Accuracy and usability of saliva and nasal rapid antigen self-testing for detection of SARS-CoV-2 infection in the general population: a head-to-head comparison

Background SARS-CoV-2 self-tests may lower the threshold of testing and produce a result quickly. This could support the early detection of infectious cases and reduce further community transmission. However, the diagnostic accuracy of (unsupervised) self-testing with rapid antigen diagnostic tests (Ag-RDTs) is mostly unknown. We therefore conducted a large-scale head-to-head comparison of the diagnostic accuracy of a self-performed SARS-CoV-2 saliva and nasal Ag-RDT, each compared to a molecular reference test, in the general population in the Netherlands. Methods In this cross-sectional study we consecutively included individuals aged 16 years and older presenting for SARS-CoV-2 testing at three Dutch public health service test sites irrespective of their indication for testing, vaccination status, and symptomatology. Participants were sampled for molecular testing at the test site and received two self-tests (the Hangzhou AllTest saliva self-test and the SD Biosensor nasal self-test by Roche Diagnostics) to perform at home within a few hours without knowledge of their molecular test result. Information on presence and type of symptoms, user experiences, and results of both self-tests were collected via an online questionnaire. For each self-test, sensitivity, specificity, positive and negative predictive values were determined with molecular testing as reference standard. Findings The SARS-CoV-2 molecular reference test positivity rate was 6.5% in the 2,819 participants. Overall sensitivities with 95% confidence intervals were 46.7% (85/182; 39.3%-54.2%) for the saliva Ag-RDT, and 68.9% (124/180; 61.6%-75.6%) for the nasal Ag-RDT. With a viral load cut-off ([≥]5.2 log10 SARS-CoV-2 E-gene copies/mL) as a proxy of infectiousness, sensitivities increased to 54.9% (78/142; 46.4%-63.3%) for the saliva Ag-RDT and 83.9% (120/143; 76.9%-89.5%) for the nasal Ag-RDT. For the nasal Ag-RDT, sensitivities were 78.5% [71.1%-84.8%] and 22.6% [9.6%-41.1%] in those with and without symptoms at the time of sampling, which increased to 90.4% (113/125; 83.8%-94.9%) and 38.9% (7/18; 17.3%-64.3%) after applying the viral load cut-off. In those with and without prior confirmed SARS-CoV-2, sensitivities were 36.8% [19/372; 16.3%-61.6%] and 72.7% [161/2437; 65.1%-79.4%] for the nasal Ag-RDT, which increased to 100% (7/7; 59.0%-100%) and 83.1% (113/126; 75.7%-89.0%) after applying the viral load cut-off. The diagnostic accuracy of the nasal Ag-RDT did not differ by COVID-19 vaccination status, sex, and age. Specificities were >99%, positive predictive values >70% and negative predictive values >95%, for the saliva Ag-RDT, and >99%, >90%, and >95% for the nasal Ag-RDT, respectively, in most analyses. Interpreting the results was considered (very) easy for both self-tests. Interpretation The Hangzhou AllTest self-performed saliva Ag-RDT is not reliable for SARS-CoV-2 infection detection, overall nor in the studied subgroups. The SD Biosensor self-performed nasal Ag-RDT had high sensitivity in individuals with symptoms and in those without a prior SARS-CoV-2 infection. The overall accuracy in individuals with symptoms was comparable to that found in previous studies with professional-performed sampling for this Ag-RDT. The extremely low sensitivity of the nasal Ag-RDT in asymptomatic individuals and in individuals who had had a prior SARS-CoV-2 infection is an important finding and warrants further investigation.