Iain E Buchan
Professor of Public Health and Clinical Informatics, University of Liverpool
Malcolm G Semple
Professor of Outbreak Medicine and Child Health, University of Liverpool
Tim E Peto
Professor of Medicine, University of Oxford
Thirty –minute, inexpensive, non laboratory tests for the SARS-CoV-2 antigen can identify people likely to pass on the virus and may augment public health efforts to reopen society.
Which tests are needed to manage COVID-19?
Rapid isolation of infectious cases is central to managing outbreaks of high-consequence diseases like COVID-19. As those infected with SARS-CoV-2 may not have symptoms, testing is essential for identifying cases.1 Three types of test are being used: nucleic acid; antigen; and antibody detection. There are also three purposes for testing: clinical testing to guide care; public health testing to control the virus; and research testing to understand immunity against future infection.2
The mainstay of testing has been reverse transcriptase polymerase chain reaction (PCR) technology – a nucleic acid test for the virus’ genetic material, usually on nose/throat swabs. Results are reported in one to three days as positive/negative. PCR amplifies complex molecules of nucleic acid, doubling it across each of a series of cycles, detecting even single fragments of dead virus. Counting the cycles needed to detect the nucleic acid – the cycle threshold (Ct) – also allows quantitation of viral load. Comparing Ct between laboratories, however, is difficult as there is currently no adoption of an iInternational sStandard.3
Lateral flow is the most common technology for detecting SARS-CoV-2 antigens, usually nucleocapsid protein.2 A monoclonal antibody to the antigen is stuck on a line across a nitrocellulose membrane at the result window of a small plastic device. The sample and buffer are placed in a well and flow laterally along the nitrocellulose picking up test ingredients. If the antigen is present the line changes colour within 30 minutes – darker if there are more antigens but designed to be read as positive/negative. Thorough swabbing of the nose/throat, sufficient mixing of the swab with the solution and timing of reading are vital for good performance of the test. Lateral flow tests (LFTs) have been validated in a wide variety of settings and viral loads.2-7
Antibody tests reflect historical rather than current infections by looking at the body’s immune response to the virus. Their value in COVID-19 management is being assessed.
Do rapid SARS-CoV-2 antigen lateral flow tests (LFTs) work?
The accuracy of LFTs have been debated – a complex topic, we recently clarified.3
In the absence of a practical gold standard test for live virus, LFTs have been compared with PCR, which looks for evidence of the virus, alive or dead, in samples from a person’s nose or throat. Whereas LFT looks for evidence that a person is shedding larger amounts of live virus and may pass it on.2
In the typical course of infection, symptoms appear a median five-day incubation after exposure to the virus, however, 30-50% of people do not show classic symptoms.8 PCR usually detects virus nucleic acid one to two days after infection, with LFT detecting antigen a day later, both before symptoms may appear. Viral levels peak at symptom onset and wane over the next few days. In the four to eight day transmission window when a person is shedding substantial amounts of virus, with or without symptoms, both PCR and LFT are likely to be positive. Thereafter, their immune system controls the virus and, for the next 17 days or longer, they often have dead virus RNA in their nose/throat giving a positive PCR and negative LFT.9
The infectiousness of individuals with different viral loads has been determined by testing their contacts.4 Most infectious individuals have moderate-to-high viral loads, which LFTs reliably detect (including current viral variants) with sensitivities >90%, whilst individuals with low viral loads are not very infectious and not detected by LFT. These studies indicate that LFT identifies at least eight out of 10 contagious individuals. LFT results are also reliably specific, with fewer than one in 1000 false negatives. A common error is to assess LFT performance against PCR as the ‘gold standard’ when PCR identifies both infectious and post-infectious periods – a starkly different reason for testing.3
Antibody tests reflect historical rather than current infections by looking at the body’s immune response to the virus.
Can society reopen sooner by using rapid antigen tests?
LFTs can be manufactured quickly, cheaply, in vast quantities and can be used with minimal training. Getting results within 30 minutes anywhere enables SARS-CoV-2 transmitters to isolate quickly. This allows safer lifting of COVID-19 restrictions with reduction in the harm to the health, social fabric and wealth of society.
A test is only as good as how it is used in a public health programme, cognisant of behaviours not just biology. Ideally, everyone should know how to test and respond to the results. The 24+ hours quicker results from LFT vs PCR is valuable when test-positive people isolate promptly and encourage their contacts to get tested. Similarly, understanding among workplace teams (e.g. fire crews) of how to test daily instead of quarantine after contact with a case can secure key services.
LFT offers the scale and speed needed to crowd-source safer reopening of society, reducing the harms from current restrictions. The challenge is to ensure equity of access to testing and support isolate for disadvantaged communities.7
1. Cevik M, Kuppalli K, Kindrachuk J, Peiris M. Virology, transmission, and pathogenesis of SARS-CoV-2. BMJ. 2020 Oct 23;371:m3862. doi: 10.1136/bmj.m3862.
2. Crozier A, Rajan S, Buchan I, McKee M. Put to the test: use of rapid testing technologies for covid-19. BMJ. 2021 Feb 3;372:n208. doi: 10.1136/bmj.n208.
3. Mina MJ, Peto TE, García-Fiñana M, Semple MG, Buchan IE. Clarifying the evidence on SARS-CoV-2 antigen rapid tests in public health responses to COVID-19. Lancet. 2021 Feb 17:S0140-6736(21)00425-6. doi: 10.1016/S0140-6736(21)00425-6.
4. Lee L, Rozmanowski S, Pang M, et al. An observational study of SARS-CoV-2 infectivity by viral load and 2 demographic factors and the utility lateral flow devices to prevent 3 transmission. University of Oxford. 2021. http://modmedmicro.nsms.ox.ac.uk/wp-content/uploads/2021/01/infectivity_manuscript_20210119_merged.pdf (accessed Feb 12, 2021).
5. Ferguson J, Dunn S, Best A, et al. Validation testing to determine the effectiveness of lateral flow testing for asymptomatic SARS-CoV-2 detection in low prevalence settings. medRxiv 2020; published online Dec 24. https://doi.org/10.1101/2020.12.01.20237784 (preprint).
6. Pray IW, Ford L, Cole D, et al. Performance of an antigen-based test for asymptomatic and symptomatic SARS-CoV-2 testing at two university campuses—Wisconsin, September–October 2020. MMWR Morb Mortal Wkly Rep 2021; 69: 1642–47.
7. University of Liverpool (Buchan I, Ed). Liverpool Covid-19 Community Testing Pilot interim report. University of Liverpool. 2020. https://www.liverpool.ac.uk/media/livacuk/coronavirus/Liverpool,Community,Testing,Pilot,Interim,Evaluation.pdf (accessed Feb 12, 2021).
8. Cevik M, Tate M, Lloyd O, et al. SARS-CoV-2, SARS-CoV, and MERS-CoV viral load dynamics, duration of viral shedding, and infectiousness: a systematic review and meta-analysis. Lancet Microbe 2021; 2: e13–22.
9. van Kampen JJA, van de Vijver DAMC, Fraaij PLA, et al. Duration and key determinants of infectious virus shedding in hospitalized patients with coronavirus disease-2019 (COVID-19). Nat Commun 2021; 12: 267.
