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What Is Viral Load?

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.

How Is the Test Used?

The test is helpful in several areas:

  • For medical researchers, the test proved that HIV is never "latent" but is always multiplying. Many people with no symptoms of AIDS and high CD4 cell counts also had high viral loads. If the virus was latent, the test wouldn't have found any HIV in the blood.
  • The test can be used for diagnosis, because it can detect a viral load a few days after HIV infection. This is better than the standard HIV (antibody) test, which can be "negative" for 2 to 6 months after HIV infection. (Fact Sheet 102 has more information on HIV antibody testing.)
  • For prognosis, For prognosis, viral load can help predict how fast HIV disease progresses without treatment. The higher the viral load, the faster the disease progression.
  • For prevention, viral load predicts how easy it is to transmit HIV to someone else. The higher the viral load, the higher the risk of transmitting HIV.
  • For managing therapy, the test shows treatment is controlling the virus. Current guidelines (Fact Sheet 404) suggest measuring baseline (pre-treatment) viral load. Treatment is "working" if it lowers viral load by at least 90% within 8 weeks. The viral load should continue to drop to less than 50 copies within 6 months. The viral load should be measured within 2 to 8 weeks after treatment is started or changed. It should be monitored every 6 months after that for patients with good adherence who have an undetectable viral load and have been clinically stable on their treatment for 2 or 3 years. According to the guidelines, treatment failure is indicated by a confirmed viral load above 200 copies while adherent to treatment.

How Are Changes in Viral Load Measured?

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.

Viral Load "Blips"

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.

What Do The Numbers Mean?

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.

Are There Limitations With The Viral Load Test?

  • Only about 2% of the HIV in your body is in the blood. The viral load test does not measure how much HIV is in body tissues like the lymph nodes, spleen, or brain. HIV levels in lymph tissue and semen go down when blood levels go down, but not at the same time or the same rate.
  • The viral load test results can be thrown off if your body is fighting an infection, or if you have just received an immunization (like a flu shot). You should not have blood taken for a viral load test within four weeks of any infection or immunization.