Understanding various COVID-19 tests
We are now well into the 5th month of the covid-19 pandemic in India and the virus seems to be relentless. We keep hearing about new diagnostic tests and therapies for Covid-19 in the media and get excited to know about the latest scientific breakthroughs that have happened.The aim of this post is to simplify the understanding of these tests for you .I have covered the standard RT-PCR test, new antigen and antibody tests .
The real time RT-PCR test : RT-PCR stands for Reverse transcriptase Polymerase Chain Reaction. This is the standard test used for detecting covid-19 infection in your body .
To understand this , look at this picture of a virus.
As you can see , the virus basically has a genetic material core surrounded by a molecular cover or envelope. This genetic material in viruses can either be RNA ( Ribonucleic acid ) or DNA ( Deoxyribonucleic acid ) . Covid-19’s core is RNA. In the process of RT-PCR, RNA is extracted from the virus, converted into DNA and amplified. This amplified copies of the DNA is then detected using fluorescent dyes by the RT-PCR machine’s computer.
It is a very sensitive test and specific test which means that the likelihood of infection present is almost certain if the test comes positive. It takes 3-6 hours to deliver the result and at present continues to be the most accurate test to detect the virus.
The sample is collected from your nose or throat and then placed in a special viral transport medium and taken to the lab. The process sample collection is painless.
The Antigen test : It is the latest test approved by the ICMR ( Indian Council of Medical Research ) . At present , it is mainly being used in containment areas to screen huge number of population with symptoms in a short time. The sample here also is taken from the nose or throat and analysed.
It is a point of care test which means , the test does not need to be performed in a lab and the results from machine can come in 15-30 mins.
The difference between RT-PCR and Antigen test is that whereas RT-PCR amplifies the genetic material of the virus and detects its’ presence, Antigen test detects the presence of viral proteins on the surface of virus.
The advantage of antigen test is that is very rapid . However the big disadvantage is that it is only 50-80% sensitive. It means that if 100 people are tested, there is a chance that only 50-80% will be detected to be positive. Hence a few may be wrongly diagnosed as “not infected” .
Thus, it is recommended that – If the antigen test is positive, the person can be considered as infected and no need to confirm with another test.
However, if the person has symptoms and still tests negative on the antigen test, the person must be re-tested with RT-PCR test.
The Antibody Test : Whenever a virus enters a body, the body as a defence mechanism produces antibodies in response. There are two types of antibodies which are produced, IgM, which is produced early on after onset of disease and IgG, which is produced later on. In Covid-19 virus infection, IGM is roughly produced after 7 days of infection whereas IgG is produced 14 days after infection.
Thus an antibody is an indirect way to detect the presence of virus. As COVID-19 is a new virus, there is no concrete data available as to what should be the blood levels of these antibodies after infection.
At present antibody test should not be used to diagnose covid-19 infection. It is mainly used in population based studies by epidemiologists to see what percent of the total population in an area might have got infected by the virus.
Also as we do not know what levels of antibodies are protective , we cannot assume that someone with positive antibodies cannot get reinfection and give “ immunity certificate “ to them.
To summarise, RT-PCR remains the “gold-standard” for Covid-19 testing. Antigen tests can give results much faster than RT-PCR but may not be able to detect all positive cases . Antibody tests should not be used to test for infection and are more useful for population studies.