As part of their efforts to expand testing and gauge the scale of the coronavirus spread, Delhi and Mumbai have allowed private labs to conduct COVID-19 testing for all without requiring prescriptions. While people in Mumbai can get both RT-PCR (real-time polymerase chain reaction) and antibody tests done, Delhiites will now be able to request these labs for antibody tests to know if they had the infection in the past.

FIT answers some common questions on antibody tests, how reliable they are and how you can get these done.

Here’s how they work.

What is An Antibody Test?

In an earlier video for FIT, Dr Shahid Jameel, a virologist & CEO of Wellcome Trust DBT India Alliance, explained, "When you get infected by a virus or any other pathogen, your body reacts to it and produces antibodies. That is what you measure in an antibody response."

Writing for FIT, he said, “Most antibody tests determine the presence (or absence) of two types of antibodies – immunoglobulin M (IgM) and IgG. Studies on COVID-19 patients show that IgM antibodies appear in the blood around 7 days after they first show symptoms; IgG antibodies take about 10 days. While the IgM antibodies disappear after 35-40 days, the IgG antibodies persist even after 2 months.”

Dr Rajesh Chawla, respiratory and critical care, Indraprastha Apollo Hospitals, told The Indian Express,

Speaking to FIT, Dr Arjun Dang, CEO of Dr Dangs Lab, Delhi shared that all kinds of tests are being offered right now - IgG, IgM as well as a combination of both. While the presence of IgM is indicative of recent exposure, IgG shows the infection happened a while back and the person is now COVID negative.

FIT also got in touch with Thyrocare which has recently launched antibody testing across India, “We are testing for IgG antibodies primarily using ELISA technology. We also test for Total antibodies using CLIA technology. Total includes IgM and IgG.”

Any patient who couldn’t get tested earlier or self-recovered can be identified with the help of an antibody test. This can give the government a clear estimation of how much of the population is actually infected or was infected.

How Do COVID-19 Antibody Tests Differ From RT-PCR Tests?

Antibody tests are not for diagnostic purposes, but only to determine if a person has developed antibodies to the disease after getting infected in the past. On June 23, the ICMR reiterated in its revised guidelines that RT-PCR (real-time polymerase chain reaction) test remains the ‘gold standard’ for detecting the infection.

A negative antibody test has no diagnostic value to know if a person has COVID-19. To know if someone is currently infected, an RT-PCR test would have to be done. A positive antibody test, however, indicates an ongoing or past infection.

Dr Shahid Jameel says, "A negative antibody test doesn’t mean you don’t have COVID-19. PCR is a more conclusive test. An antibody test will also miss anything that happens early. So people who are in early phase of the disease, who don’t show symptoms or who’ve shown symptoms for only day or two will be missed by antibody tests."

The spokesperson from Thyrocare Labs said,

The benefits these tests offer over RT-PCR are that they are fast, inexpensive, and less cumbersome, but their main purpose remains to help assess the scale of an infection spread. They could also help detect potential plasma donors who have enough antibodies which can be used in the experimental therapy being offered to some patients.

Who Can Get COVID-19 Antibody Tests? Do I Need a Doctor's Prescription?

In Delhi and Mumbai, individuals who wish to get these tests can now visit almost any of the ICMR approved private labs without a doctor’s prescription.

Most of the top private labs and hospitals in Delhi started conducting antibody tests after the government’s order from 28 June, where it asked hospitals, nursing homes, district magistrates and labs to follow the ICMR advisory.

The Indian Express reported that Delhi has already collected 23,000 samples as a part of a serological survey using the COVID Kavach Elisa testing kit and the results are expected to be announced by 10 July.

The results are shared in a few hours to a day’s time, depending on the lab, and no follow-up test of any kind is conducted regardless of the results. In an emailed response to FIT, Thyrocare Labs shared that they get the results within four hours of receiving the sample in the laboratory.

How Much Will A COVID-19 Antibody Test Cost?

There is no fixed rate and different labs are charging differently, anywhere between Rs 600-1500.

According to the IE report, Dr Lal PathLabs is offering the antibody IgG test at Rs 1,400 City X-ray and Scan Clinic at Rs 900.

Thyrocare Labs informed FIT, “The tests are priced at Rs 600 to make sure a large number of people can avail it. This is the country’s lowest.”

Understanding Antibody Test Results

The exact sensitivity and specificity (markers to indicate the accuracy of a test in detecting positive and negative cases) would depend on which kits are being used.

However, since most tests include IgG and IgM antibody detection, it is important to understand the possible scenarios when either or both of these types of antibodies are present or absent.

Dr Shahid Jameel explains that in the case of this virus, evidence so far suggests that any antibody comes roughly 7-10 days after symptoms appear (not after infection, but symptoms). Because in most cases you don’t really know about symptoms, a negative test is not a reliable indicator of a person having or not having the infection - they could just be within the 7 day period. “That has nothing to do with the quality of the test. This is inherent biology.”

It’s important to understand the possible scenarios:

• If a person is just IgM positive and IgG negative, it is an early ongoing infection with the potential of spreading it.
• If somebody is just igG positive and not IgM positive, then they had the infection in the past and are now clear of it.