Why do COVID-19 PCR and antigen exams produce completely different outcomes?

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COVID-19 exams are in higher demand than ever, and new information concerning the Omicron variant could make selecting and utilizing various kinds of exams complicated.  Molecular exams for COVID-19, just like the “RT-PCR” know-how utilized by many laboratories — often called PCR exams — are broadly thought-about the “gold standard” for recognizing probably the most infections brought on by SARS-CoV-2.

By comparability, “antigen” or “lateral flow” exams, which may be finished at dwelling and return sooner outcomes, are much less delicate than PCR exams, however may very well be helpful for figuring out circumstances when individuals are prone to be contagious. One evaluate final yr estimated fast antigen exams may solely detect on common 75% of circumstances that PCR exams can spot, with antigen exams performing greatest in symptomatic folks throughout the first week of their illness.

Some public well being officers have advocated for wider adoption of antigen testing by Americans — if they’ll handle to search out one of many fast at-home exams — as an added layer of safety which may spot extra circumstances earlier than they unfold.

But many elements can have an effect on how doubtless a COVID-19 take a look at is to precisely spot an an infection. Here’s the newest on how these two forms of exams work and why they could generally provide completely different outcomes:

A refresher: How PCR and antigen exams work

Most laboratories generate their COVID-19 take a look at outcomes by a molecular testing course of referred to as reverse polymerase chain response, or RT-PCR, that has been used for years to diagnose a wide range of ailments.

A affected person will get examined with a nasal or throat swab, or in some circumstances offers a saliva pattern. Then, utilizing chemical compounds and specialised gear, scientists combine the pattern collected from the affected person with the components wanted to create copies of the genetic materials that makes up the virus. By repeating many cycles of this course of, particular units of primer and probe chemical compounds may be focused by take a look at producers to search for distinctive indicators of SARS-CoV-2 — the virus that causes COVID-19.

“The PCR test, it’s looking for that genetic material inside the virus and you don’t have to have a huge viral load to pick up for it to be sensitive and specific, meaning it functions extremely well under a variety of circumstances to include the asymptomatic as well as the symptomatic,” stated Dr. Emily Volk, president of the College of American Pathologists.

“PCR tests are still by far the best test and those are the tests that most hospitals are using to identify COVID-19 patients,” she stated.

Salvadorans Get Tested To Prevent COVID-19 Surge
A well being employee inserts a swab right into a affected person’s nostril at a COVID-19 testing heart.

Camilo Freedman/APHOTOGRAFIA/Getty Images


In addition to PCR exams supplied at medical workplaces, pharmacies and drive-thru and pop-up testing websites, some take a look at kits enable folks to gather their very own pattern at dwelling after which ship it off to a lab for PCR testing. These are completely different than the fast at-home antigen exams that return outcomes on the spot.

The FDA has additionally licensed some take a look at choices from DetectLucira, and Cue Health that enable households, enterprise house owners or personal services to gather samples and run a sort of molecular testing just like PCR themselves, however the gear may be pricey.

Antigen exams are a preferred at-home different to these molecular exams, although they continue to be in brief provide at many shops and on-line retailers. More than a dozen firms have been granted authorization to promote antigen exams over-the-counter, which may yield outcomes at dwelling in as little as quarter-hour.

These exams work by mixing samples from affected person swabs with a buffer, a liquid that helps the virus journey throughout a strip coated with antibodies that bind to particular proteins — often called antigens — widespread to many variants of the virus. Similar to a optimistic over-the-counter being pregnant take a look at, the strip seems as a coloured line if it detects the antigen.

Girl shows COVID test results
Ariel Kahana, 10, reveals her COVID-19 antigen take a look at end result forward of the primary day of college in Moshav Talmey Yafe, Israel, Aug. 31, 2021.

Tsafrir Abayov / AP


Different forms of free or low-cost COVID exams can be found at websites across the nation. Find a location close to you.

Starting this month, the White House says it additionally plans to supply an internet site to distribute free at-home exams on-line, and would require well being insurers to reimburse for the acquisition of at-home exams.

Are exams pretty much as good at detecting Omicron?

Only a small handful of the molecular testing kits utilized by laboratories have been discovered to fail to detect COVID-19 circumstances brought on by Omicron, in line with the Food and Drug Administration, due to mutations that evade the targets picked out by the builders of these exams.

On the opposite hand, a partial goal failure seen with some PCR take a look at manufacturers, often called “S-Gene Drop Out,” has really been useful for laboratories as a result of it helps them rapidly determine potential Omicron circumstances for genetic sequencing.

Since antigen exams usually goal part of the virus referred to as the nucleocapsid, which noticed fewer mutations in Omicron, take a look at producers and well being authorities overseas say antigen exams ought to nonetheless work to detect many circumstances brought on by the variant.

