Ontario has an HIV testing program that is offered to all pregnant individuals in Ontario. The Public Health Ontario Laboratory receives all prenatal specimens (for HIV tests and other laboratory testing). Of these, individuals who had received a diagnostic HIV test within the past 220 days comprise the estimated proportion that received an HIV test. A more detailed explanation of how these estimates follows, through different metrics:
Total number of prenatal specimens tested (a)
Counts of all prenatal specimens tested (for HIV and otherwise) at PHOL were compiled by year. These are comprised of all specimens for female patients accompanied by the prenatal testing requisition or the general test requisition with “prenatal” indicated as the reason for testing. Specimens were assigned to a year based on the date the specimen was received at PHOL.
Number of unique prenatal specimens not tested for HIV (b)
PHOL automatically appends a reminder letter to all unique prenatal specimens for whom the patient has not had an HIV diagnostic serology test within the last 220 days. Counts of these letters comprise the number of unique prenatal specimens not tested for HIV.
Estimated number of prenatal HIV tests (a-b)
To estimate the number of prenatal HIV tests, the number of unique prenatal specimens not tested for HIV was subtracted from the number of total prenatal specimens tested.
Estimated proportion that received an HIV test (1-b/a)
This is estimated by calculating the proportion not receiving an HIV test (number of unique prenatal specimens not tested for HIV divided by the number of prenatal specimens tested), and then subtracting this proportion from one.
The number of prenatal HIV tests and proportion that received an HIV tests are estimates because the prenatal specimens not tested for HIV are unique to individual patients, while the total number of prenatal specimens is not.