Back to contents

CD4 cell counts

Michael Carter, Greta Hughson
Published: 25 March 2014

T-cells (or T-lymphocytes) are white blood cells that play important roles in the immune system. There are two main types of T-cells. One type has molecules called CD4 on its surface; these 'helper' cells organise the immune system’s response to bacteria, fungi and viruses. The other T-cells, which have a molecule called CD8, destroy cells that are infected and produce antiviral substances.

HIV is able to attach itself to the CD4 molecule, allowing the virus to enter and infect these cells. Even while a person with HIV feels well and has no symptoms, billions of CD4 cells are infected by HIV and are destroyed each day, and billions more CD4 cells are produced to replace them.

CD4 cell counts

Doctors use a test that 'counts' the number of CD4 cells in a cubic millimetre of blood (a very small sample). A normal CD4 count in a healthy, HIV-negative adult can vary but is usually between 500 and 1500 CD4 cells/mm3 (though it may be lower in some people). Your doctor will normally just talk about your CD4 cell count as a number.

Most people with HIV find that their CD4 count falls over time. This often happens at a variable rate, so the count can still be quite stable for long periods. It is useful to have your CD4 count measured regularly for two reasons:

In the current UK guidelines, it is recommended that you start HIV treatment when your CD4 cell count is around 350. Starting treatment at this time (rather than waiting until your CD4 cell count is lower) reduces your risk of HIV-related illnesses and some other serious illnesses as well.

In some circumstances, it is recommended that you start treatment when your CD4 count is higher. These include:

  • if you also have hepatitis B or hepatitis C
  • if you have heart problems or kidney disease
  • if you are having treatment for cancer
  • if you are over 50
  • if you want to start treatment, because you are concerned about the risk of passing on HIV to someone else (HIV treatment reduces the risk of HIV being passed on)
  • if you are ill because of HIV
  • in some cases, if you acquired HIV very recently.

However a major trial announced its results in May 2015 and this is likely to result in BHIVA's  guidelines changing. This study proved that there are advantages to starting treatment at high CD4 cell counts. We now know that starting HIV treatment earlier reduces the risk of serious illnesses, AIDS, death and HIV transmission. It’s important to feel ready for the commitment of taking HIV treatment, but when you do feel ready, there’s no reason to delay.

If your CD4 count drops below 200 to 250, or you are diagnosed with HIV at a low CD4 count, you are at increased risk from serious infections and it is very important that you start HIV treatment. At this point your doctor should also offer drugs to try to prevent such infections, such as cotrimoxazole (Septrin) for PCP pneumonia.

One effect of anti-HIV drugs may be to improve the state of your immune system. This is roughly reflected in an increase in your CD4 count. Evidence suggests that the cells' ability to fight infections is also improved. For example, people taking anti-HIV drugs who find their CD4 count rises and stays above 250 may no longer need to take additional treatments like cotrimoxazole.

Monitoring the changes in your CD4 count while you are taking anti-HIV drugs can help you and your doctor to decide whether your treatment is working, or whether it is time to try different treatment options. A fall in CD4 count would be a sign that your treatment is not working and you may need to consider switching to another combination. However, the CD4 count isn't the only consideration when making these decisions; you should also take account of your viral load results, how well you feel, whether you have any symptoms, which treatments you've used before and therefore which options remain.

In some cases, in order to help understand changes in your CD4 count, your doctor may also assess what proportion of all lymphocytes are CD4 cells. This is called the CD4 percentage. In HIV-negative people a normal result is around 40%. A CD4 percentage which falls below about 15% is understood to reflect a risk of serious infections.

Understanding the results

CD4 cell counts vary from person to person, and various factors can affect a test result on any given day, so it's important to look for the trend in your CD4 count over time, rather than being concerned about small fluctuations.

Factors other than HIV can affect your CD4 count including infections, time of day, smoking, stress and which laboratory tests the blood sample. Women living with HIV may find their CD4 count varies at different points in the menstrual cycle, and taking oral contraceptives can also affect the CD4 count.

Only about 2% of the body's CD4 cells are in the blood; the rest are in tissues such as lymph nodes. Changes in your CD4 cell count (which looks only at the blood) may reflect the movement of cells into and out of the blood, rather than changes in the total number of CD4 cells in your body.

Doctors will normally suggest measuring your CD4 count every three to six months if you have a relatively high count, have no symptoms and are not taking anti-HIV drugs. They may suggest more frequent counts if you are facing decisions such as starting treatment, if you develop HIV-related symptoms, or if the decline in CD4 cells seems to be speeding up.

Once you're taking stable HIV treatment, then it's likely that your CD4 cell count will be checked every three months or so during routine clinic visits. If your treatment has been working well for a long time, then there may be longer intervals between your CD4 cell tests. Some doctors think once a year could be sufficient.

For more information on CD4 counts, you may find our booklet CD4, viral load & other tests useful. You can read this on our website (visit or contact us to request a copy by emailing or calling 020 7837 6988.

Contact NAM to find out more about the scientific research and information used to produce this factsheet.

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.