Tests for Covid-19 in India

Subject: Science and tech

Context:

India is using RT-PCR as standard test for COVID. But recently ICMR approved ELISA test kits for Covid-19 developed by two companies.

Concept:

An ELISA test is of two types depending on the antibodies tested for immunoglobulin G (IgG) and immunoglobulin M (IgM).
IgG detects antibodies developed in later stage of infection, and IgM detects antibodies produced in early stages of infection.
Currently only IgG testing kits have been approved in India.
In India, the ELISA test for Covid-19 is only approved for serosurveys which estimate the proportion of the population exposed to infection and for surveys in high-risk areas and segments like containment zones, immune compromised individuals, and frontline and health workers

ELISA

ELISA (enzyme-linked immunosorbent assay) is a plate-based assay technique designed for detecting and quantifying peptides, proteins, antibodies and hormones.
In an ELISA, an antigen must be immobilized to a solid surface and then complexed with an antibody that is linked to an enzyme.
Detection is accomplished by assessing the conjugated enzyme activity via incubation with a substrate to produce a measureable product. The most crucial element of the detection strategy is a highly specific antibody-antigen interaction.

Types:

ELISAs can be performed with a number of modifications to the basic procedure: direct, indirect, sandwich or competitive.
The key step, immobilization of the antigen of interest, can be accomplished by direct adsorption to the assay plate or indirectly via a capture antibody that has been attached to the plate.The antigen is then detected either directly (enzyme-labeled primary antibody) or indirectly (enzyme-labeled secondary antibody). The detection antibodies are usually labeled with alkaline phosphatase (AP) or horseradish peroxidase (HRP).
A large selection of substrates is available for performing the ELISA with an HRP or AP conjugate. The choice of substrate depends upon the required assay sensitivity and the instrumentation available for signal-detection (spectrophotometer, fluorometer or luminometer).

RT-PCR

The standard test for COVID-19 so far has been the Polymerase Chain Reaction (PCR).
The test involves extracting RNA or ribonucleic acid, the genetic material of the virus, and checking if it shares the same genetic sequence as the SARS-CoV-2 virus. If it is a match, the sample is deemed positive.
It is done after taking nasal or throat swabs of the patient, from which RNA is extracted and converted to DNA, which is then amplified before being checked for confirmation of the virus.

Rapid antibody test

In this person’s plasma is tested for presence of antibodies against the antigen like COVID 19 Virus
Blood has two components, a matrix called plasma and the blood cells. The plasma minus the components of whole blood that cause it to clot is known as serum.
It takes less than 30 minutes.
It is an indirect test because it cannot find the virus, but it can determine if the immune system has encountered it.
But a rapid test involves a high risk of false results — it may detect antibodies against of some other infection and show that the sample is positive for Covid-19.
Hence this test is only used for population surveys. If a person tests positive through a rapid test, he has to undergo a confirmatory RT-PCR test before treatment.
ELISA is more accurate than a rapid test.

True Nat

This is a privately designed test that works on the same principle as RT-PCR, but with a smaller kit and with faster results.
It is small and portable, mostly running on batteries, and provides result within 60 minutes. It involves taking nasal or oral swabs.