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An Expert Explains: The number of corona tests required per day cannot be the same across India

Dr Lee Hilborne is Professor of Pathology and Laboratory Medicine at David Geffen School of Medicine at University of California-Los Angeles, and Medical Director of Quest Diagnostics at Southern California. He speaks to The Indian Express.

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Dr Lee Hilborne is Professor of Pathology and Laboratory Medicine at David Geffen School of Medicine at University of California-Los Angeles, and Medical Director of Quest Diagnostics at Southern California. He was formerly president of the American Society for Clinical Pathology, and a member of US CDC’s Clinical Laboratory Improvement Advisory Committee. He spoke to The Indian Express.

We constantly hear in every country, bar South Korea, that it is not testing enough. What does it really mean?

Testing in this context is generally driven by the need for data to make public health determinations and provide some guidance to individuals wondering if they had immunity. We need to have enough testing as communities — same in US and India — to, with reasonable certainty, understand how many people at any one time are acutely infected, whether or not they have symptoms. Since people without symptoms can spread the infection, it’s important to have this insight. It doesn’t mean we need to test everybody all the time, but we need to have a big enough sample to have a reasonable degree of certainty that the numbers are correct. Knowing who is infected will give us better information on the R0 (rate of spread), understand the risk at present, and also look at infection as efforts are made to ease social restrictions.

Then we need to know how many people have antibodies, and presumed immunity. This is important to determine when “herd immunity” exists and how far we are from that goal. That number needs to be informed by the R0 and rate of transmission because you need a greater number of people immune if the R0 is higher. I don’t think we will have approached herd immunity until there is an effective vaccine given that the numbers we are seeing now vary from under 10% to just over 20% in the US, depending on the location. Because there is variation by location, that’s the reason having the data by geographic area is important.

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Are the number of tests per day linked to determining the date of the opening of the lockdown?

Not the number per se, but the extent to which the number informs us with good probability that there is a reduced risk of continuing to spread the virus. The number of tests per day would be determined by ascertaining statistically the number of people with active infection in a region. In areas where there is more social crowding, the number of people with active infection would need to be lower because each one has a greater risk of transmitting disease due to proximity to others.

So what is that requisite number of tests per day for a country like India with a population of 1.3 billion to decide on opening the lockdown?


As discussed earlier, I don’t think it’s by India as a whole, but numbers would be different in Mumbai, Delhi, and other large cities. Probably fewer in smaller areas, and those with less risk. I am working with some colleagues now to try and determine this with greater precision.

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RT-PCR and the antibody tests — which of these two is more important for containing the spread of COVID-19?


As I mentioned, both are important because they provide different information. Long term, the antibody test may be more important but during an acute infection, the most important thing to know is who has active disease, and that’s the question the RT-PCR test answers. The need to know who has the infection in the population, even if asymptomatic, means we should be examining presumed healthy individuals, not just those that are sick.

Based on the test results that you have seen and analysed, what are the big things that have registered with you?

We know that the positivity rates we see underestimate the true infection because what is reported are those that test positive — and negative. But because many who might have mild symptoms don’t get tested, that information is very biased. Similarly, the mortality rate is overstated because the denominator includes just those tested. Also, because collection is not perfect, there are some people with the infection who may have a negative RT-PCR test. For the antibody test, I think it supports the notion mentioned above — because the prevalence of antibodies is considerably greater, even though still low, than would be suggested by those who have positive results.

What is the problem with antibody test kits? There seem to be all kinds of reports about the unreliability of these tests.

This is a Buyer Beware situation. There are reliable tests on the market and there are unreliable or “junk” tests. I don’t want to speculate on the motivation for putting out lousy tests. Normally when we bring a test to market, there are strict evaluations that come first. Because of the emergency there was a relaxation of those requirements. It does suggest that a reasonable amount of oversight is in the best interest of the public to protect us all from dangerous tests, the same as for drugs and medical devices.


Is the problem with the test kits imported from China?

Again, it’s not that the products are made in China — but rather whether they are made properly with the right amount of quality control, production standards and consistency. Because many of the initial tests were developed in China, some were good and some were bad. Purchasers should make sure that their vendors have recognised good manufacturing practices.

First published on: 28-04-2020 at 04:15 IST
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