Types of coronavirus tests

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There are three types of coronavirus tests:

  • Genetic tests look for the virus’s RNA in a nose or throat swab, or in saliva samples. The most common type is a polymerase chain reaction (PCR) test.
  • Antigen tests look for specific proteins on the surface of the virus.
  • Antibody tests are blood tests that look for signs that a person has had an infection with the virus and had an immune response. These aren’t used to diagnose an active infection.

The Centers for Disease Control and Prevention (CDC) considers PCR tests the “gold standard” of SARS-CoV-2 testing. These tests are run by laboratories at hospitals, universities, and public health agencies.

Some labs can process samples within 1 day, but sometimes it takes much longer — with people waiting a week or more to find out if they tested positive.

Antigen tests can be done more quickly — with results in as little as 15 minutes — using saliva or a nasal swab. Like PCR tests, antigen tests show whether someone has an active infection.

Although antigen tests are faster and the number of tests being run can be easily scaled up, they have a high false-negative rate — with as many as half of negative results inaccurate.

The Vermont Department of Health counts a positive antigen test as a positive case only if it’s been confirmed with a PCR test. Other states have similar procedures.

But PCR tests aren’t always accurate. Some studies have found that up to 29 percent of these tests can give false negatives.

The accuracy of these tests — both PCR and antigen — vary widely based on the test and its manufacturer.

Other factors can also affect the results: how a nasal swab or saliva sample was collected, how the sample was transported, how a person runs the test (and if they’ve been trained properly), and the equipment being used.

Moving testing out of the laboratory

The goal of testing is to identify people who have an infection with SARS-CoV-2 so they can prevent spreading it to others. But if people don’t receive their results for 2 weeks or longer, it doesn’t matter how accurate the test is. They’ve already missed the chance to self-isolate.

Laboratories can reduce backlogs to some extent by adding more equipment and technicians, or by automating procedures. Some experts have also proposed pooled testing in which samples are mixed together before testing. If a batch test positive, individual samples — or smaller groups of samples — are tested. This reduces the number of tests that need to be run.

However, Dr. Alexis Nahama, senior vice president of diagnostics at biotech company Sorrento Therapeutics Inc. in San Diego, says it’s really difficult to overcome backlogs simply by increasing the number of PCR machines.

“To really be able to run the testing at a massive scale, you need to be able to decentralize where the test is being done,” he said. “Which is why you need the doctors’ offices and the dental offices to be running the tests. You can even have minilabs that can run a lot of the tests at airports.”

A simple, fast test that can be run at schools, restaurants, airports, and stadiums would enable these places to identify people who have an infection before they enter. This would reduce the risk of transmission in public settings.

Sorrento is working on marketing a rapid test that was developed by Dr. Zev Williams and his team at the Columbia University Fertility Center in New York City.

The test can detect the presence of the new coronavirus’s RNA in a saliva sample in as little as 30 minutes. If the test is positive, the color of the fluid in the tube changes yellow.

Preliminary analysis shows that the test is highly accurate, along the lines of a PCR test. But unlike PCR tests, which require specialized equipment, Sorrento’s saliva test only requires a simple heating block.

“This test would be absolutely suitable for a doctor’s office, or a hotel before people check-in or the airport before people get on a plane,” said Dr. Mark Brunswick, Sorrento’s senior vice president of regulatory affairs.

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