1. Indication
1. Used to confirm positive HIV EIA (HIV Screening Test)
2. Mechanism
1. Direct visualization of antibodies to HIV components
1. Components: Env, Gag, Pol
2. Antibodies: p24, gp41, gp120, gp160
3. Band forms if Antibody present to that component
2. Positive Test
1. Two or more bands from p24, gp41, gp120 or qp160
3. Efficacy
1. Reliably identifies HIV-1
2. False negative causes
1. Group O protein (10% false negative rate)
2. HIV-2 (20% false negative rate)
3. Early seroconversion (less sensitive than HIV ELISA)
3. False positive causes
1. Hyperbilirubinemia
2. Connective tissue disorder
3. Polyclonal Gammopathy
4. Follow-Up evaluation for indeterminate or negative test
1. Repeat Western blot in 1-3 months
2. Consider Western blot for HIV-2
5. Indication
1. Initial HIV Screening Test
6. Efficacy
1. Highly sensitive and specific (>99%)
7. Precautions
1. High false positive rate in low risk groups
2. Never tell anyone they are HIV positive based on EIA
1. Confirm with HIV Western Blot
The HIV Western blot consists of a thin nitrocellulose strip in which are
embedded proteins claimed to be unique to HIV. Each protein is labeled with a 'p'
followed by its molecular weight in thousands. Serum is added to the strip and if there are
antibodies to a particular protein this band will 'light up'.
The HIV Western blot is not standardized and thus around the world different
combinations of bands are considered positive. Hence a positive test in one country is not
positive in another. An African would not be positive in Australia. A person from the
MACS would not be positive anywhere in the world including Africa. Yet the HIV
Western blot is considered to be highly specific and is considered synonymous with HIV
infection.
According to data presented in Lundberg et al. (JAMA 260:674-679) when the
US FDA criteria are used to interpret the HIV Western blot less than 50% of US AIDS
patients are HIV positive whereas 10% of persons not at risk of AIDS are also positive by
the same criteria.
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Dr. Alex Franko III
AFR = Africa; AUS = Australia; FDA = US Food and Drug Administration; RCX = US Red Cross; CDC =
US Center for Disease Control; CON = US Consortium for Retrovirus Serology Standardization; GER =
Germany; UK = United Kingdom; FRA = France; MACS = US Multicenter AIDS Cohort Study 19831992.
Understanding Your HIV Test Results
There are a number of different tests that can be used to diagnose HIV infection.
The most commonly used tests are called "HIV antibody tests." Antibodies are the
body's reaction to HIV.
Standard antibody HIV testing can have two stages. The most commonly used
initial screening test is either an enzyme immune assay (EIA) or the enzyme-linked
immunosorbent assay (ELISA). If the EIA/ELISA test is negative, no additional testing
is done. If the EIA/ELISA screening test is positive, that result must be confirmed with a
more specific test called "Western Blot."
Rapid HIV testing using oral fluids or blood specimens are EIA/ELISA
screening tests that provide results in 20 minutes. This kind of test has been shown to be
highly accurate (99.5%) and comparable to the blood tests that are performed in a lab. A
positive result from a Rapid EIA test is considered "preliminarily positive" and must be
confirmed with a Western Blot performed in a lab.
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Dr. Alex Franko III
HIV Antibody Western Blot Testing
HIV antibody tests can have three different results: positive, negative or indeterminate
(inconclusive).


A Positive result on a confirmed HIV antibody test means that HIV antibodies
are present and you are infected with HIV (called "HIV positive"). To read about
clinical trials for HIV-positive people, click here to learn about on-going studies
at the NYU/Bellevue AIDS Clinical Trials Unit.
A Negative result on an HIV antibody test means that most likely you are not
infected with HIV. However, it can take 3 to 6 weeks, and sometimes up to 3
months (and in few cases up to 6 months) before HIV antibodies show up on a
standard test. As a result, some people who are recently infected with HIV may
still have a negative test result during this time. This is called the "Window
Period." The window period is why it is important to be tested for HIV regularly.
An indeterminate/inconclusive result means that the antibody test was neither positive
nor negative. This may be a result of:
o
o
o
o
o
o
Recent HIV infection,
Prior blood transfusions, even with non-HIV infected blood,
Prior or current infection with syphilis, malaria, or other viruses,
An autoimmune disease such as lupus or diabetes,
Being a recipient of an experimental HIV vaccine,
Or problems with the test procedure itself, such as contamination of the
blood sample.
o If a person has an indeterminate HIV test result, the test should be
repeated. If the test results continue to be indeterminate, there are other
tests that may be done to detect an infection.
Western Blot Tests Illustrate the Window Period
Below is a photograph of a Western Blot test. On the left side of the image there are
two columns to be used as points of comparison. The column marked "NC" is an HIVnegative test result and the column marked "PC" is an HIV-positive test result.
Columns 3 to 10 show a series of tests on an individual person who became infected
with HIV to illustrate how an HIV test result can change during the window period from
HIV-negative to HIV-positive. Each column is one Western Blot test. These tests were
performed on a single person beginning with the day the person was first infected with
HIV (Column 3, Day 0) to when the person had a conclusive HIV infection (Column 10,
Day 30). Each black or dark grey horizontal stripe is representative of the presence of a
different antibody against a protein found in HIV. To be conclusive (HIVpositive), a Western Blot must have 5 horizontal stripes. An HIV infection
is not the same as an AIDS diagnosis.
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Dr. Alex Franko III
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Dr. Alex Franko III
PCR/Viral Load Testing
A PCR (Polymerase Chain Reaction) test, also known as a "viral load," is used to
measure the amount of HIV in an HIV-positive person's blood. Because this test looks
for HIV directly in a person's blood instead of detecting antibodies (the body's reaction
to HIV), it may detect an HIV infection about a week after an exposure. Therefore the
PCR test is used by researchers and health care providers to identify infections during
the window period.
Links
http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5019a1.htm
http://www.cdc.gov/hiv/topics/testing/index.htm
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Dr. Alex Franko III