tests was poor, k = 0.164. The Ag test performs well in the presence of high viral loads, whereas lower levels are missed. Considering the poor sensitivity of the method, real-time PCR remains the gold standard as front line screening for SARS-CoV-2 infection.