Also known as rapid antigen tests or point-of-care tests can be bought from the drugstore for self-testing at home. They do not require specialized equipment or the guidance of a medical professional and produce results in as little as 15 minutes. But while at-home Covid tests have helped improve access to Covid testing, their accuracy has been questioned.
How Reliable Are Rapid Covid Tests?
Compared to polymerase chain reaction (PCR) tests, rapid Covid tests are less reliable. The probability that self-testing at home using rapid Covid tests can produce a false-negative result, especially for people who do not show symptoms, is high.
While PCR tests can detect even minute traces of the SARS-CoV-2 coronavirus that causes Covid-19, rapid tests are less sensitive, requiring a heavier viral load to accurately detect infection.
Rapid Covid tests, which you can take at home, at a pharmacy, or at a designated testing facility have an overall sensitivity of 85 percent. This means 15 percent of those that use them get a false negative result. PCR tests, on the other hand, accurately detect infection 97 percent of the time.
Why Are PCR Tests More Accurate Than Rapid Antigen Tests?
There are two types of tests that are used to test for Covid infection. These are PCR tests and antigen tests. Antigen tests are what are commonly called rapid or at-home tests.
The main difference between the two tests are on what they test for, or rather what they look to detect to prove Covid infection. PCR tests look for the coronavirus’ genetic material, making copies of it to ensure the result is accurate. The tests are also conducted in a laboratory setting, by qualified medical technicians, hence they are referred to as diagnostic tests.
Rapid Covid tests, on the other hand, detect what are called antigens, which are proteins that sit on the surface of the virus. This is a less reliable way to test for the virus that requires the virus to have sufficiently replicated itself for accurate detection.
Accuracy Varies Between Rapid Antigen Test Kit Manufacturers
Research has also shown a wide variance in accuracy between antigen tests made by different manufacturers. The widest variances, some as much as 10 percent, were noted in cases where the tests returned accurate positive Covid-19 cases.
There is therefore a higher chance that a test from a specific Covid test kit brand will return a false negative result. So it isn’t simply a case of antigen tests being less accurate in general; there’s also a quality control issue that dogs these tests.
Self-testing instructions, which must be followed to the letter to ensure accurate results, also differ between antigen test kit brands. It is, thus, critical that you carefully read and follow the instructions given on the specific test kit you are using and not rely on what you have read elsewhere.
When Should You Take a Rapid Antigen Test?
Rapid antigen tests are more accurate in cases where a person shows symptoms of Covid infection. They are less reliable when used by asymptomatic individuals or those that have natural immunity or antibodies gained from a previous infection.
Because they require more virus in a mucus sample to accurately detect infection, you should self-test within one week of the onset of symptoms. As your symptoms improve and the viral load drops, rapid tests become less accurate.
Since antigen tests aren’t accurate whether or not you show symptoms of Covid infection, it is possible that you may get a false negative. That’s the big area of concern, since you may unwittingly pass the virus on to loved ones and colleagues who may be at a higher risk of serious illness.
So if you can get a PCR test done, it is important to get one done so you can be sure you are indeed Covid negative. Do so especially if you have recently come in contact with an infected person or intend to visit a person who may be more vulnerable to Covid.
If you can’t immediately get a PCR test done and aren’t necessarily exposing anyone, you can wait a day or two before self-testing again. If the second test returns a negative test again, you should probably look into getting a PCR test, especially if you are experiencing symptoms.
What If You Test Positive With a Rapid Test?
Antigen tests rarely return a false positive result, which is where the test shows you have the virus when you in fact don’t have it. So if you are currently experiencing Covid-like symptoms and an at-home antigen test returns a positive result, it is likely that you have indeed caught Covid.
You should treat a positive result from a rapid test result as you would one from a PCR test. To protect others and help control the spread of the virus, it is critical that you immediately isolate yourself. If you want to be sure the positive test result is accurate, you can self-test again in a day or two, while you isolate.
Quarantine is also essential even where you have tested positive for Covid but don’t show symptoms or feel sick and for people who have been tested but are yet to get their results.
It’s also highly advised to isolate yourself if you have recently come into contact with a person who was infected or who has since tested positive, even though you have yet to get tested yourself. For further guidance on when and how to quarantine, read this guide on the official Government of canada website.
If Antigen Tests Aren’t Reliable, Why Use Them?
Even though they are not as accurate as PCR tests – the gold standard for Covid testing, rapid antigen tests have proven to be a powerful tool in the fight against Covid.
The data they provide gives health authorities a useful estimate of the Covid case at load at a given time, which assists them in determining areas and groups that are most affected and where to best deploy available resources. Unlike PCR tests, antigen tests are also cheaper and easily accessible to the general population.
Importantly, antigen tests have proved to be highly effective in accurately detecting infection in people who have heavy viral loads. These are the people who have the highest risk of spreading the virus to others. By knowing their status, they can make the important decision to isolate, which is how we will win the fight against Covid.