Exposure categories

Each diagnostic test is assigned to an exposure category based on reported HIV risk factor information collected on the requisition form. The exposure category is meant to represent an individual’s most likely source of HIV risk. The exposure categories are mutually exclusive. When more than one risk factor is reported for a single individual, a hierarchy is used to assign an HIV test to a single exposure category.

This hierarchy is as follows:

1. Mother-to-child transmission (MTC): Being a child of an HIV-positive mother or aged less than 18 months
2. Men who have sex with men and who use injection drugs (MSM-PWID): Being male and indicating sex with men and needle use
3. Men who have sex with men (MSM): Being male and indicating sex with men
4. People who use injection drugs (PWID): Indicating needle use
5. HIV-endemic: Having lived in an HIV-endemic area or outside of Canada
6. Heterosexual – partner with identified risk (PIR): Being male or female and indicating sex with a person of the opposite sex/gender who is either HIV-positive, a person at risk of HIV, injects drugs, from an HIV-endemic area, had a blood or clotting factor transfusion, or is bisexual.
7. Heterosexual – partner with no identified risk (NIR): Being male or female and indicating sex with a person of the opposite sex/gender who has no identified risk.
8. Clotting factor (pre 1986): Indicating clotting factor pre 1986
9. Transfusion (pre 1986): Indicating a blood transfusion pre 1986
10. No identified risk: Indicating “none” or “other” or “needlestick injury” as a risk factor
11. Unknown/missing: No risk factors indicated (form not completed)

The exposure category data on this website also contain an “Other” category, which includes tests assigned to the MTC, clotting factor and transfusion categories. Tests categorized as “no identified risk” (category #10), or where the form is not completed (category #11), are excluded from the exposure category data in this report.

HIV-endemic areas are classified by the Public Health Agency of Canada as countries where the prevalence of HIV among adults (15-49 years old) is 1.0% or greater and one of the following criteria is met: at least 50% are attributed to heterosexual transmission; a male to female ratio of 2:1 or less among prevalent infections; or HIV prevalence greater than or equal to 2% among women receiving prenatal care. A list of these countries can be found here.

HIV risk factor data used to determine an individual’s exposure category is missing for about half of requisitions and marked as “none” for about 15%. These tests are excluded from breakdowns by exposure category. Due to the extent of missing risk factor information necessary for determination of exposure category, it may be more valid to focus on trends over time rather than the actual numbers or proportions.

It is unknown whether individuals with certain HIV risk factors, and hence exposure categories, are more likely to be missing information, which could introduce bias into the exposure category breakdowns. Also, provider practices for filling out the requisition forms may vary, leading to further bias. For example, some providers may ask people getting tested about their risk factors, while others may make assumptions or not ask.