Technical notes - general

The objective of this HIV update is to provide “real-time” epidemiologic data on the HIV epidemic in Ontario.

The report summarizes HIV diagnostic data from the HIV Laboratory, Public Health Laboratory – Toronto (PHLT) of the Ontario Agency for Health Protection and Promotion (OAHPP). Almost all HIV diagnostic tests in Ontario are performed through the public health laboratory system of the OAHPP. As part of its mandate, the HIV voluntary testing program uses a sophisticated information system which not only administers and monitors the diagnostic program but also provides an invaluable source of epidemiologic data (with personal identifiers removed) to characterize the HIV epidemic in Ontario. The HIV test database produced by this system includes HIV-positive tests since November 1985 and HIV-negative tests since 1992. The cases used in the present report are those who tested from the first quarter of 1996 to the fourth quarter of 2010.

The data in this report should be interpreted with caution for several reasons: 1) not all persons infected with HIV undergo testing and persons who do test HIV-positive may have been infected at any time in the past. Only statistical modeling of the data can help to estimate the true prevalence and incidence of HIV-infection  2) Although all diagnostic HIV testing is done at the PHLT, other laboratories perform testing for the purpose of establishing eligibility for life insurance, testing prospective immigrants before their arrival and screening blood, organ and tissue donors. Finally, Ontario residents may have been tested elsewhere before establishing residence in Ontario or may travel out-of-province to obtain an HIV test. These data are not available for analysis.  3) Demographic information for some cases may be missing. The prescribing physician provides the patients demographic information on the HIV test requisition form accompanying the blood specimen sent to the PHLT. Only 40% of test requisition forms, and 50% of those testing HIV-positive, received at PHLT have risk factor data provided.  We use adjustments based on the Laboratory Enhancement Program to produce a true estimate of the characteristics of persons undergoing testing.  

 

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First-time diagnoses

 

For every HIV-positive test, an attempt is made to determine whether the patient has been tested previously based on linkages with the information system using patient identifiers when available and information on the form regarding previous testing. If a link is made to a previous positive test, or there is indication on the requisition that the patient has tested positive previously, the case is not counted a first-time diagnoses. Nevertheless, through the Laboratory Enhancement Program, we have found that about 8% of apparent first-time positives have, in fact, tested positive previously in Ontario.

 

Classification by exposure category

 

Persons infected with HIV are counted only once in a hierarchy of mutually exclusive exposure categories (i.e. the presumed route of HIV infection). Where more than one risk factor is indicated for a patient, the case is classified according to an algorithm which assigns the case to the exposure category which represents the most likely source of HIV infection, as follows:

 

  • Men who have sex with men (MSM)
  • MSM and injection drug use (MSM-IDU)
  • Injection drug use (IDU)
  • Mother-to-infant (perinatal) transmission (MTC)
  • Clotting factors received prior to July 1985
  • Blood transfusion received prior to November 1985
  • Origin /residence in an HIV-endemic area
  • High-risk heterosexual
  • Low-risk heterosexual
  • Other, such as needlestick injuries
  • Unknown (not indicated, NIR)

 

The high-risk heterosexual category refers to persons reporting a history of sexual contact with a person known to be HIV-infected or at high risk of HIV infection (e.g., bisexual male [for women only], IDU, clotting factor recipient, person from an HIV-endemic region). The-low risk heterosexual category includes all other persons who have had sex with a person of the opposite sex, none of whom were reported to be HIV-infected or at increased risk of being HIV-infected and no other risk for HIV infection.


Cases assigned to the “needlestick category have not been validated on a case-by-case basis.


The mother-to-infant transmission (MTC) category now includes only confirmed HIV-infected infants and excludes possible duplicate HIV-positive cases with the same birth date, gender and geographic region (public health unit, forward sortation area of institution or medical doctor). This method may result in a slight underestimation of the number of HIV-infected infants (probably <5%) due to multiple births or false matches.

 

Due to difficulties associated with the transition to the LabWare laboratory information system in April 2010, patients with heterosexual partners at high risk of HIV-infection (bisexual males, IDUs, clotting factor recipients, or from an endemic areas) are misclassified as low risk heterosexual for the period April 26 to December 31, 2010. However, patients with HIV-positive partners remain accurately classified as high risk heterosexual.

 

Geographic region

 

The address entered into the database is that of the location of the prescribing physician’s office.

 

Adjustments of exposure category distribution

 

For cases without risk factor information, we have assigned exposure categories based on data from the Laboratory Enhancement Program. For more details on the methodology used, please see the link to the “Technical notes – Adjustment” notes also on this web site

 

Overview | Table and figures | Technical notes - general | Technical notes - adjustment

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Last Updated: May 04, 2011