ELISA
(Antibody) Test
What is it?
Traditionally the most common diagnostic test
for HIV, the ELISA test is a screening test for the diagnosis of HIV
infection based on the presence of antibodies. An ELISA result must be
confirmed by a Western Blot test, to make sure the ELISA test result was not
a false positive (due to the presence of e.g. lupus, Lyme disease, or
syphilis).
What should I do to prepare?
You don’t need to do anything to prepare for an
ELISA test, though your written consent will probably be required on test
day.
What happens?
During the test, the puncture site (your inner
elbow or back of your hand) will be cleaned with antiseptic, and an elastic
band will be stretched around your upper arm to allow blood to collect in
veins at the puncture site. A needle will be used to draw blood from the
vein, which may cause moderate pain or throbbing, though many people feel
only a prick or a slight stinging feeling. Blood is collected in a vial or
syringe. The elastic band is then removed to restore circulation, the needle
is removed, and bleeding is stopped at the puncture site.
Obtaining your test result usually takes 1-2
weeks.
How does it work?
HIV antigens are coated onto an ELISA plate
(shown below):
A serum developed from the patient’s blood
sample, containing antibodies, is added to the ELISA plate, which is then
washed clear of inactive antibodies that will not bind to antigens. A second
layer of antibodies, called a conjugate, is added to detect the primary
antibodies from the human serum. Excess antibodies are again washed clear of
the plate, and finally, a substrate (chromogen) is added to make reactions
occur. If positive: the enzyme on the antibodies, once bound to the
HIV antigens, will act on the substrate, changing its color (darker color
means more serum has bound to the antigen). If negative: no
antibodies will bind to the HIV antigens, so no enzyme will be present to
change the color of the substrate (no visible change will occur).
Risks & Errors
Patients having blood drawn may experience the
following risks: excessive bleeding, fainting or feeling lightheaded,
hematoma (blood accumulating under the skin), infection (a slight risk
anytime the skin is broken), or multiple punctures to locate veins. Possible
errors in the ELISA test may lead to a “false positive” or a “false
negative” result. Reasons for “false positives” include: women who have had
multiple pregnancies (who may possess antibodies directed against human
leukocyte antigens, confusing the test), or the presence of other diseases
such as Lyme disease, lupus, or syphilis. “False negatives” (a test failure
to register the presence of HIV), may occur during the early stages of HIV
infection, before seroconversion (an antibody response to the virus) has
occurred.
Western
Blotting
What is it?
The western blot test is a more specific version
of the ELISA, which not only indicates whether a patient is HIV-positive or
negative; it allows you to see which antibodies are directed against each
viral protein. It is given as a follow-up to a positive ELISA test.
What should I do to prepare?
No special
preparations are required.
What happens?
The same
blood-drawing procedure as for an ELISA test is followed to obtain a sample.
How
does it work?
In an HIV Western blotting, viral proteins from
a blood sample are passed through a gel. Different proteins migrate through
the gel at different speeds; typically, smaller proteins migrate through the
gel faster than larger proteins. The separated proteins are then passed
through an electric current so they can transfer onto a solid film strip in
order of their speed. Human serum is added, and any existing HIV antibodies
will bind to the HIV antigens. A chemical that reacts on contact with a
protein-antibody-enzyme layer changes band color, just as in the ELISA test.
Interpreting results is complicated, but the common “3-band rule” says that
if three or more bands appear, HIV antibodies have been detected.
Risks & Errors
The same risks of
drawing blood exist for the Western blot as for the ELISA test. However,
there is a smaller margin of error because the Western blot is more
specific: it shows exactly what antibodies are binding to what antigens.
However, the ELISA test is quicker and less expensive, and can detect
infection earlier. This is why both tests are usually used, and are
considered complementary.
Rapid
Testing
What is it?
A rapid test produces results in 5-30 minutes,
as opposed to the traditional antibody test, in which you have to come back
for a second visit to the clinic, hospital, or testing center. There are
four licensed rapid tests for HIV in the US:
-
OraQuick Rapid HIV-1 and Advance HIV ½
Antibody Tests, manufactured by OraSure Technologies, Inc.
-
Reveal G2 HIV-Antibody Tests, manufactured by
MedMira, Inc.
-
Multispot, manufactured by Bio-Rad
Laboratories
-
Uni-Gold Recombigen, manufactured by Trinity
Biotech
What should I do to prepare?
No special preparations are required.
What happens?
Rapid testing usually draws blood through a
finger prick.
How does it work?
This differs between various brands of tests.
However, the results are guaranteed to be just as accurate as the
traditional ELISA antibody test.
Risks & Errors
The same risks of drawing blood exist for the
rapid test as for the ELISA test, although less blood is drawn. A rapid test
also has a margin of error similar to an ELISA test, and an initial reaction
on a rapid test should be confirmed by a Western blot test.
Viral Load Testing
What is it?
The viral load test measures
the amount of HIV virus in your body, and is given after a person is
diagnosed with HIV. A viral load test should not be taken as a
diagnostic test. Along with the CD4 cell count, the viral load test used to
give your doctor information about the progression of HIV infection, to
predict its future course, and to guide recommendations for treatment.
Keeping viral load levels low for as long as possible has been proven to
decrease disease complications and prolong life.
What should I do to
prepare?
Talk with your doctor, who
will be the one to order the viral load test after HIV-positive status has
been diagnosed. If and when HIV therapy is begun as a response to the first
viral load test, your doctor should order another viral load test and a CD4
cell count about 2-8 weeks after you start treatment, to evaluate the effect
of the therapy. On a long-term basis, you should be prepared to re-take
viral load tests and CD4 cell counts about every 3-6 months as you are being
treated for HIV.
What happens?
The same blood-drawing
procedure as for an ELISA test or a Western blot test is followed to obtain
a sample.
How does it work?
There are two types of viral
load tests: PCR (polymerase chain reaction), and bDNA (branched DNA). The
tests use different techniques to measure the same thing, which is the
amount of HIV present in your bloodstream. However, it is very important to
use the same type of viral load test every time and not switch between
the two, since the values on test reports from the two different types
are not comparable. The viral load report sent to your doctor will indicate
which test was used each time.
The test report will show the
number of HIV copies in one millilitre of blood. This indicates the current
level of HIV, and the rate at which it is reproducing. The higher the count
and reproductive rate, the faster the disease is likely to progress. A high
viral load can be anywhere between 5,000 and one million copies or more; a
low viral load is usually between 200 and 500 copies. However, a low viral
load only indicates slow progression, not an absence of HIV or cured HIV (a
non-infected patient should have a negative or undetectable viral load,
since he or she has no circulating HIV virus!) The rate of change in viral
load between successive tests is also important: an increasing count
indicates that a worsening infection, while a decreasing count indicates
suppression of the HIV infection and improvement of the patient’s health.
Risks & Errors
The same risks of drawing blood exist for the
viral load test as for the ELISA test. A viral load test should not be
treated as a diagnostic test for HIV. There is a chance of a “false
positive” result, indicating a falsely high viral load, especially when done
by the very sensitive PCR method. There is also a chance of a “false
negative” when the result is undetectable; an undetectable result does not
mean the patient is cured of HIV.
Sources:
www.biology.arizona.edu
www.msichicago.org/ed/AIDS/hivtst3.htm
www.webmd.com
www.labtestsonline.org
www.aidsmeds.com
www.immunetics.com
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