Identification of SARS-CoV-2 in different human tissues. Validation of immunohistochemical and qPCR techniques in paraffin-embedded tissues and cytology

Since the beginning of 2020, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been responsible for a global pandemic. Although the scientific community has focused on its study, there is little knowledge on validated techniques for detecting SARS-CoV-2 in formalin-fixed paraffin embedded (FFPE) tissues archived in pathology departments. The objective of this study was to validate immunohistochemistry (IHC) and quantitative polymerase chain reaction (qPCR) for the diagnosis of respiratory coronavirus disease 2019 (COVID-19) in cytology and FFPE tissues, obtained from multiple specimen types (autopsy, incisional biopsy and surgical specimens). We also defined relevant temporary points (interval, persistence and archival times) to evaluate the correlation between those periods and viral load. A total of 43 cytology and FFPE samples from patients with a previous positive qPCR COVID-19 test in nasopharyngeal swabs were analysed. Two different qPCR techniques were evaluated from IDT Technologies (method A) and Roche Diagnostics (method B). In immunohistochemistry, antibodies directed against nucleocapsid and spike viral proteins were employed. A total of 25.58% of the evaluated samples were positive for any of the two qPCR techniques. Only one placental specimen (3.44%) with acute villitis showed strong IHC positivity for both antibodies, and the other 4 specimens (lung, brain, kidney and placenta) showed isolated weakly positive cells with IHC. A strong statistically significant correlation was observed between threshold values for qPCR method A and interval time (p = 0.01; ρ = 0.917) and persistence time (p = 0.002; ρ = 0.879) and between qPCR method B and archival time (p = 0.037; ρ = 0.900). In conclusion, the qPCR technique is a sensitive diagnostic tool for SARS-COV-2 detection for both cytology and FFPE specimens. With currently available antibodies, IHC to detect the presence of SARS-COV-2 should only be used in special cases.