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


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

By comparability, “antigen” or “lateral flow” tests, which may be achieved at dwelling and return sooner outcomes, are much less delicate than PCR tests, however might be helpful for figuring out circumstances when persons are prone to be contagious. One review final year estimated speedy antigen tests would possibly solely detect on common 75% of circumstances that PCR tests can spot, with antigen tests performing greatest in symptomatic folks inside the first week of their illness.

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

But many components can have an effect on how seemingly a COVID-19 take a look at is to precisely spot an an infection. Here’s the most recent on how these two types of tests work and why they could typically supply different outcomes:

A refresher: How PCR and antigen tests work

Most laboratories generate their COVID-19 take a look at outcomes by means of a molecular testing process known as reverse polymerase chain response, or RT-PCR, that has been used for years to diagnose quite a lot of illnesses.

A affected person will get examined with a nasal or throat swab, or in some circumstances supplies a saliva pattern. Then, utilizing chemical substances and specialised equipment, scientists combine the pattern collected from the affected person with the elements 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 substances 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,” mentioned 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 mentioned.

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 tests provided at medical places of work, 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 and then ship it off to a lab for PCR testing. These are different than the speedy at-home antigen tests that return outcomes on the spot.

The FDA has additionally approved some take a look at choices from Detect, Lucira, and Cue Health that enable households, business homeowners or personal amenities to gather samples and run a sort of molecular testing much like PCR themselves, however the tools may be pricey.

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

These tests 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 — frequent to many variants of the virus. Similar to a constructive over-the-counter being pregnant take a look at, the strip seems as a coloured line if it detects the antigen.

Ariel Kahana, 10, exhibits her COVID-19 antigen take a look at outcome forward of the primary day of faculty in Moshav Talmey Yafe, Israel, Aug. 31, 2021.

Tsafrir Abayov / AP

Different kinds of free or low-cost COVID tests can be found at websites across the nation. Find a location near you.

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

Are tests 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 accordance with the Food and Drug Administration, due to mutations that evade the targets picked out by the builders of these tests.

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

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

On December 28, the FDA said early research by the National Institutes of Health turned up indicators that antigen tests had been capable of 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 tests may solely detect Omicron at greater concentrations than had been wanted to detect Delta. But they famous that this could not essentially end in 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,” mentioned Bruce Tromberg, head of the NIH RADx effort that had run the research.

Why different tests might yield different outcomes

While PCR tests may be extremely delicate at detecting circumstances of the virus, well being authorities warning that individuals can proceed to set off constructive 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 published in October discovered folks had been shedding fragments of the virus that might be noticed by PCR tests for a mean of 30 days. However, scientists in a number of research have usually been unable to tradition reside virus — a resource-intensive manner 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 constructive PCR take a look at outcome despite the fact that they’re now not sick. 

By distinction, whereas antigen tests might not be as delicate as PCR tests in recognizing each COVID-19 case, analysis with earlier variants discovered utilizing antigen tests 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, published in August, discovered repeated antigen testing in nursing dwelling residents noticed all of the constructive circumstances that had been verified by different lab tests.

“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 not too long ago laid out steerage for utilizing antigen tests to assist determine whether or not to finish isolation after you have examined constructive for the virus however haven’t any signs. A unfavourable take a look at just isn’t required to finish isolation, however the CDC known as it the “best approach.”

CDC provides take a look at choice to finish COVID isolation


What you must 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 positive results, well being authorities have repeatedly urged Americans to hunt out PCR tests 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 constructive take a look at outcomes since August by means of a hotline they arrange, along with outcomes reported within the state’s smartphone app. 

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

Meanwhile, take a look at positivity charges have soared past really useful thresholds in each state, in accordance with a tally by Johns Hopkins University — an indication that the reported take a look at outcomes should not capturing a big share of the virus at present circulating in the neighborhood.

Testing for virus within the throat?

Anecdotal reports have emerged suggesting that swabbing the throat along with the nostril — as some antigen tests within the United Kingdom require — would possibly enhance the chances of recognizing an Omicron case. Some early studies additionally counsel that samples collected from saliva would possibly have the ability to spot Omicron circumstances earlier.

The thought has picked up traction amid preliminary research suggesting Omicron would possibly unfold sooner first within the higher respiratory tract as a substitute of the lungs.

For Americans, skilled well being care suppliers can already collect and run tests on a lot of these “oropharyngeal” samples. But at the very least for now, over-the-counter COVID-19 tests 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 manner 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 mentioned the chance that antigen tests would possibly carry out higher within the throat deserved extra investigation, however warned that not rigorously following the instructions on antigen tests 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,” mentioned Volk.

Alexander Tin

CBS News reporter overlaying public well being and the pandemic.


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