The viral load test measures the amount of HIV virus in your blood. It is the preferred way to monitor how HIV treatment is working.
There are different techniques for doing this:
The PCR (polymerase chain reaction) method uses an enzyme to multiply the HIV in the blood sample. Then a chemical reaction marks the virus. The markers are measured and used to calculate the amount of virus. This is the most widely used viral load test.
The bDNA (branched DNA) method combines a material that gives off light with the sample. This material connects with the HIV particles. The amount of light is measured and converted to a viral count. Bayer produces this test.
The NASBA (nucleic acid sequence based amplification) method amplifies viral proteins to derive a count. It is manufactured by bioMerieux.
Different test methods may give different results for the same sample. Because the tests are different, you should stick with the same kind of test to measure your viral load over time.
People who are recently infected (acute HIV infection, see fact sheet 103) have very high initial viral loads. Usually after a few months, their viral loads reach a “baseline” level.
Viral loads are usually reported as copies of HIV in one milliliter of blood. The tests count up to about 1 million copies, and are always being improved to be more sensitive. The first bDNA test measured down to 10,000 copies. Current tests detect as few as 20 copies. Ultra sensitive tests for research can detect less than 5 copies.
The best viral load test result is “undetectable.” This does not mean that there is no virus in your body; it just means that there is not enough for the test to find and count. With the first viral load tests, “undetectable” meant up to 9,999 copies! “Undetectable” depends on the sensitivity of the test used on your blood sample.
The first viral load tests all used frozen blood samples. Good results have been obtained using dried samples. This will reduce costs for freezers and shipping.
The test is helpful in several areas:
Repeated tests of the same blood sample can give results that vary by a factor of 3. This means that a meaningful change would be a drop to less than 1/3 or an increase to more than 3 times the previous test result. For example, a change from 200,000 to 600,000 is within the normal variability of the test. A drop from 50,000 to 10,000 would be significant. The most important change is to reach an undetectable viral load.
Viral load changes are often described as “log” changes. This refers to scientific notation, which uses powers of 10. For example, a 2-log drop is a drop of 102 or 100 times. A drop from 60,000 to 600 would be a 2-log drop.
Occasionally, patients sometimes went from undetectable to a low level (usually less than 500) and then returned to undetectable. These “blips” do not indicate virologic failure or that the virus is developing resistance.
There are no “magic” numbers for viral loads. We don’t know how long you’ll stay healthy with any particular viral load. All we know so far is that lower is better and seems to mean a longer, healthier life. US and World Health Organization treatment guidelines strongly recommend that all HIV-positive people be offered treatment.
People on antiretroviral treatment with undetectable viral loads are extremely unlikely to pass the virus on to others.