Antiviral Strategies

Page 11 of 18

Viral Load Tests
Fact Sheet

Viral Load
Tests

New diagnostic tests can detect and measure HIV RNA (genetic material)
in the plasma (blood) of almost all HIV-infected individuals, providing an
important new tool in monitoring HIV disease. The most sensitive test
today can measure down to 400 copies of virus, and a new test that measures down to 20-50 copies of HIV RNA could be available as early as 1998.
In addition, HIV RNA levels in plasma usually correlate with stage of disease, making them a good predictor of disease progression. Viral load tests
are also a sensitive tool for examining the effect of anti-HIV therapies.

What are Viral Load Tests?
The three main types of viral load tests currently being used are
Q-PCR, bDNA and NASBA. Q-PCR (quantitative polymerase chain
reaction) is commercially known as the Amplicor HIV-1 Monitor
Test and is made by Roche Molecular Systems. bDNA (branchedchain DNA, or Quan- tiplex) is made by Chiron, and NASBA (nucleic
acid sequence-based amplification) is made by Organon Teknika.
These tests are done by taking a sample of blood and making
multiple copies of the virus present in that sample. Through a
mathematical process, they can estimate the number of viral particles originally present in that sample.
Each test is most efficient for a particular range of HIV RNA levels.
Q-PCR 200 to 1,000,000 copies
bDNA 10,000 to 1,000,000 copies
NASBA 4,000 to 10,000,000 copies
The various tests are appropriate for different stages of HIV disease. Q-PCR is the most sensitive and can detect very low levels of
virus in the blood, but the bDNA test has been shown to be the
most accurate in quantifying high levels of virus. Every test has a
certain error level, some as high as 20%. In addition, the tests
require different amounts of blood - bDNA uses 2 ml (several
tablespoons) while NASBA and Q-PCR require only 100 and 200
microliters respectively (much less). This may be an issue for
people receiving other tests that require large amounts of blood.
In addition, Q-PCR on average gives viral load values twice as
high as b-DNA, so it is important to use the same laboratory and
test each time to accurately gauge changes in viral load.
New versions of these tests are being studied, and each "generation" of test generally provides greater sensitivity and accuracy.
Roche is developing a new ultra-sensitive test that will be able to
detect between 20-50 copies of HIV RNA, allowing us to know
earlier whether a therapy is succeeding. Similarly, Chiron is de
veloping second and third generation bDNA tests which will be
able to detect 500 and 50 copies respectively and Organon Teknika
has a second generation test which will be able to detect 400 copies.
Viral Load and HIV Reservoirs
Because viral load is only measured from the blood, it is important to know whether as HIV levels are reduced in the blood, levels are also being reduced in other parts of the body. A few recent
studies showed that HIV RNA levels in the blood do correlate with
RNA levels in semen, vaginal secretions and in the lymph tissue.
As viral load decreases in the blood, viral load also seems to decrease in these less accessible areas of the body. Further research
needs to focus on the correlation between plasma viral load and
HIV levels in the brain and bone marrow. If a strong correlation
exists among these areas, viral load will give us a clearer picture
of the overall state of disease in the body.
Interpreting the Results
People with undetectable viral load should remember that "undetectable" does not mean that the virus is gone, but that it is simply at such low levels that it is below the sensitivity of the test.
Viral load levels in the plasma may be undetectable, but currently
we are not certain virus is not present in other parts of the body
which are harder to access, such as the lymph tissue and the brain.
However, recent data do indicate that a correlation may exist between viral levels seen in plasma and other parts of the body. These
guidelines are being clarified and modified as physicians and researchers learn more about the predictive value of viral load tests.
What Can Affect the Results?
Because the tests measure viral activity, anything that affects the
production of virus can influence test results. In the normal progression of HIV disease, viral levels tend to rise slowly, and a sudden, sustained rise can indicate that the disease is likely to progress
more rapidly. However, a sudden rise can also be caused by other

National Hotline [phone]
to 0SF Area and International 415-558-9051 - Fax [phone]
Administrative Offices 415-558-8669 - Web Site http://www.projinf org
Copyright @ San Francisco Project Inform 1997 - 1965 Market St., Suite 220, San Francisco, CA 94103