A viral load test is a measurement of the amount of HIV virus in your body. Viral load is usually measured in a small sample of blood. This is usually reported as the number of copies in a millilitre (copies/mL). Information about units of measure for blood tests.

Even though less than 2% of HIV is in your blood, this is a good marker of how much HIV is in your body. Blood tests are also easier than trying to test lymph nodes or other body tissues.

Viral load tests can also check the amount of HIV in other kinds of sample like genital fluid, semen or spinal fluid.

Types of viral load test

There are three main types of viral load tests:

• PCR – polymerase chain reaction (written as PCR RNA)
• bDNA – branched DNA
• NASBA – nucleic acid sequence based amplification

These tests work in slightly different ways. PCR is the most widely used viral load test. PCR technology is also used to measure levels of HIV DNA in some circumstances.

How viral load tests work

Viral load tests multiply virus found in a small sample of blood many times so that it can be counted more easily. But this means that the individual results from any one test are not very accurate.

Viral load tests can have a 3-fold margin of error. So, if your viral load result is 30,000, the real result could be anywhere between 10,000 and 90,000 copies/mL.

• Viral load tests are described by the lowest level of HIV they can measure – or how sensitive the tests are. Different tests can measure different minimum levels of HIV.
• The main cut-off limit for viral load tests is less than 50 copies/mL. Some tests can measure lower levels (ie less than 5 copies/mL) but these are only used in research.

Using viral load and CD4 count to monitor HIV

Used together, CD4 counts and viral load results will tell a doctor 95% of what they need to know about the risk to your health from HIV and how well your treatment is working.

Viral load test results can be used together with CD4 counts to monitor HIV infection. As with CD4 counts, it is important to look at the trend of viral load over several tests to get a picture of whether there is any change.

• Never make any treatment decision based on the result of one test.
• When using HIV treatment, viral load is important, not CD4 count, because not getting viral load below 50 copies/mL will limit how long treatment will work.

Viral load tests are used in many countries but are difficult to get in others.

In some countries, viral load and CD4 tests cost much more than the drugs. New research is looking at developing new tests that will be just as good but which are not so expensive or difficult to run.

Even if you do not have access to these tests, it makes a difference that you understand how CD4 count and viral load change.