Covid-19 Antigen, rapid test
Explanation of the different types of COVID-19 tests
UC Davis Health is among the first in the country to implement an innovative and highly accurate test that can detect COVID-19 and flu viruses at the same time. Returns results in 20 minutes and the test method is considered the gold standard for coronavirus.
UC Davis Health has helped lead the way in COVID-19 testing and is now among the first in the US with a combined rapid COVID/flu test. UC Davis Health has helped lead the way in COVID-19 testing and is now among the first in the US with a combined rapid COVID/flu test.
Rapid site-of-care testing is a PCR-based method, which is similar to what UC Davis Health has used in its lab since March, running hundreds of tests a day, often delivering results within hours. Accuracy is close to 100%. What’s new is the combination of precision 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 the tests,” said Nam Tran, professor of laboratory medicine and principal investigator for the UC Davis Health site 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 our lab test. Both are PCR tests, which are considered the gold standard for testing. ”
To understand the differences in testing, it helps to understand the range of tests available for COVID-19. Diagnostic tests that determine if someone has an active COVID-19 infection fall into two categories: antigen tests, which are used primarily for rapid tests, and molecular and PCR tests.
Until now, most rapid diagnostic tests have been antigen tests. They are taken with a nasal or throat swab and detect a protein that is part of the coronavirus. These tests are particularly helpful 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 is a small lag between when someone is infected and when antigens appear. ”
That means that if a person is not near the peak of infection, but is still contagious, the tests can be negative. Depending on the quality of the antigen test and the examinees, false negatives can be as high as 20%.
“Here’s a good way to look at this,” Tran said. “The coronavirus replicates by putting its genetic material inside our cells. If you are testing that person at the stage where the virus is still replicating within cells, it has not produced enough protein or has not shed in large enough quantities to be detected by the antigen test. ”
The Centers for Disease Control and Prevention (CDC) has advised people who show symptoms of COVID-19 but are negative on a rapid antigen test to get a PCR test to confirm the results.
Positive antigen tests are considered much more accurate, but can still produce false positives. Tran said the concern is that false positives could be caused by the presence of other viruses, improper harvesting techniques, or other substances produced by the body during infection that interfere with the results. However, he said, antigen testing technology continues to improve.
Molecular / PCR tests
This is another area where there is some confusion. Not all molecular tests use polymerase chain reaction (PCR), but PCR serves as the mainstay of COVID-19 diagnostic testing. PCR has also become a common abbreviation in many media reports. Molecular tests detect genetic material, the RNA, of the coronavirus and are sensitive enough that only a small amount is needed.
Until now, the best PCR tests generally required trained personnel, specific reagents, and expensive machines. The sample is collected with a nasal or pharyngeal swab and usually takes hours to provide results. Good PCR tests like the ones used for 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 of up to 15% to 20%. Both the UC Davis Health tests, the rapid COVID-19/flu test and the laboratory test for COVID-19, are highly sensitive and specific PCR tests.
“They are capable of detecting very small amounts of viral RNA very early in an infection, so there is a low probability of false negatives, even among presymptomatic and asymptomatic COVID-19 cases,” Tran said.
The sensitivity of molecular methods can be a double-edged sword. In some cases, it can still detect the genetic material of the virus after a patient has recovered from a COVID-19 infection and is no longer contagious. Also, this coronavirus is still so new to science, nothing is certain.
“PCR is considered the gold standard for many viruses that we have seen in the past,” said Tran. “But we cannot be sure with SARS-CoV-2. Clearly, we have a lot to learn about this virus and we are all learning in real-time. ”
Antibody tests
These are not considered diagnostic tests that can determine if someone has an active COVID-19 infection. They use blood samples to look for antibodies produced by a person’s immune system to help fight COVID-19. These can detect if someone had a previous COVID-19 infection, but not if they are still positive for the virus. Tran said antibody tests may be of more value once an effective vaccine is available.
