FAQs: Human Immunodeficiency Virus (HIV) testing


How does Canadian Blood Services test for HIV?

During the blood donation process, about 450 mL is collected for transfusion and a small amount of blood is kept aside and sent to one of our laboratories for testing.

We use antibody testing and nucleic acid testing (NAT) to test every donation for HIV.

NAT is a highly sensitive method of testing that is used to detect Hepatitis C virus (HCV), Human Immunodeficiency Virus (HIV-1) and West Nile Virus (WNV) in blood. The period of time between initial infection and detection of antibodies is called the "window period". NAT reduces the window period by detecting low levels of viral genetic materials that are present soon after infection but before the body has had a chance to start producing antibodies.

How does it work?

NAT testing begins with a pool of six blood samples in the laboratory. If the result of a pool of samples is negative, all samples in the pool are negative. If the pool is positive, then the samples are tested individually until the actual positive sample is identified. When a sample is confirmed as positive, Canadian Blood Services will notify the donor, provide further information to the donor and their physician if required, and discard all the products made from that donation.

When was it introduced?

NAT was first implemented for HCV in 1999 and added for HIV-1 in 2001.

How long is the window period for HIV?

Approximately nine days.

What is the window period?

The window period is the time between donor exposure to the virus and the ability of a blood test to detect antibodies to that virus. NAT significantly reduces the window period by detecting the presence of the virus earlier.

How effective is the test?

International experience indicates that risk to a blood recipient is from window period donations. NAT is used in addition to the antibody test because it can detect the presence of the virus before antibody develops.

What is the international standard for testing HIV?

Most developed countries test using both antibody assays and NAT, but there are a few countries in Europe that use antibody assays only.

If the testing is effective, why do we need donor screening?

We carefully screen all prospective donors to minimize the potential for transmission of infectious diseases, and donated blood undergoes extensive testing for infectious diseases, blood groups and compatibility. Donor screening is intended to reduce the risk of window period infections as well as risk from infections for which there is no test. It is part of our multi-layered safety process to protect the blood supply.

If the window period is only nine days, why is the men who have sex with men deferral period so long (three months)?

Before the introduction of testing for HIV in the mid-1980s, thousands of Canadians were infected through tainted blood products. The Canadian Red Cross Society (responsible for Canada’s blood supply at that time) introduced a donor eligibility screening criterion that excluded all men who have had sex with a man even once since 1977, as was also done in the USA. This criterion was embedded into Health Canada regulations in 1992. In 2013, the criterion was reduced to a five-year waiting period, and in 2016, the waiting period was further reduced to one year. These changes were based on an extensive review of scientific and epidemiologic evidence. Equally important was the consultation with high-interest groups, including patient groups representing heavy users of blood and blood products as well as members of lesbian, gay, bisexual, trans, and queer(LGBTQ) community groups.

In June 2019, Health Canada approved our application to reduce the blood donation ineligibility period for men who have sex with men from one year to three months. This change was supported by scientific evidence and stakeholder input. These incremental changes are important steps towards being as minimally restrictive as possible while also maintaining the safety of the blood supply. Currently many large blood suppliers such as in the U.S., parts of the U.K., Australia and the Netherlands have implemented a one-year deferral. The U.K. has implemented a three-month waiting period for men who have sex with men, like Canada.

Canadian Blood Services continues to explore the possibility of moving toward a behaviour-based screening criteria. In partnership with Héma-Québec and with funding from Health Canada, we are supporting 15 research projects investigating various aspects of blood and plasma donors’ eligibility criteria and screening process. The goal of this unique research funding program is to generate adequate evidence to inform alternative screening approaches for donors, which could evolve the current eligibility criteria for men who have sex with men.

Our aim is to identify a long-term policy solution that prioritizes patient safety while minimizing the societal impact on certain groups of people.