FAQs: Human Immunodeficiency Virus (HIV) testing


How does Canadian Blood Services test for HIV?

During the blood donation process, a small amount of blood is sent to one of our centralized laboratories for testing. Every donation is tested for several markers, to detect the presence of HIV. This includes p24 antigen testing (present on HIV-1), anti-HIV-1 antibodies, anti-HIV-2 antibodies and HIV-1 and HIV-2 nucleic acid testing (NAT).

What is nucleic acid testing (NAT)?

NAT is the most precise method to directly detect the presence of viruses, including HIV.  

The time between acquisition of a new infection to the time when the virus becomes detectable using laboratory tests is called the “window period”. During the window period, there is a risk that if someone donates blood, laboratory tests would not detect the virus and the infectious agent could be transmitted to a blood recipient.  

Since NAT was introduced in 2001, it has helped shorten (but not eliminate), the window period by detecting low levels of viral genetic materials before the antigens or the antibodies are detectable.  

How does NAT work?

NAT testing begins with a pool of six blood samples in the laboratory. If the testing of the pool of samples detects viral genetic material, the samples are tested individually until the sample with the virus is identified. Test results are then confirmed with additional testing. 

Canadian Blood Services will notify the donor of the results and provide further information to the donor and their health care practitioner(s). We will also follow steps to report the results to public health (this is a mandatory requirement in Canada), discard all products made from that donation, and recall any recent donations from the donor.  

What happens if blood infected with HIV is donated within the window period?

There is a window period between exposure to HIV and the time when the virus becomes detectable using laboratory tests.  This means there is a possibility that blood may be donated within the window period of testing. Therefore, Canadian Blood Services takes a multi-tiered approach to safety, which includes comprehensive and extensive donor selection criteria and state-of-the-art testing technology.  

We ask donors about potential exposure to various transfusion transmitted infections including HIV, Hepatitis B (HPV), or Hepatitis C (HCV), over a three-month period or longer. This timeframe ensures that all available testing assays can reliably detect HIV, HBV, or HCV, if present.  

Since we began managing Canada’s blood system in 1998, there has not been a single recorded instance of blood-borne infection from either HIV, HBV, or HCV.   

Learn more about blood safety

How effective are these tests?

The combination of the donor screening questionnaire and multiple testing assays including NAT-based testing, has been highly effective in preventing transfusion-transmitted viruses (including HIV) from entering the blood supply. 

With these measures in place, the remaining estimated risk of HIV entering the blood supply is very low, at less than 1 in 12.9 million donations.

What is the international standard for testing HIV?

At present, the combination of antigen, antibody and NAT testing for HIV is considered the highest safety standard and is used in most high-income countries.  

However, internationally, these testing assays can be too expensive or unavailable in certain countries. The World Health Organization surveyed the HIV testing practices of 180 countries in 2021. At the time, blood operators in 36 countries (including Canada) used the combination of antigen testing, antibody testing and NAT. Another 19 countries use NAT in combination with antibody testing, but do not perform antigen testing. Most countries (N=90) use antigen and antibody testing without NAT. 

Given the variability in HIV screening and testing practices around the world, the window periods for testing and the risk of acquiring HIV from blood and blood products is variable, depending on the country in which a transfusion occurs.  

Individuals who may require blood or blood components while living or travelling outside of Canada are advised to consult with the health care provider(s) prescribing the transfusion, to discuss risks and benefits. Canadian data on blood safety cannot be directly applied to blood and blood products in other jurisdictions. 

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

Canadian Blood Services takes a multi-tiered approach to safety that includes a comprehensive and extensive donor selection criteria and state-of-the-art testing technology. Although modern testing is highly sophisticated and precise, no test is perfect. Additionally, not all transfusion- transmitted infections have tests available. Hence, donor screening is intended to reduce the risk of window period infections, as well as risk from infections for which there is no test. 

If the window period for HIV is only nine days, why are some individuals deferred for three months under Canadian Blood Services’ sexual behaviour-based screening practices?

During screening, all donors are asked questions about sexual behaviour. If they’ve had anal sex with new and/or multiple sexual partners in the last three months, they will be required to wait three months from when they last had anal sex to donate. The three-month time frame accounts for possible exposure to various viruses, including HIV, Hepatitis B (HBV) and Hepatitis C (HCV).  

The longest window period is for detection of Hepatitis B virus, which was considered when establishing the deferral time frames for activities that can lead to exposure of HIV, HBV, and HCV. These activities include (but are not limited to) injection needle use, previous blood transfusion, known HIV status of partner(s), sexual exposure, tattoos and piercings.  

Three months is the shortest timeframe that has been tested by blood operators around the world for questions relating to exposure to various viruses. Currently, no data exists when a shorter timeframe has been used. Since these changes are made incrementally, as we gather more evidence regarding impact of timeframes in virus detection, we hope to further re-evaluate the utility of these timeframes.  

Learn more about sexual behavior-based screening