What’s the distinction between a PCR and antigen COVID-19 check?


All COVID-19 assessments begin with a pattern, however the scientific course of goes very in a different way after that. Morsa Photos/Digital Imaginative and prescient by way of Getty Photos

(Spanish language model: https://theconversation.com/pruebas-covid-de-pcr-o-antigenos-conoce-cuales-son-las-diferencias-173483)

At this level within the pandemic, you or somebody has in all probability obtained no less than one COVID-19 check. However have you learnt which form of check you bought and the strengths and weaknesses of those totally different assessments?

I’m a molecular biologist, and since April 2020 I’ve been a part of a workforce engaged on a Nationwide Institutes of Well being-funded program referred to as RADx that’s serving to innovators develop fast assessments to detect when an individual is contaminated with SARS-CoV-2, the virus that causes COVID-19.

Two main forms of assessments are used to diagnose an infection with SARS-CoV-2: molecular assessments – higher often called PCR assessments – and antigen assessments. Every detects a special a part of the virus, and the way it works influences the check’s velocity and relative accuracy. So what are the variations between a lot of these assessments?

A person in a white coat with gloves placing samples into a red laboratory device.
PCR assessments are extraordinarily correct however require particular lab tools – just like the PCR heating machine seen right here – and might take hours or days to carry out. Karl Mumm by way of Wikimedia Commons, CC BY-SA

Searching for genetic proof

Step one for both form of check is to get a pattern from the affected person. This is usually a nasal swab or a little bit of saliva.

For PCR assessments, the subsequent step is amplification of genetic materials in order that even a small quantity of coronavirus genes within the affected person’s pattern might be detected. That is performed utilizing a method referred to as a polymerase chain response. A well being care employee takes the pattern and treats it with an enzyme that converts RNA into double-stranded DNA. Then, the DNA is blended with an answer containing an enzyme referred to as a polymerase and heated, inflicting the DNA to separate into two single-stranded DNA items. The temperature is lowered, and polymerase, with the assistance of a small piece of information DNA referred to as a primer, binds to the single-stranded DNA and copies it. The primers be certain that solely coronavirus DNA is amplified. You’ve now created two copies of coronavirus DNA from the unique one piece of RNA.

Laboratory machines repeat these heating and cooling cycles 30 to 40 occasions, doubling the DNA till there are a billion copies of the unique piece. The amplified sequence comprises fluorescent dye that’s learn by a machine.

The amplifying property of PCR permits the check to efficiently detect even the smallest quantity of coronavirus genetic materials in a pattern. This makes it a extremely delicate and correct check. With accuracy that approaches 100%, it’s the gold normal for diagnosing SARS–CoV–2.

Nonetheless, PCR assessments have some weaknesses too. They require a talented laboratory technician and particular tools to run them, and the amplification course of can take an hour or extra from begin to end. Normally solely massive, centralized testing amenities – like hospital labs – can conduct many PCR assessments at a time. Between pattern assortment, transportation, amplification, detection and reporting, it could actually take from 12 hours to 5 days for an individual to get outcomes again. And eventually, they aren’t low cost at $100 or extra per check.

Two small white plastic tests with single lines showing negative tests results and a circular depression to place a sample.
Fast antigen assessments – like the 2 seen right here displaying damaging outcomes – are a lot quicker and simpler to carry out however are much less delicate than PCR assessments. Lennardywlee by way of Wikimedia Commons, CC BY-SA

Antigen assessments

Fast, correct assessments are important to include a extremely contagious virus like SARS-CoV-2. PCR assessments are correct however can take a very long time to supply outcomes. Antigen assessments, the opposite main sort of coronavirus check, whereas a lot quicker, are much less correct.

Antigens are substances that trigger the physique to supply an immune response – they set off the technology of antibodies. These assessments use lab-made antibodies to seek for antigens from the SARS-CoV-2 virus.

To run an antigen check, you first deal with a pattern with a liquid containing salt and cleaning soap that breaks aside cells and different particles. Then you definitely apply this liquid to a check strip that has antibodies particular to SARS-CoV-2 painted on it in a skinny line.

Similar to antibodies in your physique, those on the check strip will bind to any antigen within the pattern. If the antibodies bind to coronavirus antigens, a coloured line seems on the check strip indicating the presence of SARS-CoV-2.

Antigen assessments have a variety of strengths. First, they’re really easy to make use of that folks with no particular coaching can carry out them and interpret the outcomes – even at residence. Additionally they produce outcomes rapidly, sometimes in lower than quarter-hour. One other profit is that these assessments might be comparatively cheap at round $10-$15 per check.

Antigen assessments do have some drawbacks. Relying on the scenario, they are often much less correct than PCR assessments. When an individual is symptomatic or has numerous virus of their system, antigen assessments are very correct. Nonetheless, in contrast to molecular PCR assessments, antigen assessments don’t amplify the factor they’re on the lookout for. This implies there must be sufficient viral antigen within the pattern for the antibodies on the check strip to generate a sign. When an individual is within the early levels of an infection, not numerous virus is within the nostril and throat, from which the samples are taken. So, antigen assessments can miss early circumstances of COVID-19. It’s additionally throughout this stage that an individual has no signs, so they’re extra more likely to be unaware they’re contaminated.

Extra assessments, higher information

Just a few antigen assessments are already accessible over-the-counter, and on Oct. 4, 2021, the Meals and Drug Administration granted emergency use authorization to a different at-home antigen check. The U.S. authorities can also be pushing to make these assessments extra accessible to the general public.

At RADx, the undertaking I’m part of, we’re presently conducting scientific research to get a greater understanding of how antigen assessments carry out at varied levels of an infection. The extra information scientists have on how accuracy adjustments over time, the extra successfully these assessments can be utilized.

Understanding the strengths and limitations of each PCR and antigen assessments, and when to make use of them, may help to convey the COVID-19 pandemic underneath management. So the subsequent time you get a COVID-19 check, select the one that’s best for you.The Conversation

This text is republished from The Dialog underneath a Inventive Commons license. Learn the unique article.



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