Performance of an antigen test for detection of SARS-CoV-2, influenza, and respiratory syncytial virus—a prospective study

OBJECTIVES: To assess the performance of the ALL IN TRIPLEX rapid antigen-based test for SARS-CoV-2, influenza A, influenza B, and respiratory syncytial virus. We focus on the potential of the test to be used in acute clinical settings as a marker of infectiousness, by determining the sensitivity at different real-time PCR cycle threshold (Ct) levels, as well as the quantity of absorbed viral RNA needed to produce an antigen band. METHODS: In total, 1372 patients with symptoms and signs of an acute respiratory infection were included in the study; 150 were children. All patients were tested using the nasal swab Triplex kit, as well as with PCR on a separately taken nasopharyngeal sample. The antigen test was analysed by emergency department staff, whereas PCR was performed at a central laboratory. In 226 cases, also the Triplex device medium was subjected to PCR. RESULTS: The sensitivity was 87.0% (95% CI, 66.4-97.7), 79.0% (95% CI, 72.7-84.4), 87.5% (95% CI, 67.6-97.3), and 91.2% (95% CI, 80.7-97.1) for SARS-CoV-2, influenza A, influenza B, and respiratory syncytial virus, respectively, using the nasopharyngeal sample PCR Ct 30 level as reference. Specificity was 100% (95% CI, 99.7-100.0) for all agents. A comparison with Triplex medium PCR Ct results showed a trend suggesting that less viral RNA is absorbed by the nasal swab than by the nasopharyngeal swab. Although there was no sign of this trend in children, it reached significance among the total of patients for influenza A (mean Ct value 31.8 for the Triplex swab and 26.0 for the nasopharyngeal swab, p < 0.0001). CONCLUSIONS: The observed performance of Triplex, for all four viruses and across all age groups, implies that Triplex may serve as a marker of infectiousness in acute clinical care settings.