‘Viral load’ is the term used to describe the amount of the HIV virus present in your bloodstream. Knowing how much HIV is present is an important indicator of how much your immune system is at risk of damage, how well your treatments are working, or whether you should consider starting or changing treatments.

A viral load test is a simple blood test. The result of a test is given as the number of viral copies of HIV per millilitre of blood. A ‘copy’ is what HIV produces every time it grows inside a cell: the more copies, the more virus.

The amount of virus in your blood may range from a very small number of copies in your blood (below 50 copies per millilitre of blood) to levels in the thousands, hundreds of thousands, or even millions. In some Australian states and territories the tests can measure down to 40 copies per millilitre of blood.

Understanding Your Viral Load results

Viral load is perhaps the simplest and easiest HIV test to understand as it is simply a count of the virus expressed in number per millilitre. When you first have your viral load tested, you will usually have two tests several weeks apart, which gives a result known as your ‘baseline’, and which can be used to compare changes over time. These results can be a useful guide if you are considering treatment:

a) ‘Undetectable’ viral load?

One result you can get back from a viral load test result is ‘undetectable’. Undetectable viral load does not mean that you have ‘cleared’ the virus from your body. It means that HIV is present, but in very small amounts (below the capacity of current commercial tests to accurately measure: that is, below 40 to 50 copies per millilitre of blood). Virus at such levels is replicating so slowly that little, if any, damage will be happening to your CD4 cells and immune system.

Viral load tests are slowly becoming more sensitive. However, special laboratory tests are able to detect HIV in even minute quantities. HIV infects cells which may remain active in lymph glands, known as resting cells, and has also been shown to infect small amounts of other types of cells. To totally cure or eradicate HIV, you would need to also eradicate the virus in these ‘resting cells’.

b) Detectable viral load results

You will often be told that your viral load result is ‘high’ (i.e. more than 100,000 copies per ml), ‘moderate’ (i.e. 10,000 to 100,000 copies per ml), or ‘low’ (i.e. less than 10,000 copies per ml). On their own your viral load results are no cause for alarm. For example, a high viral load result does not mean you are going to be sick tomorrow. Or a low result after your results have been undetectable for some time does not mean you have suddenly “failed” in any way.

Your viral load level is a rough guide to the likelihood of future damage to the immune system. So if your viral load is high it means that future damage is more likely. If it is low or undetectable it means future damage is less likely.

In order to make decisions about treatments, the viral load has to be read in conjunction with the CD4 cell count.

Viral load and the pattern over time is important

You may be asked to have viral load tests fairly frequently so you and your doctor can keep track of changes over time, or of any sudden variations between test results. In fact, an unexplained and significant upward trend in viral load over a number of tests may be a stronger indicator that you should consider changing or starting treatments than a single, detectable result in isolation. The magnitude of the change is important. For example, a rise of viral load from 5,000 to 6,000 does not necessarily indicate there is a problem. But a rise from 5,000 to 50,000 may suggest that the virus is beginning to replicate very rapidly for some reason, and that you should take this into consideration when thinking about starting or changing treatments

Other factors can affect viral load

No one viral load result should be considered alone. It’s the pattern over time which counts. There are a number of reasons why you may experience a sudden temporary rise, or ‘spike’ in your viral load.

These include:

• another infection (e.g. the flu, hepatitis, or another sexually transmitted infection such as gonorrhoea or syphilis); and
• recent vaccination (eg. routine travel-related vaccinations or hepatitis A or B vaccination), which can stimulate your immune system for a brief period causing only a temporary rise
  
  

Viral load and “infectiousness”

Viral load tests tell you how much virus is in your blood. But HIV is also present in other body fluids, including semen, vaginal fluids and CSF (cerebrospinal fluid)—the fluid which protects your brain. The level of virus in your blood is often different to the amounts in other body fluids. This difference can be caused by a number of factors. For this reason, blood viral load tests should not be used to judge the likelihood of HIV transmission. It is possible to have low or undetectable blood viral load, but higher levels in semen or vaginal fluids.

While research suggests an undetectable viral load reduces the risk of HIV transmission, an undetectable viral load has not yet been proven to completely eliminate the risk of transmitting the virus. The use of viral load in prevention is not a substitute for safe sex.

Viral load over time without treatment

The typical picture of viral load over time is given at left. Soon after initial infection there is a peak in viral load until the immune system responds. Then, for a period of years the immune system and the virus are engaged in a balancing act, though in nearly all cases the immune system is still being weakened. Throughout this period, the virus is still active. Eventually, the virus may overwhelm the immune system.

If you are not taking antiviral treatments, you will probably be advised to have a viral load test each time you have a CD4 or T-cell count. Comparing these results with your baseline viral load will alert you and your doctor to any changes in your immune system or your health.

Ask your doctor to explain the meaning of any changes in your viral load. It is quite common for viral load to change a bit with each test. What is important is the magnitude of the change. Doctors use a mathematical scale called a logarithmic (“log”) scale to measure the significance of any changes. It is only changes of a significant magnitude that are considered important.