You or someone you know has most likely obtained at least one COVID-19 test in the pandemic. This article aims to cover the difference between the previously used RT-PCR tests for travel and the newly introduced rapid antigen test, also called the lateral flow test.
SARS-CoV-2 infection is diagnosed using two types of tests: molecular tests (also known as PCR tests) and antigen tests.
Each identifies a distinct aspect of the virus, and the way they work affects the test's speed and accuracy. So, what are the distinctions between these tests?
Obtaining a sample from the patient is the initial step in any type of test. A nose swab or a small amount of saliva through the mouth can be used.
Amplification of genetic material is the next step in PCR tests, which allows even a little amount of coronavirus genes in the patient's sample to be discovered. This is accomplished through the use of a process known as polymerase chain reaction. The sample is taken and treated with an enzyme that converts RNA to double-stranded DNA by a health care provider. The DNA is then heated in a solution containing a polymerase enzyme, which causes the DNA to separate into two single-stranded DNA pieces.
The temperature is dropped, and polymerase binds to the single-stranded DNA and replicates it with the help of a little bit of guide DNA called a primer. Only coronavirus DNA is amplified thanks to the primers. From the original one piece of RNA, you've now made two copies of coronavirus DNA.
These heating and cooling cycles are repeated 30 to 40 times in laboratory equipment, doubling the DNA and producing a billion duplicates of the original piece. The fluorescent dye in the amplified sequence is read by a machine.
The ability of PCR to detect even the tiniest quantity of coronavirus genetic material in a sample is due to its amplifying feature. As a result, it's a very sensitive and precise test. It is the gold standard for diagnosing SARS–CoV–2 because of its near-perfect accuracy.
However, the PCR test also has some weaknesses. Performing them requires a skilled laboratory technician and special equipment, and the amplification process can take an hour or more from start to finish. Normally, only large centralized test facilities, such as hospital laboratories, can perform many PCR tests at the same time. It may take 12 hours to 5 days for results to be returned between sample collection, transportation, amplification, detection and reporting. Finally, for over £ 100 per test, they don't come cheap.
Antigen test which is rapid and recognised as an accurate testing method is essential to contain highly contagious viruses such as coronavirus i.e. SARSCoV2. The PCR test is accurate, but it can take a long time to get results. The other major type of coronavirus test, the antigen test, is much faster and the sample does not need to be sent to a lab for processing the result.
Antigens are substances that elicit an immune response in the body and cause the formation of antibodies. These tests use laboratory-generated antibodies to look for antigens against the SARSCoV2 virus. To perform an antigen test, first treat the sample with a liquid that destroys cells and
other particles. The liquid is then applied to a test strip with a thin line of SARSCoV2-specific antibody.
Like the antibody in the body, the antibody on the test strip binds to any antigen in the sample. When the antibody binds to the coronavirus antigen, a colored line indicating the presence of SARSCoV2 appears on the test strip.
Antigen testing has several advantages. First, they are so easy to use that they can be run at home or without special training and the results can be interpreted. It also provides fast results, usually in less than 15 minutes. Another advantage is that these tests are relatively cheap, costing around £15 to £30 per test.
Antigen testing is very accurate if the person is symptomatic or has many viruses in the system. However, unlike molecular PCR tests, antigen tests do not amplify what they are looking for. This means that the sample must have enough viral antigen for the antibody on the test strip to generate the signal. When a person is in the early stages of infection, there are not many viruses in the nose and throat from which the sample is taken. Therefore, antigen testing may miss early cases of COVID19. Also, because there are no symptoms at this stage, you are more likely not to know that you are infected.
Antigen tests have been approved in the UK by the Government as a replacement for RT - PCR test during travelling. On October 4, 2021, even the Food and Drug Administration issued another emergency permit for home antigen testing. The Government is also promoting making these tests more accessible to the general public.
The more data scientists know how accuracy changes over time, the more effectively these tests can be used. Understanding the strengths
and limitations of PCR and antigen testing, and how to use them, can help control the COVID 19 pandemic.