Written by Liza Farrugia
It should be no surprise that millions of swab tests are carried out across Europe on a daily basis. These tests are used to diagnose infected patients and have become essential to the effort to combat the disease. However, some might find it hard to blindly trust the accuracy of such tests without understanding how they actually work to provide positive or negative results.
Virus structure and spread
Firstly, it’s important to distinguish between the disease and the virus. The coronavirus disease (COVID-19) is caused by the virus officially named ‘Severe Acute Respiratory Syndrome Coronavirus 2’ (SARS-CoV-2), otherwise referred to as the “COVID-19 virus”. This virus is genetically similar to SARS-CoV-1 (the virus responsible for the 2002-2004 SARS epidemic) but has structural differences as well as a higher reproductive number which may explain its enhanced rate of transmission.[3]
SARS-COV-2 vs SARS-CoV-1
After initial exposure, symptoms typically develop within 5-6 days and the SARS-CoV-2 load in the respiratory tract peaks at the time of symptom onset or within the first week of illness. This means that it is most contagious just before or within the first 5 days of symptom onset and patients whose symptoms have not yet appeared still carry large amounts of the virus in their upper respiratory tract. Contrastingly, SARS-CoV-1 is most contagious during the second week of illness as opposed to the first week, thus enabling early case detection and minimising spread. Additionally, symptomatic and pre-symptomatic transmission likely plays a greater role in the spread of SARS-CoV-2 than asymptomatic transmission.
Explained: Testing methods
PCR Swab Test
Currently, the main diagnostic test for SARS-CoV-2 is the nucleic acid test which uses reverse transcription polymerase chain reaction (RT-PCR) technology to detect viral RNA in the sample. A swab is used to collect mucus from the nose or throat and the sample is then sent to a lab to be tested. To perform RT-PCR, first chemicals are mixed with the swab in order to extract the genetic material (RNA) of any virus present. Reagents called primers and probes are then added to the sample, which undergoes several controlled heating and cooling cycles in a specialised machine to convert the viral RNA to DNA and then amplify the DNA.
Results are interpreted through fluorescence – probes which emit a strong fluorescent signal on binding to the DNA. If there is a SARS-CoV-2 infection the machine will detect the fluorescent signal. RT-PCR technology can detect SARS-CoV-2 RNA in the respiratory tract for an average of 17 days. However, such samples have rarely been found to be positive beyond 9 days of infection, due to decreased levels of viral RNA.
Antigen Swab Testing
Another form of diagnostic test is antigen testing, commonly known as rapid tests. While the RT-PCR test looks for viral RNA, antigen testing works by detecting specific proteins on the virus surface. Both testing methods make use of nasal or throat swabs to diagnose an active coronavirus infection. Antigen tests are also known as rapid tests since results can be obtained within an hour, whereas RT-PCR test results can take up to a week. Antigen tests are more likely to miss an active coronavirus infection than RT-PCR tests and so negative results may need to be confirmed with the highly accurate RT-PCR test.
Antibody Tests
In response to a threat such as SARS-CoV-2, the body’s immune system makes specialised proteins known as antibodies to fight infection. Antibody tests, also known as serology tests, use blood samples to detect the presence of these specific antibodies produced, and not the virus itself. Since antibodies take several days or weeks to develop after infection and remain in the patient’s blood for weeks after recovery, an antibody test cannot diagnose an active coronavirus infection but shows if you’ve had the disease in the past.
As of 28th October 2020, the EU Commission decided on additional COVID-19 response measures, including establishing more effective and rapid testing to help combat rapid transmission. Thus, under the Emergency Support Instrument, the Commission is mobilising €100 million to directly purchase rapid antigen tests and deliver them to member states.