FAST AND ACCURATE COVID-19 TESTING

 

Different types of COVID-19 tests explained

The rapid point-of-care test is a PCR-based method, which is similar to what UC Davis Health has used in its laboratory since March, performing hundreds of tests a day, often with results within hours. Accuracy is close to 100%. What's new is the combination of accuracy and speed, plus the ease of a single test to detect coronavirus and influenza A and B viruses.

"There can be a lot of confusion about testing," said Nam Tran, professor of laboratory medicine and principal investigator at the UC Davis Health Center for clinical validation of the new test. "We want people to be confident that they can trust our new point-of-care test as much as they trust our laboratory test." Both are PCR tests, which is considered the gold standard for testing."

To understand the differences between the tests, it is helpful to understand the range of tests available for COVID-19 (FAST AND ACCURATE COVID-19 TESTING )

Diagnostic tests that determine whether someone has an active COVID-19 infection fall into two categories: antigen tests, which are mostly used for rapid testing, and molecular and PCR tests.

Antigen tests

Until now, most rapid diagnostic tests have been antigen tests. These are performed with a nasal or throat swab and detect a protein that is part of the coronavirus. These tests are especially useful in identifying a person who is at or near the peak of infection. Antigen tests are less expensive and generally faster. The downside is that they can be less accurate.

"You don't need complex and expensive test kits to detect antigens," Tran said. "That makes them cheaper and faster. The problem is that there's a small time lag between when someone gets infected and when the antigens show up."

This means that, if a person is not near the peak of infection - but is still contagious - the tests can come back negative. Depending on the quality of the antigen test and the people testing, false negatives can be as high as 20%.

"This is a good way to look at this," Tran said. "The coronavirus replicates by putting its genetic material inside our cells. If you're testing that person at the stage where the virus is still replicating inside the cells, it hasn't produced enough proteins or shed in large enough quantities to be detected yet by antigen testing."

The Centers for Disease Control and Prevention (CDC) has advised people who show symptoms of COVID-19 but test negative on a rapid antigen test to have a PCR test to confirm the results.

Positive antigen tests are considered much more accurate, but can still produce false positives. The concern, according to Tran, is that false positives may be due to the presence of other viruses, improper collection techniques, or other substances produced by the body during infection interfering with the results. However, he said, antigen testing technology continues to improve.

Molecular testing/PCR

This is another area where there is some confusion. Not all molecular tests use polymerase chain reaction (PCR), but PCR is the mainstay of COVID-19 diagnostic testing. PCR has also become a common abbreviation in many media reports.

Molecular tests detect the genetic material-the RNA-of the coronavirus and are sensitive enough to require only a very small amount of it.

Until now, the best PCR tests used to require trained personnel, specific reagents, and expensive machines. The sample is collected with a nasal or throat swab and usually takes hours to give results. Good PCR tests, such as those used in the last eight months in the UC Davis Health lab, are close to 100% accurate.

However, not all molecular tests, including PCR methods, are perfect. Some minor testing platforms have reported false-negative rates as high as 15% or 20%.

UC Davis Health's two tests, the COVID-19/gripe rapid test and the COVID-19 lab test, are very sensitive and specific PCR tests.

"They are able to detect very small amounts of viral RNA in the early stages of infection, so the likelihood of false negatives is very low, even in presymptomatic and asymptomatic cases of COVID-19," Tran says.

The sensitivity of molecular methods can be a double-edged sword. In some cases, it can continue to detect the genetic material of the virus after a patient has recovered from COVID-19.