Page 63 - AIDSBK23C

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populations soon after presumed exposures, then both false positive and false negative test
results could increase.[346]

WESTERN BLOT.-- The Western blot (WB) test is often used to confirm EIA positives
because of its high specificity. The method utilizes a substrate made by fractionating purified
HIV-1 by molecular weight, using polyacrylamide gel electrophoresis, into discrete bands that
are then transferred by electrophoretic blotting to a nitrocellulose membrane that is then cut into
strips. A patient serum, urine, or saliva specimen is placed on the strip and any HIV-1 antibodies
present will bind to the viral antigens. The bands are visualized by immunohistochemical
methods.[347] The Western blot technique can be utilized to distinguish HIV-1, HIV-2, HTLV-I,
and HTLV-II infections.[329]
Western blot testing requires high-quality reagents, careful interpretation of the band
patterns, and rigorous quality control. Thus, WB testing should be done by or referred to
qualified laboratories according to established criteria. Test strips showing no bands are
negative. Positive findings are interpreted by a number of "standard" criteria that require the
presence of two or more bands that represent specific denatured HIV-1 proteins including core
(p17, p24, and p55), polymerase (p31, p51, p66), and envelope (gp41, gp120, gp160) proteins
depending on the particular kit or method.[347]

A Western blot is positive if reactivity is detected with either:

gp41 and gp120/160 bands

or

either the gp41 or gp120/160 bands AND the p24 band