PCR versus antibody tests: What’s the difference?

PCR ascertains whether the virus is present; antibody tests tell you if you have beaten it

A Covid-19 antibody test. Photograph: Erin Schaff/The New York Times

A Covid-19 antibody test. Photograph: Erin Schaff/The New York Times

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There are two main approaches to testing for Covid-19 and they search for different things. The PCR test takes a swab from the back of the nose and throat and looks for presence of the virus itself. The other looks at a blood sample for antibodies generated in the body in response to the virus, showing whether or not the person tested has been exposed to it in the past.

PCR testing

The swab test removes a sample from a person’s nose or throat, and uses a lysis buffer reagent to smash open cells and virus particles if they are present. Then it pulls out genetic material known as RNA from the virus, which is like “mirror writing” of DNA.

The test converts RNA from the virus into DNA, which is more stable. Then it uses polymerase chain reaction (PCR) to amplify or make molecular photocopies of specific stretches of DNA, so they can be measured.

“The test is designed so it amplifies genetic sequences that come from the virus,” says Prof Anne Parle-McDermott, head of DCU’s school of biotechnology.

A shortage of lysis buffer and other reagents or ingredients used in the test has caused a testing bottleneck, and research labs around Ireland are looking at ways to make the required materials.

Antibody test

The other type of test is for antibodies in the blood also known as a serological test. It looks for molecules the immune system makes when encountering Covid-19. “Typically your body will produce one type of antibody called IgM in the week or 10 days after you have been exposed to a virus,” says Prof Paul Moynagh of Maynooth University. “Then about two weeks after exposure, you should have another type of antibody, IgG.”

Antibody tests are being developed and tested but have yet to be approved for use in Ireland. “If you make antibodies against the Covid-19 virus, the antibodies should persist long after the virus itself has gone,” he adds. “This means an antibody test should be able to tell if you have ‘met’ the virus, even if you didn’t have a test for the virus itself.”