Conducting COVID-19 Serology Testing at PHO

Serology testing examines a person’s blood to see if they have antibodies to COVID-19. Antibodies are produced by the immune system in response to infection by bacteria and viruses. Antibody testing that is specific for COVID-19 antibodies is therefore a useful way to find out whether a person has been exposed to the virus. In contrast, PCR testing (also known as polymerase chain reaction), is a type of test that tells us if someone currently has COVID-19. Find out more about the difference between these two types of tests and their application.

At an individual level, antibodies may assist in understanding if someone had COVID-19. However, not enough is known currently about COVID-19 immunity to determine if the antibody response indicates whether a person is immune to COVID-19 and, if it does, how much immunity that provides and for how long.

Applications of Serology

Clinical Serology

Due to significant gaps in the understanding of the COVID-19 immune response, serology testing currently has very limited clinical value for individual patients.

The current uses of COVID-19 serology testing are:

  • in the investigation of suspected cases of Multisystem Inflammatory Syndrome in Children (MIS-C)
  • and with advance PHO microbiologist approval, serology may be considered in patients with severe illness who have tested repeatedly negative by PCR and where serology results would be a helpful adjunctive tool for clinical/public health action and decision making

Healthcare providers who are looking to order antibody testing for one of these purposes should consult our COVID-19 Serology Test Information Sheet. 

Serology testing should NOT be used for:

  • the diagnosis of acute infection or determining if a patient is infectious
  • determining immune status of the patient


Another application for COVID-19 serology testing is called serosurveillance, where a large number of blood samples representative of the population are collected and tested for COVID-19 antibodies to better understand which groups in the population have been infected, and to what degree. Serosurveillance helps inform the public health response and helps us better understand:

  • the true infection rate
  • which age groups were most infected
  • geographic hotspots (where it was concentrated)
  • settings which were particularly impacted
  • the level of infection over time, if repeated serosurveys are conducted

Also, when a vaccine becomes available, serosurveillance can help determine priority groups for vaccination and help monitor the effectiveness of vaccination programs.

PHO is currently running a large COVID-19 serosurveillance initiative in Ontario, using samples submitted to PHO’s laboratory for testing since late March 2020. Partial support for this initiative has been provided by the COVID-19 Immunity Task Force.  Find out more about the PHO Serosurveillance Initiative


Health Care Providers

See our COVID-19 Serology Test Information Sheet for details about collecting and submitting a sample and our COVID-19 Laboratory Testing Frequently Asked Questions


PHO Rounds: COVID-19 in Pregnancy in Ontario: Findings from the First Year

The Better Outcomes Registry and Network (BORN) Ontario began surveillance of COVID-19-infected pregnant individuals in Ontario beginning March 1, 2020 to respond to the urgent public health crisis. 

See the Event Details
Updated 5 Aug 2020