Page 65 - AIDSBK23C

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is visualized separately, making interpretation simpler because of the absence of additional bands
and contaminating proteins. The antigens on the LIA strip are recombinant antigens or synthetic
peptides, rather than lymphocyte-derived viral lysates. For interpretation of a line immunoassay
as confirmation of true HIV-1 infection, the CDC requires at least two of three bands must be
present: p24, gp41, or gp160/120. The World Health Organization (WHO) guidelines require the
presence of two bands, gp41 and gp160/120, for confirmation of HIV-1 infection. The LIA can
be used to confirm HIV-2 infection by addition of an HIV-2 specific antigen such as p36.[333]

HIV-1 P24 ASSAY.-- The HIV-1 p24 assay detects the core antigen p24 which is
produced by the HIV-1 gag gene. This test is essentially the reverse of the enzyme
immunoassay for HIV-1 antibody, because the methodology makes use of an antibody to HIV
p24 coated on a solid phase that "captures" the p24 in a patient specimen. An enzyme
conjugated second antibody to p24 is then added and a standard enzyme immunoassay method
used for detection. The p24 assay can be utilized on non-lipemic or non-hemolyzed serum, on
plasma, or on cerebrospinal fluid.[356,357]
The p24 antigen can be detected in some persons only 1 week after initial HIV-1
infection, but in other persons it is transient, disappearing and then reappearing months later. In
most cases, p24 antigen can be detected 2 to 3 weeks following infection. The false positive rate
is significant because of interfering substances in serum such as immune complexes. In order for
a p24 test to be interpreted as positive, it must be repeatedly reactive and have a positive
neutralization test. A p24 test is “indeterminate” if it is repeatedly reactive but the neutralization
test is negative or invalid. Since HIV antibody is typically detectable within a week of p24
antigen positivity, indeterminate results can be followed up by repeat antigen and EIA testing in
a week. If the antigen test remains positive but the antibody test is negative, repeat EIA testing
for HIV is recommended in another 8 weeks. Testing for HIV by PCR can be helpful in this
setting.[358] False positive p24 assays are rare, but can occur from cross-reacting blood
proteins.[332]
Though p24 assay is not attractive for routine screening, it does have usefulness in
detecting HIV-1 infection in children born to HIV-1 infected mothers. The sensitivity of p24
assay at birth is 100%, but the specificity is only 18%.[359] The p24 antigen assay has a
sensitivity and specificity of 100% from 3 to 6 months of age, but specificity begins to fall after
6 months of age.[360] Loss of p24 antigen in HIV-1 seropositive persons may also signal the
onset of clinical AIDS, but only about 60% of patients with HIV-1 infection have detectable p24
antigenemia at the time of progression to clinical AIDS.[207,209] Long-term therapy with
zidovudine has been shown to decrease p24 antigenemia.[209] Free p24 antigen is often
complexed with p24 antibody, limiting detection methods, but immune complex dissociation
(ICD) with acid treatment or boiling of specimens improves detection of p24 antigen, which
increases the usefulness of the p24 antigen assay as a marker for progression of HIV
disease.[357]

HIV-1 IgA ASSAY.-- The HIV-1 IgA antibody assay on serum has been developed for
detection of perinatal HIV-1 infection, because maternal IgA antibody does not cross the
placenta. One commercially available method employs removal of IgG antibody in the specimen
followed by a simple immunoblot technique with visual interpretation. The sensitivity and
specificity of this test in infants over 6 months of age approaches that of the EIA method in