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Home » What is the distinction between a PCR and antigen COVID-19 check? A molecular biologist explains

What is the distinction between a PCR and antigen COVID-19 check? A molecular biologist explains


At this level within the pandemic, you or somebody has in all probability obtained not less than one COVID-19 check. However are you aware which form of check you bought and the strengths and weaknesses of those completely different checks?

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

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

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

In search of genetic proof

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

For PCR checks, the subsequent step is amplification of genetic materials in order that even a small quantity of coronavirus genes within the affected person’s pattern may be detected. That is carried out utilizing a way known 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 combined with an answer containing an enzyme known 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 known as a primer, binds to the single-stranded DNA and copies it. The primers make sure 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 instances, doubling the DNA till there are a billion copies of the unique piece. The amplified sequence incorporates 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 commonplace for diagnosing SARS–CoV–2.

Nevertheless, PCR checks have some weaknesses too. They require a talented laboratory technician and particular gear to run them, and the amplification course of can take an hour or extra from begin to end. Often solely massive, centralized testing services – like hospital labs – can conduct many PCR checks at a time. Between pattern assortment, transportation, amplification, detection and reporting, it might take from 12 hours to 5 days for an individual to get outcomes again. And at last, 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 checks – like the 2 seen right here displaying damaging outcomes – are a lot sooner and simpler to carry out however are much less delicate than PCR checks.
Lennardywlee by way of Wikimedia Commons, CC BY-SA

Antigen checks

Fast, correct checks are important to comprise a extremely contagious virus like SARS-CoV-2. PCR checks are correct however can take a very long time to provide outcomes. Antigen checks, the opposite main kind of coronavirus check, whereas a lot sooner, are much less correct.

Antigens are substances that trigger the physique to provide an immune response – they set off the era of antibodies. These checks 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.

Identical 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 checks have quite a lot 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 house. Additionally they produce outcomes rapidly, usually in lower than quarter-hour. One other profit is that these checks may be comparatively cheap at round $10-$15 per check.

Antigen checks do have some drawbacks. Relying on the state of affairs, they are often much less correct than PCR checks. When an individual is symptomatic or has loads of virus of their system, antigen checks are very correct. Nevertheless, not like molecular PCR checks, antigen checks 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 loads of virus is within the nostril and throat, from which the samples are taken. So, antigen checks can miss early instances 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.

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Extra checks, higher data

A couple of antigen checks are already out there 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 be pushing to make these checks extra out there to the general public.

At RADx, the venture I’m part of, we’re at the moment conducting medical research to get a greater understanding of how antigen checks carry out at numerous levels of an infection. The extra knowledge scientists have on how accuracy adjustments over time, the extra successfully these checks can be utilized.

Understanding the strengths and limitations of each PCR and antigen checks, and when to make use of them, may also 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.


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