False-positive results in antenatal HIV screening
CMAJ 1999;160;1285
In response to: L.A. Magee, et al
The concerns laid out by Dr. Magee and colleagues bring to our attention the important issue of the possibility of false-positive results of HIV screening in an otherwise healthy pregnant population.
Although the case they describe certainly led to a high level of anxiety for the patient, it is unclear how the testing and results were actually reported. Every time there is a positive screening EIA test result, a second EIA test is performed in duplicate on the same sample. If the result of either of these tests is positive, a confirmatory Western Blot test is also done. Only then should a result be forwarded to the ordering physician. If the report is positive it is likely that the patient is infected, and the test should be repeated just for confirmation. If the result is negative, then despite the first EIA result being positive the patient is not infected and no further testing needs to be done. If the result is indeterminate, which we assume was the case, the test should be repeated. The reason for indeterminate test results are either that the patient has recently been infected and not yet developed sufficient HIV antibodies or that non-HIV antibodies are cross-reacting with the HIV antigens, yielding nonspecific Western blot bands that cannot be interpreted as HIV negative.
Although the combination of the present generation of HIV EIA screening and Western blot confirmatory tests has a high sensitivity and specificity, with previous generations of tests indeterminate results were more likely in patients with multiple previous blood transfusions and autoimmune disorders. It is likely that the newer generation tests have lower rates of indeterminate results in pregnant women.
It is imperative that we develop a national surveillance system to determine whether there is indeed a higher rate of indeterminate results among pregnant women. In the meantime, we recommend that women with indeterminate results undergo a repeat test immediately. A phone call to the local public heath laboratory performing the test should enable a result within 24 hours in most places in the country. It is also important that the requisition state that the woman is pregnant. HIV viral load testing has not yet been licensed for HIV diagnosis and is not recommended.
Lindy Samson, MD
Assistant Professor of Pediatrics
University of Ottawa
Ottawa, Ont.
Susan King, MD
Associate Professor of Pediatrics
University of Toronto
Toronto, Ont.
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