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Interpretive Criteria Used to Report Western Blot Results for HIV-1-Antibody Testing -- United States

The Association of State and Territorial Public Health Laboratory Directors (ASTPHLD), CDC, and other organizations (e.g., American Red Cross (ARC) and Consortium for Retrovirus Serology Standardization (CRSS)) have recommended for antibody testing to human immunodeficiency virus type 1 (HIV-1) that duplicate repeat reactive enzyme immunoassay (EIA) screening results be confirmed by a supplemental test (1-6). This report examines the variation in Western blot (WB) interpretive criteria reported by laboratories enrolled in CDC's Model Performance Evaluation Program (MPEP) for HIV-1-antibody testing.

In a December 1990 questionnaire survey, 1218 participants in the MPEP were asked to identify the WB interpretive criteria they used. Laboratories were also provided descriptions of the various WB band pattern combinations that were representative of each organization's set of WB interpretive criteria (Table 1) and were asked to choose which WB patterns their laboratory would use to classify a specimen as HIV-1-antibody reactive.

Of 201 laboratories that performed WB and responded, 44 (21.9%) indicated that they used more than one set of WB interpretive criteria; the remaining 157 (78.1%) laboratories indicated that they used only a single set of criteria to interpret WB results. However, discrepancies in WB interpretive practices occurred even among this latter group; when survey analysts compared the interpretive criteria that the laboratory reported using (e.g., ARC, ASTPHLD/CDC, CRSS, and Du Pont*) with the band pattern that same laboratory used to classify a specimen as reactive, only 138 (87.9%) of 157 laboratories indicated a WB band pattern that was representative of the interpretive criteria used in their laboratory.

Participating laboratories submitted results to the MPEP after testing the performance evaluation samples sent to them in August and November 1989 and in February, May, and September 1990; the sets of WB interpretive criteria they used were grouped by laboratory type (Table 2). During this period, use of the WB interpretive criteria recommended by ASTPHLD/CDC increased (4,5), and use of the Du Pont and ARC interpretive criteria decreased. Additionally, laboratories of the same type did not use the same WB interpretive criteria (e.g., some health department laboratories used interpretive criteria other than those recommended by ASTPHLD/CDC). Approximately 5% of the laboratories participating in the MPEP program did not indicate which set of WB interpretive criteria they used. Reported by: Laboratory Practice Br, Div of Laboratory Systems, Public Health Practice Program Office, CDC.

Editorial Note

Editorial Note: The WB test is a more specific supplemental test (1,3,4,8) and is used by more than 90% of the laboratories participating in the MPEP that perform supplemental testing (9,10). Although all WB interpretations are based on detecting antibodies against specific viral proteins (Table 3), different organizations have promoted the use of different sets of criteria for interpreting HIV-1 band patterns in the WB test (Table 1). Consequently, different sets of WB interpretive criteria, depending on organizational requirements or varying reasons for testing, have evolved. As a result, interpretation of a given WB pattern may depend on which criteria are used by the testing laboratory.

All sets of WB interpretive criteria (Table 1) consider a WB test that has no bands as nonreactive for HIV antibody. WB band patterns that do not meet the specific criteria for reactive are termed "indeterminate." When the four sets of WB interpretive criteria are applied to a specific WB band pattern, a WB interpretation considered reactive using one set of criteria will, in most cases, also be reactive using another set of criteria. In the early and late stages of HIV-1 infection, however, antibody titers to specific proteins may vary considerably, and the use of different sets of WB criteria may result in an incomparable interpretation (e.g., an interpretation of a WB band pattern classified as reactive using one set of WB interpretive criteria may be indeterminate using another set of criteria).

The consistent use of the ASTPHLD/CDC WB interpretive criteria would have substantially reduced the number of indeterminate interpretations reported for these performance evaluation samples. A reduction in indeterminate interpretations for clinical and public health specimens may decrease error and misinterpretation of HIV-1-testing reports (11), cost and difficulty of counseling persons with indeterminate test results, and cost of specimen retesting. Therefore, CDC recommends that laboratories use the ASTPHLD/CDC interpretive criteria to interpret WB results (5).


  1. CDC. Update: serologic testing for antibody to human immunodeficiency virus. MMWR 1988;36:833-40,845.

  2. Dodd RY, Fang CT. Western immunoblot procedure for HIV antibodies and its interpretation. Arch Pathol Lab Med 1990;114:240-5.

  3. Consortium for Retrovirus Serology Standardization. Serological diagnosis of human immunodeficiency virus infection by Western blot testing. JAMA 1988;260:674-9.

  4. Committee on Human Retrovirus Testing. Report of the Fourth Consensus Conference on HIV Testing sponsored by the Association of State and Territorial Public Health Laboratory Directors (ASTPHLD). Infect Control Hosp Epidemiol 1989;10:357-67.

  5. CDC. Interpretation and use of the Western blot assay for serodiagnosis of human immunodeficiency virus type 1 infections. MMWR 1989;38(no. S-7).

  6. Du Pont Diagnostics. Human immunodeficiency virus (HIV): Biotech/Du Pont HIV Western blot kit for detection of antibodies to HIV. Wilmington, Delaware: Du Pont Diagnostics, 1987.

  7. Sandler SG, Dodd RY, Fang CT. Diagnostic tests for HIV infection. In: DeVita VT, Hellman S, Rosenberg SA, eds. AIDS: etiology, diagnosis, treatment, and prevention. Philadelphia: JP Lippincott, 1988:121-36.

  8. O'Gorman MRG, Weber D, Landis SE, Schoenbach VJ, Mittal M, Folds JD. Interpretive criteria of the Western blot assay for serodiagnosis of human immunodeficiency virus type 1 infection. Arch Pathol Lab Med 1991;115:26-30.

  9. CDC. Update: serologic testing for HIV-1 antibody--United States, 1988 and 1989. MMWR 1990;39:380-3.

  10. Valdiserri RO, Taylor RN, Hearn TL, Schalla WO, Muir HW. CDC perspective on quality assurance for human immunodeficiency virus type 1 antibody testing. Arch Pathol Lab Med 1990;114:263-7.

  11. Benenson AS, Peddecord KM, Hofherr LK, Ascher MS, Taylor RN, Hearn TL. Reporting the results of human immunodeficiency virus testing. JAMA 1989;262:3435-8.

*Use of trade names is for identification only and does not imply endorsement by the Public Health Service or the U.S. Department of Health and Human Services.

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