Viral load

The result is measured in copies (of the virus) per millilitre (ml) of blood and it can range from over 1,000,000/ml to fewer than 50/ml - which is classed as ‘undetectable’, meaning blood tests cannot detect the virus. Your doctor is likely to describe your viral load using just the number.

For somebody not on HIV treatment, a viral load above 100,000 is considered high, and one below 10,000 is considered low. But everyone’s viral load is different and it is not unusual for one person’s viral load to fluctuate quite widely. It might temporarily increase if you’re unwell with an infection, for example, and the time of day your blood sample is taken could also influence your viral load. So rather than attaching too much importance to a single test result, look at the trend in your viral load over time and discuss any concerns with your doctor.

If you have a high viral load then HIV is more likely to be transmitted to someone else if you don't practise safer sex than it is if you have a low viral load. Having an undetectable viral load can make it much less likely that HIV is passed on but this is not guaranteed – some people have become infected by people who had undetectable viral load. This is because Undetectable means just that: it cannot be detected by tests, but the virus is still there.

If you are not on medication then your doctor will compare your viral load with your CD4 count to see how well your body is fighting the virus. If you are on medication then the same tests will be carried out to decide how well the treatment is working and whether the dosage or the type of drugs need to be altered.

For further information on CD4 count and viral load, see the What's Happening? page in our Simple Science section.