CD4 cells (sometimes called T-cells, or helper cells) are white blood cells that organise your immune system’s response to infections.

Your CD4 cell count is the measurement of the number of CD4 cells in a cubic millilitre of blood (a very small blood sample), not your whole body. Your doctor will probably just talk about your CD4 cell count as a number.

The CD4 cell count of a person not infected with HIV can be between 450 and 1600. But CD4 cell counts can vary a lot between people. For example, women tend to have higher CD4 cell counts than men.

Your own CD4 cell count can also vary and can go up and down in response to stress, smoking, the menstrual cycle, the contraceptive pill, recent physical activity, or even the time of day. It will also go down if you have an infection or illness.

So rather than attach too much significance to an individual test result, it makes good sense to monitor the trend in your CD4 cell count over time. It’s also best to have your CD4 count measured at the same clinic and at roughly the same time of day wherever possible. If you have an infection, such as the flu or an outbreak of herpes, it is best to delay your CD4 count until you are feeling better.

If you have a relatively high CD4 count, no symptoms and you are not taking anti-HIV drugs, your CD4 count will only be measured once every three or four months (or every six months if it is very high).

However, if your count has been falling rapidly, or you are taking part in a clinical trial, your doctor may suggest that your count is monitored more often.

The frequency with which your CD4 count is monitored after you start HIV treatment will depend on your clinic and how high your CD4 cell count is. As a general rule, you can expect to have it checked every three to six months and more frequently if you have symptoms or become unwell.

Without HIV treatment, your CD4 cell count will probably gradually decline over time.

Monitoring your CD4 cell count will help you and your doctor make important decisions about your HIV treatment and care.

If someone’s CD4 cell count falls to 350, it is usually recommended that they start taking HIV treatment. Starting HIV treatment when your CD4 cell count is around 350 has been shown to have a number of advantages compared to waiting until your CD4 cell count is lower. The immune system is more likely to recover to normal levels in people who start treatment when their CD4 cell count is around 350.

Starting treatment when your CD4 cell count is around 350 also means that you are very unlikely to become ill because of HIV. It has also been shown to reduce your risk of developing other serious illnesses as well, such as heart, kidney and liver disease as well as some cancers.

So your doctor will start talking to you about treatment at this stage.

If your CD4 cell count is 200 or below, you should seriously think about starting treatment as you have a risk of developing some serious illnesses.

You’ll also need to start taking other treatment to reduce the risk of these illnesses developing. This is called prophylaxis. You can safely stop taking prophylaxis once your CD4 cell count increases. For example, you might be given antibiotics to prevent PCP, a type of pneumonia.

For more information on starting HIV treatment, see the NAM booklets Anti-HIV Drugs and HIV Therapy.

Once you start taking HIV treatment, your CD4 cell count is likely to increase gradually. The rate at which this happens can vary a lot between individuals. In some people, it can take months or even years for their CD4 cell count to climb towards normal levels. If you started treatment with a low CD4 cell count, it’s more likely to take a long time for your CD4 cell count to climb. But it’s good to know that even quite small increases in your CD4 cell count can have big health benefits.

Your doctor will monitor your CD4 cell count, along with your viral load, every three to six months once you are on treatment.

In addition to using a test to count the number of CD4 cells you have, doctors sometimes measure the proportion of all white blood cells that are CD4 cells. This is called a CD4 cell percentage. In HIV-negative people, the CD4 cell percentage would be around 40%.

Having a CD4 cell percentage of around 14% is thought to indicate the same risk of becoming ill as would a CD4 cell count of 200.

One circumstance when your doctor might measure your CD4 cell percentage could be if there is a big variation in your CD4 cell count between one test and the next.