|
Diagnosing
HIV Infection
HIV
ELISAs and Western Blots
Antibodies to HIV are usually detected by assays known as enzyme-linked
immunosorbent assays (ELISA or EIA). The wells of plastic microtiter
plates are coated with recombinant viral proteins (antigens), which
stick to the plastic. If serum from an infected patient is added
to the well, HIV antibodies bind to the proteins and become attached
to the plate. After washing away the serum the bound antibodies
are detected by a second antibody that is linked to an enzyme such
as alkaline phosphatase. This second antibody binds to human anti-HIV
antibodies and can be detected by reacting the plates with a substrate
for alkaline phosphatase that turns color when cleaved by the enzyme.
HIV antibody
tests are more than 99% sensitive (that is, they detect the presence
of HIV antibodies in nearly all infected patients); virtually the
only infected patients who are not detected by standard HIV tests
are those who are tested within the first few weeks after infection.
The specificity of HIV antibody tests is also better than 99%, but
some false positive tests do occur.
Therefore, all positive ELISAs are confirmed by a second test such
as the western blot. Though not as sensitive as the ELISA, the western
blot is more specific and allows the laboratory to identify the
specific HIV proteins to which the antibodies are reacting. Only
tests that are positive by ELISA and by a second, confirmatory test
are reported as HIV positive.
HIV tests performed
in the United States detect antibodies to both HIV-1 and HIV-2.
Antibodies to most of subtypes of HIV-1 group M are detected by
the current tests, but they are less reliable at detecting infection
with more distantly related strains of HIV-1 belonging to groups
O and N. (Group M accounts for the vast majority of HIV-1 infections;
groups O and N are found predominantly in western Africa.)
4/15/01
Copyright 2001
by HIV and Hepatitis.com. All Rights Reserved
|
