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Current Trends Update: Public Health Service Workshop on Human T-Lymphotropic Virus Type III Antibody Testing -- United States

The enzyme immunoassay (EIA) serologic tests to detect antibody to human T-lymphotropic virus type III (HTLV-III) are highly sensitive and specific, according to reports presented at a U.S. Public Health Service Workshop on HTLV-III Antibody Testing on July 31, 1985. The tests are currently being used at blood banks, plasma collection centers, health departments, and selected clinical centers throughout the United States.

The U.S. Food and Drug Administration reported cumulative HTLV-III antibody test data from more than 1.1 million units of blood collected at 155 centers through June 16, 1985. Of these, 2,831 (0.25%) were reported as positive based on a repeatedly reactive EIA test. The pattern of positive tests varied slightly in different regions of the country and by test kit used.

The Atlanta Region of the American Red Cross (ARC) and CDC reported data from testing more than 51,000 blood donors, of whom 0.23% were repeatedly reactive by the Abbott EIA method.* Among the specimens from 106 blood donors with repeatedly reactive tests, 34 (32%) were strongly reactive (ratio of specimen absorbance to cutoff value 7.0 or greater). EIA tests categorized as strongly reactive correlated highly with both positive Western blot tests (94%) and culture for HTLV-III/lymphadenopathy-associated virus (LAV) (56%).

Of 220 donors whose tests were initially reactive and subsequently negative, as well as a random sample of 50 with an initially negative EIA test, none had either a positive Western blot test or positive culture. Among those donors notified and interviewed to date, 16 (89%) of 18 with strongly reactive EIA tests had identifiable risk factors for HTLV-III/LAV infection, while none of 20 with weakly reactive tests had identifiable risk factors.

To determine the sensitivity of the Abbott EIA test in high-risk persons, virus isolations were attempted from homosexual men attending a clinic for sexually transmitted diseases in San Francisco, California. None of 70 men with negative HTLV-III antibody tests had a positive culture, while 43 (60%) of 72 with repeatedly reactive tests were culture-positive. Among the 72 EIA-positive sera in this portion of the study, 70 (97%) were considered to be highly reactive. Ninety-seven percent of those EIA-positive specimens tested to date have had a positive Western blot test.

Data from other blood banking organizations paralleled the findings of the ARC/CDC study in suggesting that approximately one-third of EIA-positive sera from blood donors were strongly reactive, regardless of the test kit used. Donors with strongly reactive EIA tests were also highly likely to have positive Western blot tests and to have positive EIA tests by other test kits.

Weakly reactive EIA tests correlated poorly with positive Western blot tests and were judged to be nonspecific for HTLV-III/LAV infection. The reason for nonspecific test reactivity is unknown, but proposed refinements in the test may eliminate many of the low level reactions. Reported by Center for Drugs and Biologics, U.S. Food and Drug Administration; AIDS Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Based on available data, only about 0.25% (1 in 400) blood donors have repeatedly reactive EIA tests to HTLV-III antibody. Approximately 0.08% (1 in 1,200) donors were found to have strongly reactive EIA tests, and these donors were likely to have other test results (Western blot, HTLV-III/LAV culture) that suggested they had been infected with HTLV-III/LAV.

Thus, in less than 5 months, serologic tests for HTLV-III antibody have been introduced and demonstrated to be highly useful in screening donated blood. Screening performed during this period may have removed as many as 1,000 potentially infectious units of blood from the U.S. blood supply. Continued use of this highly sensitive test procedure for HTLV-III antibody, in combination with voluntary avoidance of donation by members of high-risk groups, will virtually eliminate the risk of acquired immunodeficiency syndrome (AIDS) transmission by the nation's blood supply. Discussions and evaluations of other potentially appropriate and useful applications of this test are under way. *Use of trade names is for identification only and does not imply endorsement by CDC or the U.S. Public Health Service.

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