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Introduction: Highly sensitive reagents for detecting SARS-CoV-2 antigens have been developed for accurate and rapid diagnosis till date. In this study, we aim to clarify the frequency of false-positive reactions and reveal their details in SARS-CoV-2 quantitative antigen test using an automated laboratory device. Methods: Nasopharyngeal swab samples (n = 4992) and saliva samples (n = 5430) were collected. We measured their SARS-CoV-2 antigen using Lumipulse® Presto SARS-CoV-2 Ag and performed a nucleic acid amplification test (NAAT) using the Ampdirect™ 2019 Novel Coronavirus Detection Kit as needed. The results obtained from each detection test were compared accordingly. Results: There were 304 nasopharyngeal samples and 114 saliva samples were positive in the Lumipulse® Presto SARS-CoV-2 Ag test. All positive nasopharyngeal samples in the antigen test were also positive for NAAT. In contrast, only three (2.6%) of all the positive saliva samples in the antigen test were negative for NAAT. One showed no linearity with a dilute solution in the dilution test. Additionally, the quantitative antigen levels of all the three samples did not decrease after reaction with the anti-SARS-CoV-2 antibody. Conclusions: The judgment difference between the quantitative antigen test and NAAT seemed to be caused by non-specific reactions in the antigen test. Although the high positive and negative predictive value of this quantitative antigen test could be confirmed, we should consider the possibility of false-positives caused by nonspecific reactions and understand the characteristics of antigen testing. We recommend that repeating centrifugation before measurement, especially in saliva samples, should be performed appropriately.

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Objectives Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is rapidly spreading globally. Early diagnosis plays an essential role in controlling the infection. Therefore, early and accurate SARS-CoV-2 detection assays along with easy operation are required. The aim of this study was to compare the clinical performance of the Ampdirect™ 2019-nCoV Detection Kit (SHIMADZU assay), which does not require RNA purification steps, with that of the preexisting SARS-CoV-2 detection assays, which use a purified RNA template. Methods A total of 71 samples (65 nasopharyngeal specimens and 6 sputum specimens) were collected from 32 individuals, including patients infected with SARS-CoV-2 and those with suspected infection. The sensitivity and kappa (κ) coefficient were assessed between the SARS-CoV-2 detection assays using the reference standard, which was defined as a true positive result by any one of the four SARS-CoV-2 detection assays. Results The overall sensitivity and κ coefficient of the SHIMADZU assay were 86.0% (95% confidence interval [CI]: 77.9–94.2) and 0.83 (95% CI: 0.69–0.96), respectively. In particular, among the 18 samples collected within 10 days from symptom onset, the sensitivity and κ coefficient of the SHIMADZU assay were 100% and 1.0, respectively. Conclusions Although a relatively small number of samples was evaluated, the SHIMADZU assay showed good analytical performance and as such would be highly useful for the detection of SARS-CoV-2. The test can be performed easily and quickly and has the potential for future applications in situations where a highly sensitive diagnosis is required.