false positive. We do not believe this to be the case. First, in confirming these specimens as positive we eliminated 63 other specimens that were repeatedly reactive on routine EIA at the Red Cross but did not ultimately meet our criteria for positivity. Second, all six specimens not only were repeatedly reactive in routine EIA but also were reactive in a more specific recombinant protein EIA (Cambridge BioScience Corporation, Hopkinton, Mass.), conducted at the Bureau of Laboratories and Research, Federal Centre for AIDS, Ottawa. Third, all six specimens were positive in the Western blot assay according to the much more stringent set of criteria in current use. Finally, all six specimens were independently confirmed as well by radioimmunoprecipitation assay (RIPA) at the Federal Centre for AIDS. We do not know precisely the joint false-positive rate of routine EIA, recombinant protein EIA, current Western blot assay and RIPA, but it is likely to be orders of magnitude lower than the early data cited. Even if it were only one order of magnitude lower the positive predictive value in Turner's example would rise from 67% to over 95%. Martin T. Schechter, MD, MSc, PhD Department of Health Care and Epidemiology Faculty of Medicine University of British Columbia Penny J. Ballem, MD, FRCPC Noel A. Buskard, MD, FRCPC Canadian Red Cross Society Blood Transfusion Service, Vancouver Centre and Department of Medicine Faculty of Medicine University of British Columbia Vancouver, BC Michael V. O'Shaughnessy, PhD Bureau of Laboratories and Research Federal Centre for AIDS Department of National Health and Welfare Ottawa, Ont.
References 1. Meyer KB, Pauker SG: Screening for 628
CAN MED ASSOC J 1991; 144 (6)
HIV: Can we afford the false positive rate? NEngl JMed 1987; 317: 238-241 2. Burke DS, Brandt BL, Redfield RR et al: Diagnosis of human immunodeficiency virus infection by immunoassay using a molecularly cloned and expressed virus envelope polypeptide. Comparison to Western blot on 2707 consecutive serum samples. Ann Intern Med 1987; 106: 671-676
HIV antibody testing: CMA counselling guidelines
relations with their husbands? What would Chateauvert and colleagues' attitudes be toward these questions if these young women were their daughters? It seems to me that were this attitude followed without exception it would mean that the victims were being put on trial, with the death sentence a likely outcome. K. Lorne Massey, MD Department of Pathology
University Hospital Saskatoon, Sask.
I was distressed to read, in their References
letter (Can Med Assoc J 1990; 143: 827-828) responding to comments (ibid: 827) about this document,' that Drs. Michel Chateauvert, Anne Duffie and Norbert Gilmore can find very few indications for HIV antibody testing of a patient whose blood has infected a health care worker such as a laboratory technologist. One of the reasons offered is that "a true negative result cannot be distinguished from a false negative one at the time of testing." Although this is, I suppose, true in a strictly theoretical sense, in practical terms a negative result is exceedingly likely to be true. The predictive value of a negative result for a disease with a prevalence of less than 1% is very high indeed, in the order of 99.5% or greater.2 The second suggestion is that no action is going to be taken on the basis of positive or negative results of HIV antibody testing. Many of the staff in the laboratory for which I am responsible are young women, recently married, with every intention of raising a family. Should we counsel them that their actions will have no effect on whether they will come down with AIDS, whether they will become HIV carriers or whether they can pass the infection on to others? Does this, in turn, mean that we should give them: no adlvice concerning thbeivr
1. Chateauvert M, Duffie A, Gilmore N: Human Immunodeficiency Virus Antibody Testing: Counselling Guidelines from the Canadian Medical Association, CMA, Ottawa, 1990 2. Galen RS, Gambino SR: Beyond Normality: the Predictive Value and Efficiency of Medical Diagnoses, Wiley, New York, 1975
[The CMA responds.] We are very concerned that Dr. Massey has misunderstood our letter. We did not say that there are few indications for testing but that only in rare situations, such as when authorized by law or by the doctrine of necessity, is testing without a patient's informed consent justifiable. We agree that a negative result of HIV antibody testing is exceedingly likely to be true when HIV antibodies are absent. However, the test cannot distinguish between a person who is not infected and one who is infected but has not yet produced antibodies. This is why a negative result at the time of exposure for the patient who is the source of the exposure cannot reassure the exposed worker or guide decision making. Regardless of the test result the exposed worker must be counselled. We would never suggest that no action be taken on the basis of positive or negative results of HIV antibody testing. Counselling is For prescribing information see page 809
References 1. Meyer KB, Pauker SG: Screening for 628
CAN MED ASSOC J 1991; 144 (6)
HIV: Can we afford the false positive rate? NEngl JMed 1987; 317: 238-241 2. Burke DS, Brandt BL, Redfield RR et al: Diagnosis of human immunodeficiency virus infection by immunoassay using a molecularly cloned and expressed virus envelope polypeptide. Comparison to Western blot on 2707 consecutive serum samples. Ann Intern Med 1987; 106: 671-676
HIV antibody testing: CMA counselling guidelines
relations with their husbands? What would Chateauvert and colleagues' attitudes be toward these questions if these young women were their daughters? It seems to me that were this attitude followed without exception it would mean that the victims were being put on trial, with the death sentence a likely outcome. K. Lorne Massey, MD Department of Pathology
University Hospital Saskatoon, Sask.
I was distressed to read, in their References
letter (Can Med Assoc J 1990; 143: 827-828) responding to comments (ibid: 827) about this document,' that Drs. Michel Chateauvert, Anne Duffie and Norbert Gilmore can find very few indications for HIV antibody testing of a patient whose blood has infected a health care worker such as a laboratory technologist. One of the reasons offered is that "a true negative result cannot be distinguished from a false negative one at the time of testing." Although this is, I suppose, true in a strictly theoretical sense, in practical terms a negative result is exceedingly likely to be true. The predictive value of a negative result for a disease with a prevalence of less than 1% is very high indeed, in the order of 99.5% or greater.2 The second suggestion is that no action is going to be taken on the basis of positive or negative results of HIV antibody testing. Many of the staff in the laboratory for which I am responsible are young women, recently married, with every intention of raising a family. Should we counsel them that their actions will have no effect on whether they will come down with AIDS, whether they will become HIV carriers or whether they can pass the infection on to others? Does this, in turn, mean that we should give them: no adlvice concerning thbeivr
1. Chateauvert M, Duffie A, Gilmore N: Human Immunodeficiency Virus Antibody Testing: Counselling Guidelines from the Canadian Medical Association, CMA, Ottawa, 1990 2. Galen RS, Gambino SR: Beyond Normality: the Predictive Value and Efficiency of Medical Diagnoses, Wiley, New York, 1975
[The CMA responds.] We are very concerned that Dr. Massey has misunderstood our letter. We did not say that there are few indications for testing but that only in rare situations, such as when authorized by law or by the doctrine of necessity, is testing without a patient's informed consent justifiable. We agree that a negative result of HIV antibody testing is exceedingly likely to be true when HIV antibodies are absent. However, the test cannot distinguish between a person who is not infected and one who is infected but has not yet produced antibodies. This is why a negative result at the time of exposure for the patient who is the source of the exposure cannot reassure the exposed worker or guide decision making. Regardless of the test result the exposed worker must be counselled. We would never suggest that no action be taken on the basis of positive or negative results of HIV antibody testing. Counselling is For prescribing information see page 809
