SARS-CoV-2 testing is a term used for different applications including diagnostics, screening, and surveillance. This post describes the differences between them.
What is COVID-19 Diagnostic Testing?
Diagnostic testing enables the identification of individuals with an active case of COVID-19. It is used when there is a reason to suspect that an individual may be infected with the SARS-CoV-2 virus, either people who are experiencing COVID-19 symptoms or ones who have recently been exposed to someone with COVID-19. Diagnostic testing is used on:
- Symptomatic individuals
- Individuals identified using contact tracing
- Individuals who indicate that they were exposed to someone with a confirmed or suspected case of COVID-19, and
- Individuals present at locations where a person was later confirmed to have COVID-19
It is also used to determine resolution of an infection once symptoms abate.
What is Screening Testing?
Like diagnostic testing, screening for SARS-CoV-2 identifies infected individuals; however, screening tests are used when there is no reason to suspect infection. This includes screening of asymptomatic individuals without known exposure to the virus, but where there is a need to make decisions based on the results. Screening tests are used to proactively help control the transmission of SARS-CoV-2 by identifying asymptomatic individuals who could unknowingly spread COVID-19 to others.
Examples of screening testing include:
- Back to work screening
- Back to school screening
- Screening in high risk environments, including aged care facilities and airports
Screening testing results are used to determine who may enter the site and whether any protective measures should be taken. The ability to pool samples from asymptomatic individuals is critical to allow for cost-effective testing of a large number of individuals. For accurate detection in pooled samples, a very sensitive RT-PCR assay is needed as the samples are diluted by each other in the pool prior to testing.
What is Surveillance Testing?
Unlike diagnostic testing or screening, surveillance testing is used to obtain information at a population level, rather than an individual level. The purpose of surveillance testing is to understand how a SARS-CoV-2 outbreak arose and allow health authorities to make plans to contain the outbreak for the future. Surveillance testing includes monitoring environmental sources for signs of infection in the population and analyzing the genetic variants across an outbreak to identify the origins of the infection.
Wastewater-based Disease Surveillance
Monitoring sewage for SARS-CoV-2 enables cost-effective surveillance of entire communities, signaling whether the coronavirus is present in the population and whether transmission is increasing or declining. Advantages of wastewater-based disease surveillance include that it:
- Allows the collection data from all people in a community, including people who lack access to healthcare
- Provides near-real-time information on SARS-CoV-2 prevalence in a community. In communities with delayed reporting of test results, wastewater results can provide advance notice of infection dynamics (Peccia, 2020).
Molecular surveillance by sequencing the viral genome from samples which have tested positive for SARS-CoV-2 enables the tracking of mutational changes in the SARS-CoV-2 virus. Large-scale, whole genome next generation sequencing of the SARS-CoV-2 virus:
- Monitors genetic changes in the virus
- Supports contact tracing by identifying people infected with the same genetic variant
- Helps identify diagnostic and therapeutic targets through analyzing the impact of genetic changes on externally presented proteins
- Provides information about transmission dynamics, host response and severity of symptoms, and the rate of evolution of the virus
For example, the new highly contagious COVID-19 variant VUI 202012/01 was identified and characterized with targeted NGS sequencing.
Not all SARS-CoV-2 tests have the same purpose, but each of them contributes to our understanding of the pandemic and prepares us for better management of COVID-19.
Peccia, J., Zulli, A., Brackney, D.E. et al. Measurement of SARS-CoV-2 RNA in wastewater tracks community infection dynamics. Nat Biotechnol 38, 1164–1167 (2020). https://doi.org/10.1038/s41587-020-0684-z