Canadian Task Force on Preventive Health Care

Please note: In 2003, the CTF updated its Grades of Recommendations to include an "I Recommendation" for situations where insufficient evidence exists to allow a recommendation to be made. (Formerly, these situations were captured under a "C Recommendation".) This change is not retroactive, and all "C Recommendations" made prior to 2003 have not been reevaluated in light of the new "I" recommendation grade. For a discussion of these recommendation grades, please link to the 2003 article in the Canadian Medical Association Journal here.

Screening for HIV Antibody

Prepared by Elaine E.L. Wang, MD, CM, FRCPC, Department of Pediatrics, University of Toronto

Objective
To make recommendations about routine screening for the human immunodeficiency virus (HIV) antibody among general and high risk populations in Canada.

Burden of Suffering
By October 7, 1991, 5,349 AIDS cases had been reported in Canada, including 63 pediatric cases. Under reporting by up to 20% may result from efforts to maintain patient confidentiality and the complexity of the reporting procedure. Behaviours that increase risk of HIV infection include having many sexual partners, receiving anal intercourse, sharing needles during injection drug use and receiving blood or blood products prior to introduction of universal HIV antibody screening by blood banks. Homosexual or bisexual men, prostitutes, injection drug users, people receiving blood products between 1978 and 1985, sexual contacts of people with HIV infection and people from countries with a high HIV prevalence are at high risk. Of all cases reported in Canada, 78% were associated with homosexual or bisexual activity only, 1.4% with injection drug use only, and 3.5% with both. 95% of AIDS cases belonged to an identified risk group.

Options
Routine and targeted screening were considered. Screening measures included conducting histories of sexual and drug behaviour and laboratory detection of antibodies (enzyme-linked immunosorbent assay [ELISA] kits), with confirmation of positive results using Western blot, radioimmunoprecipitation or immunofluorescence assays. Treatment included zidovudine (AZT), aerosol pentamidine prophylaxis for Pneumocystis carinii pneumonia, and counselling.

Outcomes
Sensitivity and specificity of screening tests, delayed onset of acquired immuodeficiency syndrome (AIDS), opportunistic infections, P. carinii pneumonia, high risk behaviours.

Evidence
MEDLINE was searched to September 1991 using the MeSH headings "HIV antibody tests", "AIDS" and "sex-education", "sex-counselling", "sex-behaviour", "attitude", "screening", "truth-disclosure", "counselling" and "patient education".

Values
The 13-member Task Force of experts in family medicine, geriatric medicine, pediatrics, psychiatry and epidemiology used an evidence-based method for evaluating the effectiveness of preventive health care interventions. Recommendations were not based on cost-effectiveness of options. Patient preferences were not discussed.

Background papers providing critical appraisal of the evidence and tentative recommendations prepared by the chapter author were pre-circulated to the members. Evidence for chapter topics was presented and deliberated upon in 1- to 2-day meetings, 2 to 3 times per year from November 1992 to January 1994. Consensus was reached on final recommendations.

Benefits, Harms, and Costs
Sexual histories are routinely obtained by <50% of physicians. Barriers to obtaining accurate information include worries about self-incrimination, prejudice, illiteracy, race, ethnic background, language, and socioeconomic status. Risk factors were identified in <60% of HIV-positive parturient women. Targeted screening (e.g., in sexually transmitted disease clinics) has low sensitivity and specificity. Added counselling did not improve questionnaire sensitivity in identifying seropositive women.

Cohort studies of homosexual men found statistically significant reductions in risk behaviours of those aware of their seropositive status. Studies of the effects of counselling persons with hemophilia or those using injection drugs have shown similar reductions. These studies are limited due to the specialized populations and difficulties in measuring and validating sexual behaviour.

ELISA kits are easy to use and inexpensive. They have high sensitivity, but lower specificity due to cross reactions with other HIV antigen components. Repeated positive tests must be confirmed with the Western blot, radioimmunoprecipitation or immunofluorescence assay. Combined sensitivity of the 2 tests is almost 100%. These tests are not accurate for neonates.

Potential adverse effects of screening arise from false positive results which could lead to discrimination, prejudice, legal prosecution, rejection and reduced social support, depression and anger and from false negative results which could lead to treatment delays.

Treatment with AZT delays the onset of AIDS in asymptomatic persons with CD4 lymphocyte counts of <0.5 x 10⁹. 100 patients would need to be treated for 1 year to delay the onset of opportunistic infection in 4 patients. No data are available on survival, quality of life and adverse effects. No survival difference exists between symptomatic persons given early vs late AZT, despite a delay in disease onset.

An RCT of aerosol pentamidine prophylaxis reported a 70% risk reduction for P. carinii pneumonia among asymptomatic patients with CD4 counts of <0.2 x 10⁹/L, patients with AIDS and patients with advanced symptomatic HIV infection with no history of P. carinii pneumonia.

Risk of vertical transmission from mother to fetus is about 30%. Maternal awareness of seropositive status could result in avoidance of procedures or behaviours that may increase transmission risk, earlier diagnosis of AIDS-related illnesses and earlier treatment of infected infants.

Recommendations
Recommendation grade [A, B, C, D, E] and level of evidence [I, II-1, II-2, II-3, III] are indicated after each recommendation. Citations in support of individual recommendations are identified in the guideline text.

Poor evidence exists to include or exclude, in the PHE of asymptomatic persons in the general population, history-taking regarding sexual practices and injection drug use, and counselling [C, III].
Good evidence exists to offer screening with ELISA and confirmatory tests to asymptomatic persons at high risk [A, I] (homosexual and bisexual men, prostitutes, injection drug users, people with sexually transmitted diseases, people who received blood products between 1978 and 1985, sexual contacts of persons who are HIV-positive, and persons from countries with a high prevalence of HIV infection). Patients with seronegative results should be re-tested after 6 months.
Poor evidence exists to include or exclude voluntary screening with ELISA and confirmatory tests of asymptomatic pregnant women [C, II-2] or asymptomatic persons at low risk [C, III].
Fair evidence exists to include voluntary screening with ELISA and confirmatory tests of children of women who are HIV-positive [B, II-2].

Validation
This report was externally peer-reviewed. The U.S. Centers for Disease Control recommend screening high risk groups. In 1985, they recommended screening and counselling of women in high-risk groups, and testing of patients being treated for tuberculosis. The 1989 U.S. Preventive Services Task Force found fair evidence to recommend screening of high-risk populations and pregnant women, and insufficient evidence to include or exclude screening of low risk populations.

Sponsors
The Canadian Task Force on Preventive Health Care developed this guideline with funding from Health Canada.

Source Document
Wang E.E.L. Screening for HIV antibody. In: Canadian Task Force on the Periodic Health Examination. Canadian Guide to Clinical Preventive Health Care. Ottawa: Health Canada, 1994; 708-18.