What is viral load?
Viral load is the amount of HIV in the blood.
The more HIV you have in your blood, the faster your CD4 cell count will fall, and the greater your risk of developing symptoms of HIV infection or AIDS-defining illnesses.
People taking anti-HIV drugs normally see an increase in their CD4 cell count as viral load falls. If you’re taking treatment, monitoring your viral load gives an indication of how well your treatment is working.
What is the viral-load test?
Viral-load tests estimate the number of HIV particles circulating in the liquid, or plasma, part of the blood. They do this by looking for HIV’s genes, which are called HIV RNA.
The result of a viral-load test is described as the number of copies of HIV RNA per millilitre. For example, a viral load of two hundred would be written as 200 copies/ml. But your doctor is likely to describe your viral load using just the number and in this resource viral load is just given as a number.
There are several different viral-load tests, or assays, in use at the moment. These tests use different techniques to measure the number of HIV particles, but the tests are equally reliable at showing whether your viral load is low, medium, or high. The PCR (polymerase chain reaction) assay is the one most commonly used in the UK.
Viral-load tests used in this country are equally accurate at measuring types of HIV (called subtypes) found in other parts of the world, for example Africa and Asia.
It’s now usual to use what are called ultra-sensitive viral-load tests. These are able to detect viral load as low as 40 or 50 copies/ml (depending on which test is being used). If your viral load is below 40 or 50, it is said to be ‘undetectable’ and, for most people, getting an undetectable viral load is one of the key goals of anti-HIV treatment. It's important to remember however, having an undetectable viral load doesn't mean that you have been cured of HIV. This is considered in a lot more detail in the HIV treatment section.
Understanding your viral-load results
For somebody not on HIV treatment, a viral load above 100,000 is considered high, and one below 10,000 is considered low.
But your viral load can seem to fluctuate quite widely from one test to another if you are not on treatment, even though this has no implications for your health.
Indeed, doctors have looked at viral-load changes in people not on anti-HIV treatment and have found that two separate tests on the same sample of blood can give widely different results. So you shouldn’t get too worried if your viral load increases from 5,000 to 15,000 when you’re not on treatment. Even an increase from 50,000 to 100,000 isn’t necessarily that important if you’re not on treatment. Although it appears that your viral load might have doubled, it’s within the margin of error for the test.
Rather than attaching too much importance to a single viral-load test result, look at the trend in your viral load over time. The time of day your blood sample is taken could influence your viral load, and your viral load might temporarily increase if you’re unwell with an infection before falling back again. Similarly, some vaccinations can cause a temporary variation in your viral load.
You should, however, be concerned if your viral-load results over several months show an upward trend, or if the increase is greater than threefold. For example, an increase from 5,000 to 15,000 isn’t significant, but an increase from 5,000 to 25,000 is.
Undetectable viral load
All viral-load tests have a cut-off point below which they cannot reliably detect HIV. This is called the limit of detection, and the tests used in the UK have a lower limit of detection of either 40 or 50 copies/ml. If your viral load is below 40 or 50 it is said to be undetectable.
But just because the level of HIV is too low to be measured doesn’t mean that HIV has disappeared from your blood. It might still be present in the blood, but in amounts too low to be measured. As viral-load tests only measure levels of HIV in the blood, the viral load in other parts of your body, for example your lymph nodes or sexual fluids, might be detectable.
Why it's good to have an undetectable viral load
Having an undetectable viral load is desirable for two reasons. It means that you are at a very low risk of becoming ill because of HIV, and also that there is a very low risk that you will become resistant to your anti-HIV treatments.
HIV can only become resistant to a drug if it continues to reproduce whilst you are taking that drug. If the reproduction of HIV is kept at very low levels, the appearance of drug resistance should be delayed, hopefully indefinitely. This means that your anti-HIV drugs go on working.
Because of this, HIV doctors now stress that an aim of anti-HIV treatment should be to get HIV viral load down to undetectable levels as soon as possible, ideally within six months of starting HIV treatment.
Some people take three to six months to reach this point, others go below detection within four to twelve weeks, and others may never reach this goal.
For a discussion on whether people who have an undetectable viral load are infectious, see the section called Sex.
If your viral load is undetectable, there’s a chance that your viral load might occasionally increase above 40 or 50 (depending on which test your clinic is using) to 100 or 200 in a single test before falling back to being undetectable. These are called viral-load blips and they do not indicate that your HIV treatment is failing. Indeed, many blips seem to be due to testing 'errors' at the laboratory.
Viral load in women
Women seem to have lower viral loads than men with the same CD4 cell counts. This doesn’t have any effect on the rate of HIV disease progression and the reasons for it aren’t properly understood. It’s been suggested that women might have lower viral loads due to a superior immune response to infections; or that viral production is naturally lower in women.