Q & A: What you need to know about using COVID-19 rapid antigen tests

The province of Ontario has changed eligibility for PCR tests as it continues to deal with surging cases.

With fewer people able to book an appointment for a PCR test, demand has increased for rapid antigen tests that can be taken at home.

Zahid Butt, an assistant professor in the School of Public Health Sciences at the University of Waterloo, spoke with CTV Kitchener about what people need to know when using rapid antigen tests.

CTV Kitchener: Why is there such a high demand for rapid tests?

Zahid Butt: The message was out there about if you test negative on your rapid test that you can you can go and attend parties. So, I think that's when it started.

I think the messaging wasn't clear in the sense that you would have to know you'd have to tell people about the pros and cons of the rapid test. So first of all, we have to talk about how sensitive that test is. The rapid test is not as sensitive as the PCR test, which means that even if you test negative, there will be a certain proportion of people who are false negative, which means that they're actually positive for testing.

The test depends on how much of the virus is in your body when you are actually testing yourself. So it may or may not detect the virus.

Also, it depends on when you do the test. For example, if you do the rapid test and then you go to a gathering like two or three days after, the issue here is that you might get infected within two or three days. So you are actually infected and then go to the party.

If you test positive on the rapid test, you should get a PCR test for confirmation, because that's more accurate.

Another thing about the rapid tests, especially with Omicron, is that the sensitivity is only 50 per cent, which means that if you test 100 people, only 50 of them would test positive, which means the sensitivity really low for the Omicron variant.

CTV Kitchener: So is it fair to say that it's only 50 per cent accurate?

Butt: There's a new study, which has not been peer reviewed, says that the sensitivity of the test is only 50 per cent, which means that a lot of people test negative but they're actually positive.

You have to be very careful when you're interpreting these tests. The rapid tests are not a replacement for vaccination or booster vaccination or not falling public health orders. Having a rapid test doesn’t protect you against COVID-19. It just tells you whether you have the virus or not.

CTV Kitchener: Is there one brand that's better than the other, or is it more or less the same?

Butt: You'd have to go and do some research to see which brands have sensitivity is higher than others. You'd have to then do a comparison and say that this is this is probably the best rapid test out there.

Obviously it depends on whether that test is available. If that particular rapid test is not available, then we'll have to use another test, which may not have the same sensitivity as this one.

CTV Kitchener: When should people use a rapid test? Should they use it when they're not showing any symptoms or if they are? Because, right now, the general public can't go get a PCR test, we have to rely on these rapid tests. When is the best time to use them?

Butt: The best time to use them is actually when you have symptoms.

The issue here is the availability of the tests and that's why you see the recommendation from public health authorities is that, if you have symptoms, you stay at home you self isolate, you don't have to go get a test.

CTV Kitchener: So if I have, for example, a cough and a runny nose, I should take a rapid test?

Butt: You should use your rapid test if you have symptoms of the COVID-19 virus.

If you had enough tests out there, rapid tests, then you could, in a sense, get yourself tested if you're going to a gathering, or even if you are not symptomatic.

But, the issue here is the availability of the tests out there, rapid tests and PCR. You have long lines of people trying to get a test.

CTV Kitchener: Should people use them if, let's say they went to a party, they don't have any symptoms. Should they don't use it when they come back for peace of mind, whether or not they were exposed to COVID-19?

Butt: Certainly that would depend on the availability of tests. If you have rapid tests widely available and you are concerned that you might have been infected from the particular event, you can do the test. But generally now it's better to test if you are symptomatic, because you don't have enough tests out there. This is a distribution issue.

CTV Kitchener: With rapid tests being not that accurate and could give false negatives, why should we bother using these rapid tests if they're not giving that accurate result?

Butt: The purpose of these rapid tests is more for screening. If you want to go for confirmation that you will have to go for a PCR test.

The utility of these tests is that that, if you're symptomatic, the chances of you getting positive on this test is higher, as compared to if you're asymptomatic. So it's basically more for your own peace of mind, where if you're symptomatic, you want to know whether you have COVID-19 or not, then yes, you can go ahead and get the test.

CTV Kitchener: So if I take a rapid test, it comes back negative, two days later I take another rapid test and it comes back negative, is it safe to assume that it's not COVID-19?

Butt: Generally speaking, if you repeat a rapid test and it doesn't turn out to be positive, it's likely that you don't have the disease.

But as I said, if you want to really confirm, you would have to go for a PCR. The issue here is because you have these limits of testing capacity, they're actually not giving PCR tests everyone. They're advising people to basically stay at home and self-isolate. It's only for high risk groups or even the targeted professions that are more likely to contract COVID-19.

CTV Kitchener: Should people who do test negative multiple times, should they continue to isolate themselves even if they tested negative on a rapid test?

Butt: It depends on the guidelines. For healthcare workers, you can isolate, isolate for five days from the beginning of symptoms, and then you can come back to work.

For people who are unvaccinated or immunocompromised, they're saying that you should self-isolate for 10 days. The CDC has recommended a negative test before you join the workforce. But, we are still waiting Canada to determine whether you need a negative test.

Some answers have been edited for length and/or clarity