Antigen testing for COVID-19 using image-based assessment of oral specimens

While numerous diagnostic tests for COVID-19 have been developed for clinical and public health use, most of them provide binary or one-dimensional information on SARS-CoV-2 infection in pursuit of speed and ease of use. As their readouts are largely dependent on the specimen collection procedure, reliable diagnosis is still difficult. Here we report the development of a prototypical method for the immunocytochemical diagnosis of SARS-CoV-2 infection using oral specimens and fluorescent nanobodies against the viral spike and nucleocapsid proteins. Our cytological approach for the detection of SARS-CoV-2 infection was validated by our finding that at least half of SARS-CoV-2 RNAs in oral specimens were localized in the cellular fraction. Mapping antigens on sampled cells provided qualitative image data to which appropriate statistical texture analysis could be applied for the quantitative assessment of SARS-CoV-2 infectious status. A comprehensive comparative analysis revealed that oral cavity swabbing by medical workers provides specimens for COVID-19 diagnosis that yield comparable diagnostic accuracy as self-collected saliva specimens. Our diagnostic strategy may enable medical workers to acquire a wealth of information on virus–human cell interactions for multifaceted insight into COVID-19.