Coronavirus: Rapid test vs. PCR test. What you need to know

Credit: DaytonDailyNews

Hundreds of people showed up for the second Clark County Combined Health District’s free COVID-19 testing clinic Wednesday.

Credit: DaytonDailyNews

The point-of-care tests are a fast and cheap, though less accurate, way of detecting the virus compared to sending nasal swabs to a lab.

The rapid type of coronavirus test that Gov. Mike DeWine took on the tarmac before greeting President Donald Trump could be more widely available in Ohio if the state succeeds with a group purchasing agreement with six other states for these types of tests.

Gov. Mike DeWine said Friday the state is looking to expand the use of “rapid” antigen tests and pooling test specimens as ways to boost overall testing capacity, the governor said on Friday.

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So far, more than 90% of the 1.6 million tests administered in Ohio have been “PCR” tests, which analyze the genetic make up of the virus from a nasal or throat swab. Ohio State University Dr. Peter Mohler called the PCR test the “gold standard” but noted that results take longer.

Ohio is banning together with other states to purchase antigen tests, which give results within 20 minutes but are not as accurate. The states would purchase a combined 3.5 million tests.

“When we deploy an antigen test, we have to think long and hard about how we’re going to deploy it. It has certain advantages ... we also know the reliability is not nearly as good as the PCR tests that most Ohioans have had,” DeWine said.

Mohler likened the two tests to a high-powered telescope and a pair of binoculars — two tools worth using.

In a matter of hours on Thursday, DeWine had both types of tests. He got a false positive from the antigen test and was cleared by the PCR test, which OSU ran twice.

DeWine was scheduled to greet President Donald Trump on an airport tarmac in Cleveland on Thursday. He took the required antigen test to be cleared for that contact with the president.

When the results came back positive for coronavirus, DeWine canceled his plan to see the president, called his wife and adult children, and returned to Columbus where he had the second, more accurate test administered.

Another testing strategy Ohio is pursuing is pooled testing: samples from a handful of people are tested all at once; if it’s negative, all are cleared; if it’s positive, further testing is done. It’s a way to expand testing capacity by 80%, Mohler said.

Results from Ohio’s antibody testing study are expected to be finalized by early September, Lt. Gov. Jon Husted said. That study is examining blood samples from more than 1,000 participants to determine what percentage of the population have had COVID-19 and may have immunity to it.

The American Red Cross is running antibody tests on donors and has found between 1.4% and 1.8% of donors have had the disease, Husted said.

Why testing matters

Testing serves several purposes in responding to the pandemic. It can provide a diagnosis for a person who might need care. It lets people know if they are infectious so they isolate before spreading the virus any further and so their close contacts can be aware they were exposed and thus could also be sick. It also adds to the data available on how and where the novel coronavirus is spreading, so public health officials can adapt measures as needed.

For the past week, Ohio Department of Health has reported there have been an average of 22,000 tests performed in Ohio a day, more than double where the state was six weeks ago. There are however can be lags in getting lab results in a timely manner as demand surges. In addition, some groups such as nursing homes have struggled to get the quick, cheap and routine testing they’ve long requested, given the impracticalilty of a nursing home relying on drive-thru testing.

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These rapid tests are now on the market and have gotten more accurate than in the beginning of the pandemic but test makers haven’t aggressively ramped up manufacturing. DeWine said the group purchasing plan demonstrates to testing manufacturers that there’s a significant demand for this type of tests, and the pooled agreement also has the potential to help states buy these tests in a cost effective way.

Rapid antigen tests vs. PCR tests

If you’ve gotten a coronavirus test — such as at a drive-thru or pop-up site — you likely received a PCR test, which involved someone taking a sample from your nose with a swab and sending it to a lab.

PCR (polymerase chain reaction tests) are highly accurate tests that work by extracting and amplifying genetic material from the novel coronavirus, according to Mohler.

They are much more sensitive than a rapid test, meaning PCR tests can pick up very tiny amounts of virus so they have a high likelihood of catching an infection and not giving a false negative. These tests are a more expensive option, they take a more complex supply chain and take more human labor as well as laboratory capacity to complete — all of which are in high demand right now as everywhere in the U.S. tries to run more tests.

Rapid tests like the one DeWine used are what are called antigen tests.

“They can be done similar to how you would do a flu tests or a strep test,” Mohler said. “You can imagine having these in a doctor’s office or even a nursing home and to be able to test whether someone has it in, say, 15 or 20 minutes. The disadvantage is that they’re just not as sensitive as that molecular PCR test that we’ve been hearing about over the last few months.”

Both antigen tests and PCR tests have a place in the toolbox when it comes to responding to the pandemic, Mohler said.

This method of testing is nearly immediate, which lets contact tracing start sooner with a virus where time is of the essence. When there’s a positive test result received quickly, it raises the likelihood that an infectious person can be isolated before they get a lot of other people sick.

The accuracy of available antigen tests have improved since the start of the pandemic but they still aren’t as accurate as PCR tests. A positive antigen test for the coronavirus is pretty trustworthy but a negative test needs treated with more caution because the person might actually still be infectious.

“The point-of-care tests that we have available right now for COVID still suffer from some sensitivity issues,” said Dr. Roberto Colón, vice president of quality and safety for Premier Health. “And that means that when they have a negative test, if you still have concerns for COVID or if it was still a patient that you suspected you may have COVID, you may need to either repeat the test again or have another test to actually confirm that it indeed was negative. ”

Colón said the timing of antigen tests in the disease course is important. Later in the disease, people shed less antigen and someone with symptoms as little as seven days could have a false negative.

“That’s not something we typically see with viral PCR. That’s something that’s very different between those two tests that has to be taken into consideration when we look at the strategy,” Colón said.

“The best place to use these antigen tests is large groups of population screening,” he said, where these tests can be quickly deployed and then health officials can figure out which patients in the group need a secondary screening.

Finalizing deal

With the plan to get more of these tests to Ohio, the bipartisan group of three Republican and four Democrats are talking with test makers Becton Dickinson and Quidel to purchase 500,000 tests per state or a total 3.5 million tests.

Several steps that would have to pass for the plan to translate to tests on the ground in Ohio. What the governors did was sign a non-binding agreement and would need to hammer out a final contract with all the relevant parties before testing machines and related supplies were ordered. More states could sign onto the plan.

The Rockefeller Foundation is guiding the testing strategy and offering credit if needed to finance the plan.