How does a Covid-19 test work?

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The common cold will not produce a positive result on a Covid-19 diagnostic test.
How does a Covid-19 test work?

We've been asked by readers whether a Covid-19 test can return a positive result if the person tested only has the common cold rather than Covid-19.

This is based on the fact that the common cold can be caused by a number of human coronaviruses, which are related to (but not the same as) the new coronavirus which causes Covid-19.

But it's a misunderstanding of how the main type of test for Covid-19 works.

Understanding the types of test

There are two main types of diagnostic test which can detect the presence of viruses like SARS-CoV-2 (the virus which causes Covid-19). Diagnostic tests tell you whether a person has the virus now, and they're what we're normally talking about with programmes like the UK's contact tracing systems.

Additionally there are antibody tests, also called serology tests, which can detect if someone has had the virus in the past, but not whether they still do.

The two diagnostic tests are PCR tests and antigen tests.

PCR (polymerase chain reaction) tests detect the virus's RNA (genetic material) in a sample. First, various substances are added to the sample (usually a nose or throat swab) taken from the person tested. These substances, enzymes known as "reverse transcriptase" and "DNA polymerase", work to make many copies of any viral RNA present.

This is so enough copies are present to be picked up when that sample is then tested. This testing involves specially designed "primers" and "probes" which attach themselves to specific sequences of the virus's genetic code, and send out a signal that indicates the genetic material has been found. These primers are designed to target unique segments of the virus's genome.

Antigen tests, by contrast, do not detect the viral genetic material, but rather usually proteins in the virus. The presence of antigens normally triggers an immune response by the body. Antigen tests are "not widely used for coronavirus testing currently".

These terms are sometimes (confusingly) used interchangeably. Professor of Immunology and Infectious Disease at the University of Edinburgh, Eleanor Riley has said: "Tests for the virus (current infection) are often called "antigen" tests – where antigen refers to some component of the virus, typically the external (coat) protein of the virus. However, the test being used for COVID-19 is actually looking for viral RNA (which is technically not a viral antigen).

"So when people talk of "antigen" tests and others talk of tests for viral RNA or "PCR tests" they are actually talking about the same thing."

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Abbas Panjwani
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