Coronavirus is a new challenge for the medical fraternity and scientists world over, be it in its prevention or detection. There is lot of debate in media regarding the two types of tests to detect the virus infection. One is RT-PCR based and the other antibody based. Some of the opinions are by well-known experts, and therefore, should be respected whether one agrees or not.
But there are many others who are talking about it without proper understanding, and few others have a sole aim of spreading sensationalism. It is, therefore, imperative to understand the scope and limitations of both these tests.
The controversy of one test over the other is misplaced. Both are important, but their purposes are different. RT-PCR is the confirmatory test for the doctor to do, if someone walks into his cabin with clinical symptoms. The same test will certify if the patient is fully recovered or not.
The antibody test will give data on how many people are exposed to the virus at some point of time—a method to gauge the extent of virus spread in the community.
RTPCR and coronavirus
First, some scientific fundamentals. The core technique behind RT-PCR is PCR, which is abbreviation of Polymerase Chain Reaction. It is essentially a chain reaction, conducted by a special enzyme “polymerase”, by which a targeted fragment of DNA is doubled at every cycle of reaction. For example, if there is only one copy of target gene in a test tube, in the first cycle of PCR, there will be two, in second cycle four… and so on. If we conduct 20 such cycles, there will be 220 copies, a number large enough to detect. Needless to say, it’s extremely sensitive. Example: from a single strand of your hair, PCR can detect if it originated from your body or not.
However, this technology can be used only for DNA as starting material. It cannot be directly used for detection of RNA viruses, like a coronavirus. That’s why, RNA needs to be converted into DNA (by another enzyme called Reverse Transcriptase) before it is subjected to PCR. That’s why the name RT-PCR. With sophistication of technology and instrumentation, RT-PCR can actually give us a quantitative estimation of the number of viruses in a given sample, though this data is not required for diagnostic purposes.
As the fragment of DNA subjected to this repeated amplification is very specific to that organism (there is very little likelihood that same fragment is present in its closest relative), this test is a rare combination of sensitivity and specificity.
The anti-body test
On the other hand, the antibody test is based on a biological phenomenon. In response to entry of any ‘foreign invader’ in our body, antibodies are generated by our immune system, which are again specific to that invader. Therefore, if we can detect the presence of specific antibodies in a blood sample, it can be safely presumed that at some point, that individual is exposed to that invader. “At some point of time” is noteworthy and we will discuss it later. Also, it is important to know that unlike DNA, antibodies cannot be copied in a test tube, and therefore, this test is generally less sensitive than PCR.
Importantly, the human body takes some time to generate antibodies after it is exposed to an invader. Antibodies continue to be present in the blood for some time even after the invader is completely eliminated from the body and there are no clinical symptoms. The human body is also capable of generating immunological memory after first infection, so that next time, if same invader re-enters, the body is better prepared to eliminate the infection, a principle behind all vaccinations. In some cases, this memory (which we typically call “immunity”) is life long, in other cases, it is short termed.
In practice, the antibody based test can be performed in most of the pathology laboratories, while RT-PCR needs special instrument, which many labs may not have.
How they are applied in Covid-19 cases
Let’s apply all these fundamentals on the ongoing controversy of RT-PCR vs antibody testing for coronavirus.
Take a case of a person with clinical symptoms typical of Covid-19 infection with high fever, dry cough, etc. for the 24 hours to 48 hours. The doctor takes throat swab and blood sample of this person. If the RT-PCR of throat swab shows presence of the virus (and therefore, infection), it can be concluded that a patient is Covid-19 positive. But the antibody test of the same patient will most likely be negative, as anti-Covid-19 antibodies take a duration of 5-8 days to be in sufficient quantities to be detected. Therefore, if we apply only the antibody test (and not RT-PCR), this patient will be declared Covid-19 negative, even though he is actually infected.
Let’s now take another case. A person is healthy, relaxing at his home in lockdown. He had mild fever and cough two weeks back but he recovered without any medication. One fine morning, suddenly healthcare workers knock at his door and inform him that his colleague, with whom he shares his workstation, has been diagnosed Covid-19 positive. If his throat swab and blood is subjected to both the tests, respectively, it is most likely that his RT-PCR will be negative and the antibody test positive. You are likely to get same results if your spouse was Covid-19 positive a few weeks ago and is now fully recovered, and you never had even the mildest symptoms.
In another scenario, virus infected person hides his symptoms, continues to mingle with his friends and relatives, before being admitted in a hospital because he is unwell. In such a case, many of the people whom he has closely interacted with are likely to be antibody positive but RT-PCR negative, even though none has any symptoms.
Where RT-PCR is conclusive
Therefore, RT-PCR is a conclusive test for a patient who has clinical symptoms and the doctor needs to make a diagnosis if he is suffering with normal flu or Covid-19. After all the clinical symptoms disappear, the doctor will conduct the RT-PCR test again, before the patient is declared to be recovered from Covid-19 infection. The antibody test is not conclusive both times. At the beginning, the antibody test may come negative (as antibody level is too low to be detected), and after complete recovery, the antibody test will remain positive for some period of time.
On the other hand, if someone wants to collect the data on how many people are exposed to virus in last few weeks or months, irrespective of whether they had symptoms or not, the antibody test will give you the overall “exposure” to virus within community while RT-PCR will be negative, (except in those who have ongoing clinical symptoms).
RT-PCR will give an idea if people within the immediate proximity of a newly-confirmed patient are infected by the virus. However, in this case, the first negative result does not completely rule out lack of infection. Therefore, two weeks quarantine/isolation of these people (who were in proximity with patient) is essential.
The vast majority (by some estimates 70-80 per cent) would have sub-clinical infection (they have never shown any symptoms). In such a scenario, the antibody test data will give an important insight on the spread of the virus, various factors affecting the collective immunity, etc. It is not possible to gather this insight with RT-PCR.
Vijay Chauthaiwale is in-charge, foreign affairs department, Bharatiya Janata party. Views are personal.