Viral load is the term used to describe the amount of HIV in your blood. The more HIV there is in your blood (and therefore the higher your viral load), then the faster your CD4 cell count will fall, and the greater your risk of becoming ill.

Viral load tests measure the amount of HIV’s genetic material in your blood. The results of a viral load test are described as the number of copies of HIV RNA in a millilitre of blood. But your doctor will normally just talk about your viral load as a number. For example, a viral load of 10,000 would be considered low; 100,000 would be considered high.

If you are not taking HIV treatment, your viral load will be monitored at your regular HIV clinic appointments. This is because the level of your viral load can provide important information about the way that HIV might affect your health if it is left untreated. Amongst people with the same CD4 cell count, those with a high viral load tend to lose CD4 cells and become ill faster.

When you’re not taking HIV treatment, the level of your viral load can fluctuate between tests. Often increases in your viral load are nothing to worry about. Even a doubling in your viral load might not be significant.

Vaccinations, such as a flu injection, and infections can cause a temporary increase in your viral load. Your doctor should take this into consideration when looking at your results.

Like your CD4 count, it’s best to look at the trend in your viral load over time. When viral load results over several months show a continuing increase, or when the increase is greater than threefold, there may be a cause for concern.

For example, an increase from 5000 to 15,000 shouldn’t cause you to worry when you are not on treatment. A rise from 50,000 to 100,000 may not be significant – it’s still within the margins of error for the test. But a rise from 5000 to 25,000 is significant. It represents a fivefold increase in the amount of virus in your blood since your last viral load test.

Your doctor will probably want to confirm this trend with a repeat test.

When you’re thinking about starting HIV treatment, one of the factors your doctor will discuss with you is your viral load. As mentioned earlier, it is recommended that people start HIV treatment when their CD4 cell count is around 350. One group of people who are particularly recommended to start treatment at this time are those with a viral load of 100,000 or more.

Your viral load should start to fall once you start HIV treatment. The aim of HIV treatment is an undetectable viral load. Your viral load should have fallen to undetectable levels within three to six months of starting HIV treatment.

Once you have started treatment, your doctor will check your viral load a month later, and again twelve weeks after your treatment started. Then your viral load will be monitored every three to six months, along with your CD4 count.

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’. And 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 body. It might still be present in the blood, but in amounts too low to be measured. Viral load tests only measure levels of HIV in the blood, which may be different from the viral load in other parts of your body, for example in your gut or your lymph nodes.

Having an undetectable viral load is desirable for a number of reasons.

First of all, it means that you have a lower risk of becoming ill because of HIV. It also reduces your risk of developing some other serious illnesses as well. There is now good evidence that HIV itself can increase the risk of cardiovascular disease (illnesses like heart disease and stroke).

Secondly, having an undetectable viral load means that the risk of HIV becoming resistant to the anti-HIV drugs you are taking is very small.

Finally, having an undetectable viral load reduces the risk of you passing on HIV to someone else. This is discussed in more detail later in this booklet.

If your viral load hasn’t fallen to undetectable levels within three to six months of starting HIV treatment, then your doctor will discuss with you changing your anti-HIV drugs.

If you are taking HIV treatment and your viral load falls to undetectable, but then becomes detectable again in later tests, you will probably need to change your HIV treatment.

Having a detectable viral load when you are taking HIV treatment can mean that your HIV will become resistant not only to the anti-HIV drugs you are taking, but also to other similar drugs as well.

Before you start HIV treatment or change your anti-HIV drugs because of a detectable viral load you should have a resistance test.

These are blood tests that can tell which anti-HIV drugs have the best chance of working for you.

The results of resistance tests are most accurate if your viral load is 200 or above.

For more information, see the NAM booklet Adherence and Resistance.

People with an undetectable viral load sometimes experience what are called ‘blips’ in their viral load. Their viral load increases from undetectable to a low but detectable level before becoming undetectable again on the next test.

Viral load blips do not necessarily show that your HIV treatment is no longer working.

There are a number of theories about the reasons for blips. These include ‘errors’ at the testing lab or having an infection like a cold or the flu. One study found that blips were more likely to happen in winter, which might support the theory that infections are a possible cause.

If your viral load is detectable on two consecutive tests, then it’s a good idea to discuss with your doctor possible causes and whether you need to change your treatment.

If you have a high viral load in your blood, then you might also have a high viral load in your semen or vaginal fluid. People with high viral loads are more infectious to other people.

As well as reducing viral load in your blood, HIV treatment also reduces viral load in semen and vaginal fluid.

There’s been a lot of debate recently about the infectiousness of people who are taking HIV treatment and who have an undetectable viral load in their blood.

This is a controversial subject and new information is becoming available all the time. You can keep up with the latest developments by signing up for NAM's weekly email bulletin, HIV Weekly.

HIV treatment is very effective at preventing mother-to-child transmission of HIV. If you are pregnant, or thinking about becoming pregnant, then it’s a good idea to discuss your HIV treatment 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.