COVID-19: The tests reveal all

Updated - April 29, 2020 10:45 am IST

Published - April 28, 2020 11:30 pm IST

Dr. Anantharaman

Dr. Anantharaman

What are the important timelines and symptoms characteristic of COVID-19 infection we need to understand before testing?

After exposure to SARS-CoV-2, the incubation period is typically 5 to 6 days. It can range from 2 to 14 days. In the first week, only viral RNA and antigen particles will be present.

During the second to third weeks, the initial antibody response in the form of IgM will be seen which may be present for another 2 weeks. IgG antibody response will appear after 2 weeks in the blood and provides long term immunity. Currently we do not know how long the long term immunity lasts as this is a novel virus.

From the available literature, the COVID-19 infection will be asymptomatic/mild in 81%, severe in 14% and critical in 5% of cases as shown. But the infection can be transmitted from persons through droplets, even from those with mild symptoms or asymptomatic carriers.

What are the two major types of tests?

The antigen test is the diagnostic test for COVID-19 in which the viral RNA is directly identified by real time reverse transcriptase polymerase chain reaction (rRT-PCR).

This is an extremely sensitive test to identify whether someone is currently infected and active.

The antibody test is to identify the immune response to earlier infection. By identifying the type of antibody positivity (IgM or IgG or both or none) we can determine whether the individual had exposure to infection or not, still has active infection or has developed long-term immunity.

What is the best diagnostic test to identify if a person is currently having active COVID-19 infection?

The rRT-PCR antigen test is the best diagnostic test to identify if someone is currently infected and active with SARS-CoV-2. In this, a nasopharyngeal (nasal) or oropharyngeal (mouth) swab is taken and sent in a cold storage box to the central laboratory, where the viral RNA is converted to complimentary DNA (cDNA) using reverse transcriptase, and amplified following the addition of a primer and fluorescent dye and DNA building enzymes.

A positive test will be indicated by the raise in fluorescence.

Depending on the equipment and method used, the time taken may vary between a few hours to a day to get the reports. This test is very sensitive and specific and can detect even if a single viral RNA particle is present in the swab.

Some of the reasons why we get a false negative (negative test when the patient is actually COVID-19 positive) may be improper swab sampling, prolonged time to get sample to lab causing degradation of RNA.

What is the best test to identify if a person had exposure (with or without symptoms) to COVID-19 and their immunity response?

The test for antibodies will give information on whether a person had exposure to SARS-CoV-2 at any point in time and whether immunity has developed. When exposed to SARS-CoV-2 our immune system will form proteins called antibodies to neutralize the virus, initially antibody response will be IgM and long-term durable response will be IgG. Presence of IgG antibody alone will indicate development immunity.

Why are the currently available point of care (POC) rapid testing kit (COVID-19 antibody test) results variable?

There has been a lot of interest in the POC rapid antibody testing as it is quick (under 15 minutes). It can be done by anyone without much training, and if the antibody test is positive and if the individual is shown to be immune, he or she can be reassured. At healthcare providers’ and business level, they can return to work, ánd at government policymakers’ level, they can come out of lockdown and plan future vaccination programsThe currently available POC rapid testing kits have been brought into the market without quality assurance by the regulatory authorities of the countries in which they were manufactured and they lack the validation process.

Several of the testing kits have been fast-tracked due to increasing demand, hence lacking in quality, with variable sensitivity from 30% to 80%. The main concerns raised by several European countries and India are that these kits are showing a higher percentage of false negative reports (i.e. results of the test is negative while the patient is COVID-19-positive).

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