Viral load

What is viral load?

Viral load is the term used to describe the amount of HIV in a body fluid. Your clinic will measure the viral load in your blood.

When you first become infected with HIV, your viral load will be very high. After four to six months, as your immune system starts to respond to HIV, your viral load will drop and then stabilise, increasing slowly during the time before symptoms develop.

The level at which viral load stabilises (the ‘set point’) is considered an important indicator of how fast HIV might progress – the higher this level is, the faster someone may become ill because of HIV.

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 HIV-related illnesses. Your HIV viral load will also increase much faster if you start becoming ill because of HIV.

You can find out more about how and when your viral load will be monitored in Frequency of testing.

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 that is how we talk about it here.

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. These can measure all subtypes of HIV equally accurately.

It’s now usual to use what are called ultra-sensitive viral load tests. These are able to detect viral load as low as 20, 40 or 50 copies/ml (depending on which test is being used). If your viral load is below 50, it is said to be ‘undetectable’.

For most people, reaching an undetectable viral load is one of the key goals of 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 HIV treatment.

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. Like your CD4 cell count, your viral load results may fluctuate quite widely from one test to another. This won’t necessarily have any effect on your health

Indeed, doctors have looked at viral load changes in people not on 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 5000 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.

Once you’re on treatment your viral load should stabilise. However, 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 (such as a flu vaccination) can cause a temporary variation in your viral load.

It may be more significant if your viral load results over several months show an upward trend, or if the increase is more than three times a previous viral load test result. For example, an increase from 5000 to 15,000 isn’t significant, but an increase from 5000 to 25,000 is. If you’re on HIV treatment, a significant increase can indicate that your treatment is not working as effectively as it should.

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. Tests used most commonly in the UK have a lower limit of detection of either 40 or 50 copies/ml, but there are some very sensitive tests that can measure below 20 copies/ml. If your viral load is below 50 it is said to be undetectable. The aim of HIV treatment is to reach an undetectable viral load.

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 higher.

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 develop resistance to your anti-HIV drugs.

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 stress that an aim of 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 will take longer, especially if they have started treatment with a high viral load. Your viral load four weeks after starting HIV treatment is a good indicator of whether it will become undetectable.

For more information on whether people who have an undetectable viral load are infectious, see Sex.

Viral load blips

If your viral load is undetectable, there’s a chance that your viral load might occasionally increase to detectable levels (above 20, 40 or 50, depending on which test your clinic is using) – to between 50 and 1000 – in a single test before falling back to being undetectable. These are called viral load ‘blips’ and they do not necessarily indicate that your HIV treatment is failing.

If you have two or more tests showing a viral load over 50, or if you have a lot of ‘blips’, your doctor will review the treatment you are on to see if it is still working effectively and talk to you about the possibility of changing treatment.

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.

If you are pregnant, or thinking about becoming pregnant, it’s a good idea to discuss your options with your doctor. If you have an undetectable viral load while you are pregnant and at the time of delivery, then the risk of you passing on HIV to your baby is very low, but it is very important that your health and viral load, and that of your baby, are monitored while you are pregnant and after you have given birth. You can find out more about this in Having a baby.

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.