The human immunodeficiency virus (HIV) is a retrovirus that
infects cells of the immune system, destroying or impairing their function. As
the infection progresses, the immune system becomes weaker, and the person
becomes more susceptible to infections. The most advanced stage of HIV
infection is acquired immunodeficiency syndrome (AIDS). It can take 10-15 years
for an HIV-infected person to develop AIDS; antiretroviral drugs can slow down
the process even further.
HIV is transmitted through unprotected sexual intercourse
(anal or vaginal), transfusion of contaminated blood, sharing of contaminated
needles, and between a mother and her infant during pregnancy, childbirth and
breastfeeding.
Many HIV-positive people are unaware that they are infected
with the virus. For example, in 2001 less than 1% of the sexually active urban
population in Africa have been tested and this proportion is even lower in
rural populations. Furthermore, in 2001 only 0.5% of pregnant women attending
urban health facilities are counselled, tested or receive their test results.
Again, this proportion is even lower in rural health facilities. Since donors
may therefore be unaware of their infection, donor blood and blood products
used in medicine and medical research are routinely screened for HIV.
HIV-1 testing is initially by an enzyme-linked immunosorbent
assay (ELISA) to detect antibodies to HIV-1. Specimens with a nonreactive
result from the initial ELISA are considered HIV-negative unless new exposure
to an infected partner or partner of unknown HIV status has occurred. Specimens
with a reactive ELISA result are retested in duplicate.If the result of either
duplicate test is reactive, the specimen is reported as repeatedly reactive and
undergoes confirmatory testing with a more specific supplemental test (e.g.,
Western blot or, less commonly, an immunofluorescence assay (IFA)). Only
specimens that are repeatedly reactive by ELISA and positive by IFA or reactive
by Western blot are considered HIV-positive and indicative of HIV infection.
Specimens that are repeatedly ELISA-reactive occasionally provide an
indeterminate Western blot result, which may be either an incomplete antibody
response to HIV in an infected person or nonspecific reactions in an uninfected
person.
Although IFA can be used to confirm infection in these
ambiguous cases, this assay is not widely used. In general, a second specimen
should be collected more than a month later and retested for persons with
indeterminate Western blot results. Although much less commonly available,
nucleic acid testing (e.g., viral RNA or proviral DNA amplification method) can
also help diagnosis in certain situations.[64] In addition, a few tested
specimens might provide inconclusive results because of a low quantity
specimen. In these situations, a second specimen is collected and tested for
HIV infection.
Modern HIV testing is extremely accurate. A single screening
test is correct more than 99% of the time.The chance of a false-positive result
in standard two-step testing protocol is estimated to be about 1 in 250,000 in
a low risk population.Testing post exposure is recommended initially and at six
weeks, three months, and six months.
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