False-positive results in antenatal HIV screening
CMAJ 1999;160;1285
See response from: L. Samson, S. King
We read with interest the article by Lindy Samson and Susan King on evidence-based guidelines for HIV screening in pregnancy [full text].1 Along with Proffitt and Yen-Lieberman,2 they offer reassurance with respect to low rates of false-positive and false-negative results with the use of enzyme-linked immunosorbent assay (EIA) and Western blot analysis in combination (sensitivity
99.0%, specificity 99.9%). However, we recently became aware of the particular risk that pregnant women bear for false-positive results.
We cared for a woman initially seen because of Rheus isoimmunization who consented to routine HIV screening at 20 weeks' gestation. The initial EIA result was reactive, as was the result of a repeat test 2 weeks later, at which time a supplemental EIA test (nonreactive) and Western blot analysis (indeterminate) were requested. One week later, the result of a third screening EIA test was nonreactive. A month later, another screening EIA test gave a reactive result, a supplemental EIA test result was nonreactive, and a Western blot analysis result was indeterminate for all 3 determinants of HIV-1; the patient's HIV viral load was < 500 copies/mL. At 34 weeks' gestation (when all results were finally to hand) the patient was reassured that she was HIV negative. Twelve weeks had passed since the first positive test result. Understandably, the patient was under considerable stress during this time.
Although false-negative results with EIA screening tests are well recognized, false-positive results are less so. We had been unaware that multiparity, multiple previous transfusions and autoimmume disorders are all risk factors for false-positive reactions (in non-pregnant populations) because of anti-HLA-DR or other antibodies.36 Pregnant women are often multiparous, may have a prior history of ante- or post-partum hemorrhage requiring transfusion, and belong to the gender and age groups in which autoimmune phenomena are most common.
Physicians who provide antenatal care should be aware of the occurrence, impact and causes of false-positive HIV screening test results to better counsel pregnant women before testing and to deal more effectively with "borderline" results.
Laura A. Magee
Assistant Professor of Medicine
Obstetrics and Gynaecology
Kellie E. Murphy
Assistant Professor of Obstetrics and
Gynaecology (MaternalFetal Medicine)
Peter von Dadelszen
Clinical Fellow in MaternalFetal Medicine
University of Toronto
Toronto, Ont.
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References
- Samson L, King S. Evidence-based guidelines for universal counselling and offering of HIV testing in pregnancy in Canada. CMAJ 1998;158(11):1449-57.
- Proffitt MR, Yen-Lieberman B. Laboratory diagnosis of human immunodeficiency virus infection. Infect Dis Clin North Am 1993;7:203-19.
- Dock NL, Lamberson HV, O'Brien TA, Tribe DE, Alexander SS, Poiesz BJ. Evaluation of atypical human immunodeficiency immunoblot reactivity in blood donors. Transfusion 1988;28:412-8.
- Kuhnl P, Seidl S, Holzberger G. HLA-DR antibodies cause positive HTLV-III antibody ELISA results. Lancet 1985;1:1222-3.
- Mortimer PP, Parry JV, Mortimer JY. Which anti-HTLV-III/LAV assays for screening and confirmatory testing? Lancet 1985;2:873-7.
- Yu Sk, Fong CKY, Landry ML, Hsiung GD, Solomon LR. A false positive HIV antibody reaction due to transfusion-induced HLA-DR4 sensitization. N Engl J Med 1989;320:1495-6.
