Covid: when can antigenic self-testing be preferred over laboratory PCR?

To detect SARS-CoV-2, the virus responsible for Covid, we now have access to, among other things, reverse transcription-polymerase chain reaction (RT-PCR) tests, but also rapid antigenic tests (which may exist in the form of self-tests).

What is the difference between these two types of tests? And which one should you use?

How it works …

PCR tests are used to diagnose infections with SARS-CoV-2. This type of test looks for the genetic material of the virus (in this case RNA and not DNA as in our species).

RT-PCR initially converts viral RNA into DNA, so that it can be read by our cells, and "amplifies" this genetic text, making billions of copies of it. This makes it possible to detect them reliably.

Since this test can multiply tiny amounts of viral genetic material, it is considered the gold standard for detection. It can detect infection at earlier stages than other tests such as rapid antigenic tests.

Rapid antigenic tests detect viral proteins. The proteins bind in the test solution to antibodies that specifically recognize them and become fluorescent to indicate their presence.

These antigenic tests:

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

. And how effective?

While both tests are more likely to correctly detect an infection when the person's viral load is high, PCR tests are more sensitive than rapid antigen tests.

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An Australian study comparing the sensitivity (i.e. a correct diagnosis of SARS-CoV-2 infection when you have it) of a type of rapid antigenic test versus a PCR test showed that 77% of the positive antigenic test results were consistent with the PCR test results.

This proportion reached 100% when people were tested within a week of onset of symptoms.

The therapeutic Goods Administration provides a list of approved rapid antigenic tests, the results of which are consistent with the PCR test in 80-95% of cases, provided the test is conducted within one week of onset of symptoms. Some of these tests are classified as having very high sensitivity, with 95% agreement with PCR tests. (the list of all tests and self-tests whose performance corresponds to the prerequisites defined by the High Health Authority in France can be consulted here, ndlr)

Perform an RT-PCR laboratory test if:

PCR is the test of choice in these situations because it is more accurate to diagnose infection.

Consider rapid antigenic self-testing if:

Rapid antigenic testing is seen as one of several screening tools. In other words, it may indicate that you may be infected but a PCR test is needed to confirm the result.

Although a negative result on the rapid antigen test does not guarantee that you are not infected, it still offers better protection to your contacts than the absence of a test. (if you have a positive result, you need to confirm it immediately with a PCR test and isolate yourself, ndlr)

How often do I have to do rapid antigenic testing?

It depends on why you take the test. If you are part of a monitoring program, do the test when asked.

If you have no symptoms, taking the test two to three times over a week can help improve the sensitivity of the test because the viral load varies. The sensitivity of the test will be highest when the viral load is at its peak.

What about the detection of Omicron?

This new highly mutated variant appears to be still detected by PCR and rapid antigenic tests.

Normally, a PCR test indicates whether or not you have an infection with SARS-CoV-2, but not which variant you have contracted. That's not his goal. Sequencing of the viral genome is necessary to know this.

However, some PCR tests look for a specific genetic sequence that is missing in the Omicron variant (called target defect of the S gene). These particular PCR tests can therefore not only detect an infection but also give an indication that it is likely to be Omicron.