A prospective evaluation of three saliva qualitative antigen testing kits for the detection of SARS-CoV-2 in Japan

Introduction: Rapid qualitative antigen testing has been widely used for the laboratory diagnosis of COVID-19 with nasopharyngeal samples. Saliva samples have been used as alternative samples, but the analytical performance of those samples for qualitative antigen testing has not been sufficiently evaluated. Methods: A prospective observational study evaluated the analytical performance of three In Vitro Diagnostics (IVD) approved COVID-19 rapid antigen detection kits for saliva between June 2022 and July 2022 in Japan using real-time reverse transcription polymerase chain reaction (RT-PCR) as a reference. A nasopharyngeal sample and a saliva sample were simultaneously obtained, and RT-PCR was performed. Results: In total, saliva samples and nasopharyngeal samples were collected from 471 participants (140 RT-PCR-positive saliva samples and 143 RT-PCR-positive nasopharyngeal samples) for the analysis. The median Ct values were 25.5 (interquartile range [IQR]: 21.9-28.8) for saliva samples and 17.1 (IQR: 15.5-18.7) for nasopharyngeal samples (p<0.001). Compared with saliva samples of RT-PCR, the sensitivity and specificity were 46.4% and 99.7% for ImunoAce SARS-CoV-2 Saliva, 59.3% and 99.1% for Espline SARS-CoV-2 N, and 61.4% and 98.8% for QuickChaser Auto SARS-CoV-2, respectively. The sensitivity is >90% for saliva samples with a moderate-to-high viral load (Ct<25), whereas the sensitivity is <70% for high-viral-load nasopharyngeal samples (Ct<20). Conclusion: COVID-19 rapid antigen detection kits with saliva showed high specificities, but the sensitivities varied among kits, and the analytical performance of saliva qualitative antigen detection kits was much worse than that of kits using nasopharyngeal samples.