From nose swabs to blood samples: Everything you wanted to know about COVID-19 tests

The most common test for COVID-19 is the Reverse Transcription Polymerase Chain Reaction (RT-PCR)—a very sensitive test that looks for the RNA of the virus in a sample

Published: 15th April 2020 06:30 PM  |   Last Updated: 15th April 2020 06:30 PM   |  A+A-

A medic collects swab from the mouth of a woman resident for COVID-19 test in a containment zone at Kurla in Mumbai on Tuesday | PTI

Express News Service

Ever wondered why some COVID-19 tests require nose or mouth swabs while others use blood samples? Here's a handy guide on how COVID-19 testing works.

What is the most common test for COVID-19?

The most common test for COVID-19 is the Reverse Transcription Polymerase Chain Reaction (RT-PCR)—a very sensitive test that looks for the RNA of the virus in a sample. This can detect as little as one virus particle in swabs taken from inside the mouth or nose. At the start of the outbreak, most countries relied on RT-PCR tests which take a minimum of five hours to show results.

Is there any other method through which COVID-19 can be confirmed in a person? If yes, which states in India are doing that?

Presence of COVID-19 can be confirmed through rapid diagnostic serological kits which use blood samples for the tests. The most common type of rapid diagnostic test marketed for COVID-19 includes a test that detects the presence of antibodies in the blood of people believed to have been infected with COVID-19. Antibodies are produced over days to weeks after infection with the virus.

Another type of rapid diagnostic test detects the presence of viral proteins (antigens) expressed by the COVID-19 virus in a sample from the respiratory tract of a person. The antigens detected are expressed only when the virus is actively replicating; therefore, such tests are best used to identify acute or early infection.

The rapid kits can give the results within 25-30 minutes. ICMR has permitted the test in India but it has not started as rapid diagnostic tests are yet to be delivered by suppliers.

What is RT-PCR pool testing and which states are doing this?

Meant to save the number of kits used, a pooled testing algorithm involves the PCR screening of a specimen pool comprising multiple individual patient specimens, followed by individual testing only if a pool screens positive. As all individual samples in a negative pool are regarded as negative, it results in substantial cost savings when a large proportion of pools tests need to be tested.

A feasibility study was conducted at ICMR’s Virus Research & Diagnostic Laboratory at King George's Medical University, Lucknow, and it was demonstrated that performing real-time PCR for COVID-19 by pooling 5 samples is feasible when the prevalence rates of infection are low.

Andaman and Nicobar islands was the first region in India to start pool testing for COVID-19 and now the most populous state Uttar Pradesh too has started it.

What are the limitations of RT-PCR tests?

RT-PCR is very specific and sensitive. However, once a patient has recovered, the virus is eliminated and this test can no longer tell if they had been infected. This creates significant uncertainty especially if someone has self-isolated due to mild and unclear symptoms.

Also, even if the RT-PCR test itself only takes several hours, it can take days before the result is known due to sample transport and processing.

However, current antibody tests for the novel coronavirus haven’t fully been tested yet to make sure they are reliable, which is why the WHO recommends the RT PCR test.

What is door to door screening and which states are doing this?

All cities and districts where COVID-19 cases have been detected are required to send teams of healthcare workers to every household for health surveys in areas ranging from 3 km, 5 km and 10 km radius (depending on the number of cases) or in the whole area identified under the cluster containment strategy.

The teams ask people whether anyone has a history of international travel or is suffering from any influenza like illnesses. If people with symptoms are identified, they can be further assessed and tested for COVID-19.

It has been implemented in all states where COVID-19 cases have been detected. As of now, there are 377 districts where COVID-19 cases have been identified.

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