Which COVID-19 test is the best?

Rapid corona tests have one considerable benefit. This is what you need to know:

Israel's rapid coronavirus testing program in use. (photo credit: MARC ISRAEL SELLEM/THE JERUSALEM POST)
Israel's rapid coronavirus testing program in use.
(photo credit: MARC ISRAEL SELLEM/THE JERUSALEM POST)

Corona testing has turned into a regular task, as we see by the long lines at the testing centers. But what do we really know about these tests? Prof. Amos Adler, director of the microbiology laboratory at Tel Aviv Sourasky Medical Center, explains in the "Experts’ Clinic" podcast how exactly they work, and what the limitations are.

For way too long, corona testing has been part of our weekly routine, but lately, with places limiting entrance to holders of the Green Pass, it’s the main topic of our daily conversation.

Walla!, a sister publication of The Jerusalem Post, invited Prof. Amos Adler to its “Experts’ Clinic” podcast. He heads the microbiology laboratory at Tel Aviv Sourasky Medical Center, and we asked  him to explain the difference between the current corona tests, how reliable their results are, and who should take serological tests. Here are his answers.

 
(Credit: Avshalom Sassoni)(Credit: Avshalom Sassoni)
Which test is more effective, the rapid antigen test or the PCR?

The speed tests, called antigen tests, have a reputation for being of lesser quality. But, according to Adler, they hold a significant advantage over the more accurate PCR test

"The PCR test is super sensitive,” explained Adler "and will know if you have RNA in the airways when you’re sick and contagious, and even when you have long since stopped being contagious. That is, it can be positive weeks and in some cases even months after you were infected.”

According to him, in this case the sensitivity of the test is problematic, as it means that in some situations people are labeled as positive even when it’s irrelevant. This is why coronavirus testing is not currently performed at the end of isolation if someone isn’t symptomatic. 

The antigen test, on the other hand, is ostensibly a less sensitive test, but it has an advantage: it can detect the vast majority of people who have an infectious viral load. 

“Its insensitivity is actually an advantage in this regard," explains Adler. 

Therefore, in communal settings where the risk of corona is relatively low, the antigen test has an advantage: it doesn’t reveal  those who don’t want to be diagnosed because they’re no longer contagious. 

It does show us most patients at the stage who are contagious, he stressed, and this is in addition to being a quick, easy-to-use and available test.

Should a serological test be done before a third vaccination?

Even now, when the third vaccine has proven effective in stopping the wave of illness, there are quite a few Israelis who are debating whether or not to get vaccinated with the booster dose. There are also many Israelis who prefer to do a serological test to check if they have enough antibodies in their blood even without an additional vaccine. However, according to Adler, it’s difficult to draw an unequivocal conclusion from the serological tests.

The test can give two types of results: negative (no antibodies, meaning it’s likely that the protection from the vaccine has expired) or positive. However, "we know that people who have had antibodies have also been infected,” Adler said. “Apparently a correlation has been found between the level of antibodies and the risk of infection after vaccination but our ability to predict it isn’t very good as this system is very complex and antibodies are only one part of this complex picture." 

He added that if your antibody test is positive your ability to predict your risk of being infected is still far from perfect.