When is it okay to do a rapid antigen test for Covid instead of RT-PCR?

There are two types of COVID-19 tests – the RT-PCR test and the rapid antigen test (representative)

With Christmas just around the corner and COVID-19 case numbers are increasing, it is important to continue getting tested if you have symptoms, have been exposed to the virus, or move into a high-risk environment.

Now we have access to PCR tests (known as RT-PCR, or reverse transcription polymerase chain reaction, tests) and Rapid antigen tests to detect SARS-CoV-2, the virus that causes COVID.

So which test should you use? And what’s the difference?

This is how the tests work

In Australia, PCR tests are used to diagnose SARS-CoV-2 infections. This test looks for genetic material from SARS-CoV-2.

RT-PCR converts viral RNA to DNA and amplifies the genetic sequence, creating billions of copies before those copies can be detected.

Because the test can amplify tiny amounts of viral genetic material, it is considered the gold standard and can detect infection at an earlier stage than other tests such as rapid antigen tests.

This is how the PCR test works.

Rapid antigen tests instead of detection viral proteins. The proteins bind to antibodies in the solution, which become fluorescent to indicate the presence of the proteins.

Rapid antigen tests are:

  • faster than PCR testing (15-20 minutes versus hours to days to get a result)

  • can be done at home, as opposed to waiting in line and waiting for a swab to be analyzed in the laboratory.

However, they are less sensitive than a PCR test because there is no amplification process.

How to do a rapid antigen test at home.

How effective are they?

While both tests have a high likelihood of correctly detecting an infection if the person has a high viral load, PCR tests are more sensitive than rapid antigen tests.

A Australian study Comparing the sensitivity (correct diagnosis of SARS-CoV-2 infection, if you have it) of a type of rapid antigen test versus a PCR test, found 77% of positive antigen test results that are consistent with the PCR test results .

This rose to 100% when people were tested within a week of symptoms appearing.

the Remedy management provides a list of approved rapid antigen tests that have 80-95% identical results to the PCR test, provided the test is performed within one week of the onset of symptoms. Some of these tests are classified as very sensitive, with 95% agreement with PCR tests.

Which test do when?

Take a RT-PCR test if you:

  • Have symptoms of COVID

  • a known exposure to someone with COVID. to have

  • Do a rapid antigen test and get a positive result as PCR confirmation is required

  • must be released from quarantine or isolation by your health department

  • are required by a health department to get permission to travel to a location.

In these situations, PCR is the test of choice because it can more accurately diagnose an infection.

Consider a Rapid antigen test if you:

  • plan to visit a sensitive location (e.g. an elderly care facility)

  • plan contact with someone at high risk of COVID (e.g. an elderly person or someone on immunosuppressive treatment) and want to protect them

  • Have symptoms of COVID but cannot reach a PCR test site

  • going to an event where a lot of people will mingle, especially if it’s indoors where the Risk of transmission is significantly higher

  • want to quickly check whether you may have a SARS-CoV-2 infection

  • Are part of a regular COVID surveillance program (some workplaces require this, especially in situations where the person is not fully vaccinated).

The rapid antigen test is considered a screening tool. In other words, it may suggest you may be infected, but a PCR test is needed to confirm the result.

A negative rapid antigen test result is no guarantee that you are not infected, but it does offer your contacts more protection than no test.

How often should I do a rapid antigen test?

It depends on what reason you are taking the test. If you are participating in a surveillance program, take the test when prompted.

If you don’t have symptoms, take the test two to three times over a week can improve test sensitivity as the viral load increases and decreases. Test sensitivity is highest when the viral load peaks.

How does the Omicron variant affect testing?

The heavily mutated Omicron variant appears can still be recognized both by PCR and by rapid antigen tests.

Usually a PCR test will tell whether or not you have SARS-COV-2 infection, but not which variant you have. Genome sequencing is needed to find out.

However, some PCR tests look for a specific genetic sequence that is missing in the Omicron variant (referred to as S-gene target failure). These special PCR tests can not only detect a positive result, but also whether it is likely to be the Omicron variant. The conversation

(Author: Thea van de Mortel, Professor, Nursing and Deputy Head (Learning & Teaching), School for Nursing and Midwife, Griffith University)

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