2. 8 What is a viral load test?
Viral load tests measure the amount of HIV in a blood sample. This is reported as the number of copies of HIV per millilitre (copies/mL). Information about units of measure for blood tests.
When viral load is very low on treatment (less than 200 copies/mL), the risk of transmission during sex becomes zero. This is even if you don’t use a condom. See: Undetectable = Untransmittable (U=U).
Viral load is a good marker of how much HIV is in your body. This is even though less than 1% of HIV is in your blood. Blood tests are also easier than trying to test lymph nodes or other body tissues.
Viral load tests can be used on other samples. For example, for genital fluid, semen or spinal fluid.
Types of viral load test
- PCR. The main test uses polymerase chain reaction (written as PCR RNA). This is the most widely used type of test.
Two older tests, are now used more rarely for HIV (if at all).
- bDNA. Branched DNA is now used less often for HIV
- 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. Each stages doubles the amount of virus to make it easier to count. But the individual results only provide a rough estimate.
Viral load tests can have a 3-fold margin of error. So, if a viral load result of 30,000, the real result could be anywhere between 10,000 and 90,000 copies/mL.
- Each viral load test has a sensitivity cut-off. This is the lowest level of HIV that the test can measure. It is a measure of how sensitive the test is.
- The main cut-off limit for viral load tests is 50 copies/mL. Many tests now have a lower cut-off of 20 copies/mL.
- Special tests can measure down to 5 copies/mL or even to 1 copy/mL. These are only used in research.
Using viral load and CD4 count to monitor HIV
Used together, CD4 counts and viral load results show the risk to your health from HIV. They also show how well HIV treatment (ART) is working.
Viral load test results are the main way to monitor ART.
- On ART, viral load is more important than the CD4 count. This checks that viral load gets below 50 copies/mL. Otherwise, drug resistance can develop.
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 ART. Less expensive tests are being developed.
Even if you do not have viral load tests, it helps to understand how CD4 count and viral load are related.
Last updated: 1 August 2021.