Coronavirus | ‘RT-PCR will remain the gold standard for all viral infections'

A negative test on an antigen based assay is not reliable. Even if there is infection, there is a 50% chance that the antigen test will be negative, says Dr. P. Srinivasan, technical director at Neuberg Diagnostics.

Updated - June 21, 2020 08:10 pm IST

Published - June 21, 2020 08:00 pm IST

P. Srinivasan

P. Srinivasan

Dr. P. Srinivasan, technical director at Neuberg Diagnostics and Chairman & Co-Founder of Be The Cure Registry and Jeenomics (Next Generation Sequencing HLA Laboratory) of Jeevan Stem Cell Foundation, Chennai, spoke to G. Ananthakrishnan on current testing options for the novel coronavirus SARS-CoV-2, when to think of taking the test, state of antibody testing.

What is happening currently with COVID-19 testing in Chennai and other places?

Testing goes on. I am involved with Neuberg [Diagnostic laboratories] and we do on an average about 200 tests a day. It is RT-PCR [Reverse Transcriptase-Polymerase Chain Reaction] which is the gold standard. Three different things have been floating around. We heard a lot of noise about the antibody test. Recently there is talk on the antigen test. RT-PCR will remain the gold standard for all viral infections. Even [with] RT-PCR one needs to understand... can it give a negative report when someone really has the infection? The answer is yes. One reason is when the test is done too early, when there is no viral material in the swab, and two, when the swab is not properly taken. One needs to remember, how long is the test valid?

The COVID PCR is not even like a one rupee note. The rupee note is the same after one week. The PCR test loses its validity the moment the test is done. You can still pick it up the next day and it could turn out to be positive.

The validity of a test is restricted to the day of testing. That is one disadvantage with it and the test is really expensive.

Now that we have crossed three months into the infection, what is the state of antibody tests using ELISA or other methods?

It all started with rapid testing [for antibodies], and any rapid test for any disease has its limitations. The same thing happened with HIV when it started. Now, of course, far more rugged, reliable rapid tests are available, but it took ten years. The antibody tests for COVID are like infection, you have an IgM and IgG, the M arrives around seven days or so after picking up the infection. The G appears a little later and remains longer.

The antibody kits that are available right now are all predominantly IgG, almost all, or it is a total kit, both for M and G. There is no point in using it to diagnose the disease. We are looking at picking up patients with active disease. Testing for an IgG does not make sense. But in a population if you want to see, what percentage of the population is infected for epidemiological reasons, the antibody tests may be useful at this stage.

The antigen test - basically in a PCR we are looking at viral RNA and in the antigen test, what we call antigen is again viral protein. That is picked up by whatever technology we use. But if you clearly read the information on the antigen test, if it says positive, you can be hundred per cent sure that it is positive. However, if the test is negative, you need to follow it up with PCR. Because a negative test on an antigen based assay is not reliable. So if you look at sensitivity it is 50% to 80%, even if there is infection, there is a 50% chance that the antigen test will be negative. So if these people walk on the roads, they infect others.

Personally, I don’t think the antigen test is suitable at this point to test people for infection until the sensitivity increases.

Could you walk us through the cost factor for testing? If you didn’t get access to government testing, what are the costs involved?

I have no clue on the antigen and antibody testing because they are not suitable. When you come to RT-PCR, the charges are ₹3,000 per test. And honestly, I don’t have my own lab, and if I did I would hesitate to offer the test and refer you to somebody else. The reason is that right from the beginning, there has been a lot going on about fixing the price.

What people forget is this. Even close to 100 days after the epidemic, RT-PCR kits are still being imported. Even though people are talking that it is made in India and it is cheaper. The cost of the test has not come down drastically. Availability has become easier, because of cargo flights.

The second thing, there are three processes involved. Collect your sample and put it in the fluid called the viral transport medium. That costs X. Then we have to extract it in the lab which costs Y. Then comes the PCR testing which costs Z. The X+Y+Z costs more than what people are talking about in the media right now.

And there is always the cost per reportable result. It is not like a pack of 20 biscuits that costs ten rupees and each biscuit is 50 paise. In any test, all the more in RT-PCR, if I say the kit can do 50 tests, it will come down to 48 due to positive and negative controls. Then there are 10% to 15% of tests that need to be repeated. When it is inconclusive or doubtful. It is therefore not X divided by 100 to arrive at the overall cost. The cost is probably X divided by 75 or 80. The cost which everybody says needs be below ₹3,000, or should be ₹2,500 or ₹2,200, I honestly think it is unreasonable. But to set the record straight, the test costs ₹3,000.

Why is it difficult to make our own tests and not import?

It is not difficult. Actually, a few companies have made them. Any start-up has its own problems and issues of sensitivity and specificity. Probably it will take another 100 days to provide the number of tests we are looking for. COVID has demonstrated something phenomenally good in health care. When pushed to the corner, we are capable of coming up with the best that is possible, at a cost that others cannot think of. On PCR kits, it will take some more time, it will happen, possibly in another 30 or 40 days.

So we can make them more affordable then?

It should become more affordable. The government should put a cap on the price at which companies can sell the kit. Essentially we are getting into a control raj, but it is required. They are not capping the price of consumables. A pair of nitrile gloves is now being sold at double the price today. But the government wants to cap only the cost that the patient has to pay.

Those who have antibodies from infection, there is the view that they may be better placed to resume activity…

The ICMR study shows that, I think it is some 0.83% of the population that tested positive for IgG. That essentially means the majority of India is not infected. The bottomline is that if you need to test, you need to have symptoms. Wait three to four days with symptoms before you test. If you rush you may get a negative result.

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