Confirmation Testing for HIV Infection
The recommended confirmation test for HIV infection is Western blot or indirect immunofluorescence assay (IFA) following a reactive screening test such as enzyme-linked immunosorbent assay (ELISA) or rapid HIV test. 1, 2
Diagnostic Algorithm
- Initial screening should be performed using a sensitive test such as ELISA or rapid HIV test 1, 2
- All reactive screening tests must be confirmed with Western blot or indirect immunofluorescence assay before a definitive diagnosis can be made 1, 2
- If confirmatory testing results are negative or indeterminate, follow-up testing should be performed on a blood specimen collected 4 weeks after the initial reactive test 1, 2
- Specimens reactive on screening tests are considered "preliminary positive" until confirmed 1
Modern Testing Approaches
- Fourth-generation assays that incorporate HIV p24 antigen detection along with antibody testing are now recommended as initial screening tests 1
- The newer testing algorithm associated with fourth-generation assays may not require Western blot confirmation 1
- Instead, individuals with reactive results on fourth-generation assays are further tested with an antibody immunoassay that distinguishes HIV-1 from HIV-2 antibodies 1
- If the differentiation assay is negative, further testing with nucleic acid amplification testing (NAAT) is recommended to rule out acute HIV-1 infection 1
Special Considerations
- For infants under 15 months born to HIV-positive mothers, definitive diagnosis should be based on laboratory evidence of HIV in blood or tissues by culture, nucleic acid, or antigen detection 2
- HIV-2 testing should be considered in persons from endemic regions or their sexual partners 2
- Testing for HIV-2 is recommended when there is clinical evidence of HIV disease but negative HIV-1 antibody tests 2
Common Pitfalls and Caveats
- False-negative results can occur during the "window period" (recent infection within 6 months) 2, 3
- False-positive results can occur with screening tests, which is why confirmation is essential 3, 4
- Some authorities recommend that a positive oral rapid test result be routinely confirmed with a whole blood rapid test due to potential for higher frequency of false-positive results 1
- Never disclose a positive HIV diagnosis without confirmatory testing 2
- Indeterminate Western blot results may require additional testing such as repeat Western blot, HIV culture, or polymerase chain reaction testing 5
Accuracy of Testing Methods
- Currently licensed ELISA tests have greater than 98% sensitivity and specificity for HIV 3
- The combination of ELISA screening followed by Western blot confirmation makes false-positive HIV-1 results extremely rare 4
- Rapid diagnostic tests may have lower sensitivity compared to ELISA when used as screening assays, with one study showing sensitivity of 77.5% compared to ELISA 6
The diagnostic approach to HIV has evolved over time, but Western blot or indirect immunofluorescence assay remains the gold standard for confirmation of HIV infection following reactive screening tests. However, newer algorithms using fourth-generation assays and differentiation immunoassays are becoming more common, especially in resource-limited settings.