Analytical Sensitivity and Effectiveness of Different SARS-CoV-2 Testing Options

Author(s): Nico Lelie, Marco Koppelman, Harry van Drimmelen, Sylvia Bruisten

We prepared Severe Acute Respiratory Syndrome-Coronavirus-2 (SARS-CoV-2) working standards from a pool of swab fluid samples for comparing the analytical sensitivity of different molecular detection methods and rapid antigen assays. The following 50% Limits of Detection (LOD) (and 95% Confidence Interval (CI)) were estimated in RNA copies/mL: Roche cobas PCR 1.8 (1.0-3.3), Hologic Aptima TMA 6.6 (4.4-9.9), DRW SAMBA 15 (7-30), Molgen LAMP 23 (13-42), Fluorecare antigen 50,000, Abbott Panbio antigen 75,000 and Roche antigen 100,000 copies/mL. One 50% Tissue Culture Infectious Dose (TCID50) of culture fluid was estimated to be equivalent to approximately 1000 RNA copies (2700- 4300 International Units) in our standard. When assuming this level as a proxy for the start of contagiousness in a log-linear ramp up phase model with 10-fold and 10,000-fold rise of viral load per day for the original B.1 (Wuhan) type and B.1.617.2 Delta variant respectively we estimated relative time points of first detectability of early infection from the 50% LODs. The four molecular assays would be able to detect the B1 (Wuhan) type 40-66 hours earlier than the 1000 copies/mL infectivity threshold, whereas the three antigen tests would become positive 41-48 hours later. Our modeling of analytical sensitivity data confirms that molecular assays are more reliable than antigen assays for identifying early infected asymptomatic individuals who are potentially infectious. However, if our assumption of approximately 4-fold more rapid viral replication after exposure to the Delta variant and the currently circulating less pathogenic Omicron variant is correct daily antigen self-tests may be a more practical alternative.