The Centers for Disease Control and Prevention (CDC) has updated its HIV testing algorithm to take into account improvements in the tests available.
Some of the changes include
(1) antigen detection of p24 in screening tests
(2) revising use of Western Blot testing in patient evaluation, partly due to the possibility that this can give misleading results in patients with HIV-2 infection
(3) improved antibody detection
Specimen: serum or plasma
Initial screening test: for antibodies to HIV1 or HIV 2 and for p24 antigen
If the screening test is negative, then no further testing is performed. If the person has had a significant exposure then repeat testing may be performed at appropriate intervals.
If the screening test is positive then testing consists of an HIV-1/HIV-2 antibody differentiation immunoassay.
Antibody to HIV-1
Antibody to HIV-2
Interpretation
positive
negative
HIV-1 antibodies detected
negative
positive
HIV-2 antibodies detected
positive
positive
HIV-1 and HIV-2 antibodies detected (dual infection or undifferentiated)
Western blot testing may be performed to confirm antibodies to HIV-1. However, the Western Blot may be falsely negative in early infections. HIV-1 RNA testing can be used to resolve a discrepancy.
A positive test for antibodies to HIV-2 may be submitted for confirmatory testing (HIV-2 DNA/RNA PCR).
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