COVID-19 viral tests can tell you if you have COVID-19 right now and are potentially infectious to others.
Serology tests, also known as antibody tests, can be used to determine if you have antibodies for the virus that causes COVID-19.
Last updated: May 12, 2022
A viral test will tell you if you have COVID-19 at the time of the test and are potentially infectious to others. Viral tests look for the presence of the virus and require a swab of your nose or throat.
Find when to get a test
Rapid antigen tests can be used at home and results are usually ready in about 20 minutes.
Rapid antigen tests are available for free at many community pharmacies. You may also get rapid antigen test from a community testing centre if you have symptoms.
Rapid tests are done by collecting a sample from each nostril, using a swab.
Rapid antigen testing kits should be handled and administered by adults. The tests are not recommended for children under 5 years because if the test is not carried out by a healthcare provider the swab can injure their nose.
Rapid antigen tests are less sensitive than PCR tests. This means that the chances of a false-negative test are higher with rapid antigen tests than PCR tests. Rapid antigen tests work best in people with symptoms and are recommended for use by people who currently have symptoms of COVID-19.
It can take 3 to 7 days after infection for there to be enough antigens to be detected by the test.
These COVID-19 tests use polymerase chain reaction (PCR) technology to look for the genetic material of the virus. PCR COVID-19 tests work by measuring how much genetic material, if any, is in someone's sample.
SARS-CoV-2 is the coronavirus that causes COVID-19. COVID-19 PCR tests only detect SARS-CoV-2, not other types of coronaviruses.
The following video shows how the viral testing process works.
How COVID-19 tests are processed in BC laboratories
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Sample Collection: A sample is collected from either a nasal swab or saliva from a mouth rinse and gargle.
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Extraction: Any SARS-CoV-2 genetic material in the sample is isolated.
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Conversion of RNA to DNA: SARS-CoV-2 has genes or genetic material in the form of RNA. PCR tests detect DNA, so the RNA is converted to DNA using heat and enzymes.
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Amplification: Since the sample is small, there won’t be enough genetic material to test yet. The genetic material needs to be amplified. This creates millions of copies of the genetic material.
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Detection: Once amplified, the PCR test can detect if SARS-CoV-2 genetic material is present.
Cycle threshold values (Ct values) are the number of cycles or rounds of amplification needed to detect SARS-CoV-2.
When there is less virus in the sample, more rounds of amplification are needed which results in a
high Ct value. When there is more virus in the sample, fewer rounds of amplification are needed which results in a
low Ct value.
Ct values don’t tell us much in themselves without proper context. Ct values depend on many factors, examples include:
- Where the sample is collected from. For example, a nasal swab or saliva using the mouth rinse and gargle method
- When you get tested after exposure to COVID-19 (see image below)
- Sample quality
- Sample storage
- Sample transportation time.
The blue line in this image shows how the amount of SARS-CoV-2 viral material in your body can change over time. When there’s a low amount of virus in your body, the Ct value is high. This means more amplifications are needed to detect the virus.
Ct value cut-offs determine if a sample is positive or negative. Cut-offs vary by the
type of PCR test. Typically, a threshold of about 35 cycles is used to determine if someone has COVID-19. This means if there is no virus detected after 35 cycles, the sample is considered indeterminate or negative.
Laboratories and manufacturers set cut-off Ct values when they develop the test. Many PCR test technologies report whether the virus was detected or not, not the exact Ct value.
There are currently ten different PCR tests used in BC. You can find out more about each test on the manufacturer webpages:
False positive test results mean someone tests positive when they do not have COVID-19.
False negative test results mean someone tests negative when they do have COVID-19.
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Both false positives and false negatives are rare when the sample is taken correctly and at the correct stage of infection.
- Although rare, false negatives can happen when you don’t yet have enough virus in your body to be detected by a PCR test. This means you could have a negative COVID-19 test result, but show symptoms later.
An antibody test, also known as a serology test, looks for specific antibodies in your blood.
When you are exposed to a new virus, your immune system creates proteins called antibodies to help protect you from infection. Your immune systems also creates antibodies in response to vaccination. The immune system remembers these antibodies can they can be made quickly and in large numbers if your body is exposed to the same virus again. This can help your body to defend itself against getting seriously ill from COVID-19.
Antibody testing is used for limited clinical and research reasons and select outbreak investigations. Health care providers may use antibody testing for patients in hospital who have less common symptoms of COVID-19 but need to have their COVID-19 infection confirmed to help their medical care. Health care providers and public health work together to determine when antibody tests should be used.
Serology testing can also be used at a population level to assess what proportion of the population is protected because they have antibodies for COVID-19.
COVID-19 antibody (serology) testing: Information for patients
Serology testing is not recommended for outpatient care, and should only be used for specific clinical and research purposes.
Serology tests will not tell you:
- If you have enough immunity to prevent a COVID-19 infection
- If you currently have COVID-19
- How well your COVID-19 vaccine will protect you against COVID-19
- If you need another COVID-19 vaccine
Serology tests can detect different types of antibodies. Many of the COVID-19 vaccines in BC only use a harmless piece of the virus (a protein) to create an immune response. This means, your body will only make antibodies for the specific protein found in the vaccine. Some serology tests will detect antibodies created in response to COVID-19 infection and not vaccination. Some can detect antibodies that are from either previous infection or vaccination.
