A Breakdown of the Reported Sensitivities of SARS-CoV-2 Diagnostic Assays and Implications for the Sensitivity Required for Rapid Antigen Tests

Figure Caption

Top Tier: The viral load of SARS-CoV-2 in patients charted as determined in hospital settings across several regions of the world.[1-4] In three studies, the pool includes a limited number patients with mild to moderate symptoms in hospitals, while a fourth study includes a larger sample pool taken across a hospital network on the US East Coast.[4] The latter is considered more representative of the viremia in the general population since it likely includes asymptomatic patients.

Middle Tier: We report the results determined and published by the FDA of molecular tests from several manufacturers validated against an FDA reference panel.[5] Tests were performed in a blinded fashion and results reported to the FDA for confirmation of the assay outcomes.

Bottom Tier: As of October 2020, results using FDA blind reference panels have not been reported for rapid antigen tests, thus we chart the LOD as reported in the manufacturer’s package inserts.[6-9] Manufacturers used different inactivated virions samples for their analytical determination. Hence, the viral load of some samples (RNA equivalent/mL) cannot be determined based on the manufacturer’s disclosures and have thus been estimated.

Discussion

Interestingly, the general pattern for molecular tests shown above is that manufacturer reported LODs are 1-1.5 logs lower than the LODs determined using the blind FDA reference panels.

With this in mind, it is reasonable to anticipate there may be a similar pattern for rapid antigen tests between LODs determined by the manufacturer using their own reagents vs the reagents provided by a standard FDA reference panel. Hence, an LOD ~106 RNA/mL remains a very desirable milestone for rapid antigen tests to achieve when validated using a standardized FDA reference panel.

What is truly needed, yet still unavailable, is a rapid antigen test with an LOD ~105 RNA/mL or lower as a test with an LOD of 104 RNA/mL would detect approximately 67% of true positive cases in the infectious stage, up from the ~31% with the current antigen tests on the market.[4]

The WHO recommendation for a rapid COVID-19 POC device and a broader Target Product Profile description can be found in Ref [10].

References

[1]        Y Pan et al., Viral load of SARS-CoV-2 in clinical samples, The Lancet Infection 20, (2020), 411-412

[2]        S. Iwasaeki et al., Comparison of SARS-CoV-2 detection in nasopharyngeal swab and saliva, Journal of Infection 81, (2020), e145-e147

[3]        K.K.K To et al., Temporal profiles of viral load in posterior oropharyngeal, saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study, Lancet Infect Dis (2020) 20: 565–74

[4]        R. Arnaout, R. A. Lee, G. Rye Lee, C. Callahan, C. F. Yen, K. P. Smith, R. Arora, J. E. Kirby, SARS-CoV-2 Testing: The Limit of Detection Matters, bioRxiv 2020.06.02.131144; doi: https://doi.org/10.1101/2020.06.02.131144

[5]        SARS-CoV-2 Reference Panel Comparative Data (accessed September 21, 2020): https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/sars-cov-2-reference-panel-comparative-data

[6]        Veritor System For Rapid Detection of SARS-CoV-2 https://www.fda.gov/media/139755/download (accessed Jul 23, 2020)

[7]        Sofia SARS Antigen FIA https://www.fda.gov/media/137885/download (accessed Jul 23, 2020)

[8]        BinaxNow COVID-19 Ag card, IN195000 Rev1 2020/08 (accessed 2020-09-01)

[9]        B.D. Grant et al., SARS-CoV2 Coronavirus Nucleocapsid Antigen-Detecting Half-Strip Lateral Flow Assay Toward the Development of Point of Care Tests Using Commercially Available Reagents, Anal. Chem. 2020, 92, 11305−11309

[10]     World Health Organization, COVID-19 Target product profiles for priority diagnostics to support response to the COVID-19 pandemic v.0.1 https://www.who.int/publications/m/item/covid-19-target-product-profiles-for-priority-diagnostics-tosupport-response-to-the-covid-19-pandemic-v.0.1 (released July 31, 2020)