There are currently three types of tests in use that measure viral load:
Only one of these products, AMPLICOR HIV-1 MONITOR Test , is currently approved by the US Food and Drug Administration, and has been used by six of the last eight pharmaceutical companies for their new drug discovery trials. One advantage of the AMPLICOR HIV-1 MONITOR Test and NucliSens HIV-I QT Test is their sensitivity. AMPLICOR HIV-1 MONITOR Test can detect as few as 50 copies of viral RNA per mL of plasma and NucliSens HIV-I QT Test as few as 80 copies/mL.
All three tests measure viral load from a blood sample. The amount of virus in the blood represents only two per cent of the total virus in the body. However, recent studies have shown that viral load can be used to accurately predict HIV disease progression. Some virus is found in the lymphatic system and other body tissues.
AMPLICOR® Products (PCR technology)
The AMPLICOR HIV-1 MONITOR can monitor the viral load in the blood through all stages of HIV infection.
In this test, the single stranded HIV RNA present in the sample is converted to DNA. This process is called reverse transcription.
The DNA strands are then amplified by Polymerase Chain Reaction (PCR), detected and measured. This test measures from 400 to 750,000 copies of virus particles by the standard method and down to 50 copies/mL by the UltraSensitive method.
With the bDNA assay, numerous probes bind to the HIV RNA The quantity of HIV-1 RNA is measured at the level found in the sample without amplification of the material.
The bDNA Version 2.0 test can detect from 500 to one million copies of HIV per sample. The version 3.0 test is in beta sites currently and is expected to reach 50 copy sensitivity. It requires a much larger sample of blood than the other two tests - 2mL (several tablespoons) compared to only 100 and 200 microlitres required for NASBA and AMPLICOR HIV-1 MONITOR respectively.
During a NASBA assay a primer is added and annealing, extension and amplification of HIV RNA takes place. Post-amplification, chemicals are added to the solution which cause a chemical reaction resulting in the emission of light (luminescence). The intensity of the luminescence is measured to calculate the viral load.
NASBA is able to detect 400 to ten million copies of virus particles per sample in the standard assay and down to 80 copies in the more sensitive version of the assay.
The techniques for testing for viral load are constantly being updated, modified and improved. Each new generation of test is designed to be more accurate and more sensitive than the previous version.
Roche Diagnostics has developed a more sensitive test, AMPLICOR HIV-1 MONITOR with the UltraSensitive Sample Preparation Method, that is able to detect to 50 copies of HIV particles. This increased sensitivity gives physicians and patients with HIV a more accurate measure of viral load, resulting in better management of the disease and allows for more effective treatment. Each individuals management can then be reviewed in light of their viral load and in conjunction with their general health, CD4 cell count, previous therapies and other factors.
Testing in Different Clinics
It is advisable to use the same clinic/laboratory and assay method for serial tests of the same individual. This will avoid variations between methods which may cause variation in results.
Physicians are also advised to adopt one procedure for collecting, storing and transporting samples. Ideally samples should be separated and frozen within six hours of collection. The Roche AMPLICOR HIV-1 Monitor test has the advantage of not requiring that the sample be frozen. It can be stored and transported in refrigerated conditions or in wet ice containers. This allows samples to be drawn at outlying clinics and easily transported to the testing laboratory.
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