On December 28, the FDA stated early research by the National Institutes of Health turned up indicators that antigen exams had been in a position to detect Omicron however might need “reduced sensitivity” to identify circumstances of that variant in comparison with different strains.

The NIH’s early analysis discovered that in test-tube research, some over-the-counter antigen exams may solely detect Omicron at increased concentrations than had been wanted to detect Delta. But they famous that this might not essentially lead to a big drop in real-world efficiency — which is now being tracked by researchers.

“A change in sensitivity in the laboratory is not a guarantee that there’s a change in sensitivity from a clinical point of view. I would not recommend that people suddenly abandon their antigen tests,” stated Bruce Tromberg, head of the NIH RADx effort that had run the research.

Why completely different exams might yield completely different outcomes

While PCR exams may be extremely delicate at detecting circumstances of the virus, well being authorities warning that folks can proceed to set off optimistic PCR take a look at outcomes for weeks and even months after they might really feel they’ve recovered — “long beyond the period of expected infectiousness.”

One CDC cohort examine revealed in October discovered folks had been shedding fragments of the virus that may very well be noticed by PCR exams for a median of 30 days. However, scientists in a number of research have usually been unable to tradition stay virus — a resource-intensive means that scientists can attempt to assess if somebody is definitely able to spreading COVID-19 to others — from samples collected later than 9 days after signs start.

This is a part of why the CDC exempts individuals who have recovered from COVID-19 prior to now 90 days from its worldwide pre-flight testing requirement — they could nonetheless get a optimistic PCR take a look at end result regardless that they’re now not sick. 

By distinction, whereas antigen exams will not be as delicate as PCR exams in recognizing each COVID-19 case, analysis with earlier variants discovered utilizing antigen exams two days in a row may reliably spot many circumstances of the virus when folks had been very infectious. 

One examine co-authored by CDC scientists, revealed in August, discovered repeated antigen testing in nursing dwelling residents noticed all of the optimistic circumstances that had been verified by different lab exams.

“The appropriate tool in this pandemic to answer the question, ‘Do I need to isolate? Am I infectious and a risk to my neighbors?’ is using a rapid antigen test because it only stays positive as long as you are infectious,” epidemiologist Dr. Michael Mina, the chief science officer at eMed, instructed “CBS Mornings.” 

The CDC additionally lately laid out steering for utilizing antigen exams to assist resolve whether or not to finish isolation after you’ve got examined optimistic for the virus however don’t have any signs. A detrimental take a look at shouldn’t be required to finish isolation, however the CDC referred to as it the “best approach.”


CDC provides take a look at possibility to finish COVID isolation

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What it is best to do with outcomes from an at-home take a look at

After you first get sick, due to antigen testing’s decrease sensitivity in addition to the small risk for false optimistic outcomes, well being authorities have repeatedly urged Americans to hunt out PCR exams to verify whether or not they’re contaminated with the virus. 

Some native well being departments are additionally urging residents to voluntarily report at-home take a look at outcomes to them. A spokesperson for Washington state’s well being division says they’ve obtained greater than 1,310 optimistic take a look at outcomes since August by a hotline they arrange, along with outcomes reported within the state’s smartphone app

Unlike PCR exams, that are usually carried out by laboratories that report summaries of their outcomes to authorities businesses, public well being consultants and watchdogs have warned for months that rising use of at-home exams may hinder correct monitoring of COVID-19 circumstances within the U.S.

Meanwhile, take a look at positivity charges have soared past advisable thresholds in each state, in line with a tally by Johns Hopkins University — an indication that the reported take a look at outcomes are usually not capturing a big share of the virus at present circulating locally.

Testing for virus within the throat?

Anecdotal studies have emerged suggesting that swabbing the throat along with the nostril — as some antigen exams within the United Kingdom require — may enhance the chances of recognizing an Omicron case. Some early research additionally recommend that samples collected from saliva may be capable of spot Omicron circumstances earlier.

The concept has picked up traction amid preliminary analysis suggesting Omicron may unfold sooner first within the higher respiratory tract as a substitute of the lungs.

For Americans, educated well being care suppliers can already acquire and run exams on all these “oropharyngeal” samples. But no less than for now, over-the-counter COVID-19 exams have solely been greenlighted by the FDA to be used with nasal swabs. 

“We generally do not believe that throat swabs are appropriate for self-collection. If performed incorrectly they can cause harm,” the FDA’s Toby Lowe instructed take a look at builders in October.

Another means to enhance the chances of efficiently detecting the virus is to easily take one other at-home dwelling take a look at 24 hours later.

Volk stated the likelihood that antigen exams may carry out higher within the throat deserved extra investigation, however warned that not rigorously following the instructions on antigen exams can result in errors that hinder a take a look at’s accuracy.

“Today I would say, still follow the instructions of the rapid test that you’re using and collect it the way they tell you to collect it,” stated Volk.

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