Coronavirus Outbreak Explained: What Is Antibody Test And What Role Does It Play In The Fight Against COVID-19?

New Delhi: “We have a simple message for all countries: test, test, test. Test every suspected case. The most effective way to prevent infections and save lives is breaking the chain of transmission. And to do that, you must test and isolate”, Dr Tedros Adhanom, Director General of the World Health Organisation had said in a media briefing. Testing is considered as one of the best ways to know who, where, and how many are infected with the highly contagious disease COVID-19 caused by the Novel Coronavirus known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

For COVID-19 testing, there are different kinds of tests available; while tests like RT-PCR (Reverse transcription-polymerase chain reaction), TrueNAT test directly for the virus, rapid antibody test checks for the presence of antibodies in the blood. To understand what are antibodies, why is it important to check for it and how is it done, NDTV speaks to experts.

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What Is An Antibody Test And How Is It Done?

Antibody test, also known as a serology test determines whether the human body had contracted COVID-19 in the past and now has antibodies to fight against the virus. It examines if an individual’s immune system – body’s defence against illness – has responded to the infection.

Antibody test is not a COVID-19 diagnostic test. It is typically done to see if someone has been exposed to the virus and checks the presence of these antibodies. In the case of COVID-19, over 98 per cent of patients develop antibodies, Dr Rajesh Parikh, Director Medical Research and Neuropsychiatrist at Jaslok Hospital and Research Centre.

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Antibody test is like a regular blood test under which a sample is collected through a finger prick. The test is simpler and quicker when compared to COVID-19 diagnosis test – RT-PCR or TrueNAT. The test costs around Rs. 500 and since the interpretation of the test is easy and doesn’t require highly specialised machinery and staff, the result comes within half an hour.

What Are Antibodies?

Antibodies are proteins produced by a human body and used by the immune system to identify and neutralise foreign objects such as bacteria and viruses. Explaining the term in detail, Dr Mugdha Tapdiya, Senior Consultant, Internal Medicine at Fortis Hospital in Vasant Kunj said,

Antibodies are the immune response that our body develops against any infection. After contracting the COVID-19 infection, it takes one to three weeks for antibodies to develop. There are two kinds of antibodies – IgM (Immunoglobulin M) and IgG (Immunoglobulin G).

In the first week of contracting the virus, the antibody can give false negatives so it’s suggested to undergo the test only after eight days of getting infected or even later.

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What Is The Role Of Two Different Antibodies?

While IgM develops in an early phase of the infection, IgG antibodies are more likely to show up later – when an individual has recovered from the infection.

IgM is the initial response of the body to any infection. Typically, in the case of COVID-19, IgM will start getting positive after the first week of contracting the virus. Over time that is after three weeks of contracting the virus, the IgG which is the later response to any infection starts turning positive. IgM tells us that an individual has been exposed and he/she is either entering early immune phase or established immune phase, said Dr Ravindra Mehta, Chief of Pulmonary and Critical Care Medicine at Apollo Hospitals in Bengaluru.

IgM disappears after six weeks whereas IgG stays for a longer period of time. However, as of now, it’s not clear for how long IgG gives immunity. Also, developing these antibodies doesn’t mean an individual has developed long term immunity against the COVID-19 and cannot contract the virus again. There IS not enough evidence to prove how long the immunity will last.

How Specific And Sensitive Is Antibody Test?

There are two important markers of the value of a test – sensitivity and specificity. Sensitivity is the ability of a test to correctly identify those affected with the disease and not give false negatives. The more sensitive a test is, fewer false negatives will turn up. On the other hand, specificity is to identify those without the disease and not declare false positives, explained Dr Rajesh Parikh.

The antibody test lacks sensitivity but is highly specific. According to Dr Mugdha Tapdiya, while the sensitivity rate is 60 per cent to 70 per cent, specificity is better and is around 80 per cent to 85 per cent.

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Why Are States Doing Antibody Test?

Clearly antibody test is not a COVID-19 diagnosis test so why are states across India procuring rapid antibody test kits and conducting the test? Explaining the same, Dr Mugdha Tapdiya said,

It was being done to check whether there has been a community transmission of Coronavirus or not. We cannot be doing RT-PCR tests in a large population as it requires certain expertise to collect samples and it’s a long process. So for the surveillance of the population, the antibody test is good.

What Are The Pros And Cons Of Antibody Test?

The three basic pros are the test is simple, quick, and cheap. Along with this, the other advantage is for healthcare workers who are fighting the battle against COVID-19 from the frontline. Explaining the benefit, Dr Mugdha Tapdiya said,

We are having a shortage of healthcare workers in states due to a high rate of COVID-19 infection. Once an individual contracts the virus, he/she has to remain in isolation for two to three weeks or maybe more because the healing period is long. With antibody test, we can actually test if they are immune. If they have had these antibodies, we can probably send them to work with all the precautions.

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On the negative side, the low sensitivity rate which means the test can give false negatives while checking for antibodies is the disadvantage of the test.

What Is The Role Of Antibody Test In Convalescent Plasma Therapy?

The importance comes in convalescent plasma therapy (CPT) in which antibodies from the blood of a recovered patient are taken and administered into a critically ill patient of COVID-19. To undertake CPT, you need to check whether or not the COVID-19 recovered patient has developed antibodies against the virus, said Dr Rajesh Parikh.