How accurate are COVID-19 tests?

Alligators in New Orleans. (Dr. Noorali Bharwani)
Alligators in New Orleans. (Dr. Noorali Bharwani)

By this time everybody knows the basic facts about the COVID-19 pandemic. COVID-19 is a communicable respiratory disease caused by a new strain of coronavirus. It was first reported in China.

Scientists are still puzzled by the origin of this virus. It is possible the virus began in animals. At some point one or more humans picked up the virus and now it has spread all over the world.

The disease spreads from person to person through infected air droplets. This happens during sneezing and coughing. It can also be transmitted when humans have contact with hands or surfaces that contain the virus and touch their eyes, nose, or mouth with the contaminated hands. Frequent hand washing is important.

Since there is no vaccine or treatment for COVID-19, infection can be prevented by observing personal hygiene.

Currently in Canada, there are three ways of testing for COVID-19 (Canada.ca).

Molecular PCR and point-of-care testing (swabs from nose or throat)

Molecular PCR and point-of-care tests detect the virus and diagnose COVID-19. If you are being tested for a possible current case of COVID-19, you will receive one of these tests.

Molecular PCR test – uses swabs to collect samples from the nose or throat which are sent to a laboratory for testing. Results are ready in 1-3 days.

Point-of-care test – involves a rapid test done at the time and place of care, such as a hospital or doctor’s office. This test uses swabs to collect samples from the nose or throat which are then tested on-site. Results are ready in 30-60 minutes.

Antibody testing (blood test – serology)

Health care providers may have access to blood tests, called serological tests, which are used to see who may have already had the virus.

These tests detect the presence of antibodies produced by your immune system in response to an infection. A positive serological test means that someone has been exposed to the virus in the past, but cannot reliably indicate how long ago that exposure occurred. As a result, they are not used to diagnose a case of COVID-19 in early stages of infection, since they don’t detect the virus itself.

This kind of test help us estimate how many people have had COVID-19, determine which public health measures need to be in place and better understand how much the virus has been circulating in the community. Positive serology does not equate with lifelong immunity.

How accurate are these tests?

Accuracy of the test depends on the timing of the test.

A positive test means that you have COVID-19 and must follow public health measures. False positive results are also possible. Health care professionals make their diagnosis based on a number of factors, including test results and symptoms.

If the test is negative but you have symptoms then you have to ignore the test results and follow quarantine guidelines. A negative test can indicate that a person is not infected, but it doesn’t always mean you’re COVID-19-free.

If you’re tested too soon after exposure, the virus may not be detected because the person is still incubating the virus and it is not detectable at the time of the test and will ultimately become infectious.

Test accuracy can vary during the course of your illness. Timing of the sample collected is important. Test positivity does not always entail infectivity. Results of RT-PCR test may remain positive for many weeks after the clinical disease has resolved. It is believed this is unlikely to be associated with replicating virus on cell culture and are unlikely to be infectious.

Stay safe, wash your hands frequently, wear a mask and maintain social distancing.

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