CD4 cell count
CD4 positive T-lymphocytes (CD4 cells) are a type of white blood cell. CD4 cells are also known as “helper T cells” because they play an important part in directing the immune system to respond to infections.
HIV multiplies in these cells and destroys them. Initially, after infection with HIV, CD4 cells are replaced as rapidly as the virus destroys them. Over time, however, the body cannot continue to replace CD4 cells as needed, and the CD4 cell level in the blood falls.
The level of CD4 cells (CD4 cell count) in the blood gives an indication of which stage of HIV disease a person has reached. The normal CD4 cell count in the blood is about 800 to 1000 cells per microlitre (written as 1000/ul or 100/mm3).
A rough guide is as follows:
- 200-500ul: early stages of HIV disease; moderate immune suppression.
- Below 200/ul: significant immune suppression; high risk of opportunistic infections.
- 50/ul or less: severe immune suppression; usually all the signs of AIDS will be present.
In children, normal CD4 counts are age-dependent. Therefore CD4 counts are often expressed as a percentage of normal, rather than an absolute number.
A CD4:CD8 ratio is sometimes reported. In a normal immune system, CD4 cells are in slight excess of CD8 cells so the ratio is slightly higher than 1.
As CD4 cells are lost and some compensatory increase in CD8 cells occurs, the ratio falls to lower than 1.
In resource-poor settings, an absolute lymphocyte count may have to be used. A lymphocyte count of less than 1250 cells/mm3 gives an indication of significant immune deficiency.
The CD4 count is also influenced by other serious infections. If a person has TB, for example, the CD4 cell count may drop while the TB is active, and improve when it is treated.
A person’s first CD4 count is known as the "baseline” CD4 count.
After the baseline, a CD4 cell count is usually done every six months if you are in the earlier stages of HIV disease.
If you have more advanced disease, the CD4 cell count may be done more frequently.
The CD4 cell count may be used to help decide when to start antiretroviral treatment.
If you are having antiretroviral therapy, the CD4 cell count is usually done every three months. On highly active antiretoviral therapy the CD4 cell count can increase, typically by 100 cells/ul over the first 12 months.
The viral load refers to the level or concentration of HIV in the blood. The level of the virus in the blood (viral load) indicates how fast the virus is multiplying and also how well or how poorly the virus is being controlled by the immune system.
There are several tests that can be used to measure the viral load. These tests can measure any level from as low as 50 viruses per millilitre of blood (50 virus copies/ml) to as high as 10 million viruses per millilitre.
The higher the viral load, the more rapidly the virus is multiplying and the more quickly a person will develop AIDS.
As a rough guide:
- viral load higher than 100 000 copies/ml: rapid disease progression
- viral load between 10 000 and 100 000 copies/ml: average disease progression
- viral load lower than 10 000 copies/ml: slow disease progression.
The viral load increases if another illness such as ‘flu is present, since the other illness will divert the immune system away from control of the HIV infection. It is therefore best to avoid doing a viral load test during another illness.
A person’s first viral load is known as the "baseline” viral load.
A viral load test may be done every six or twelve months if you are in the earlier stages of HIV disease. Along with the CD4 cell count, the viral load may be used to help decide when to start antiretroviral treatment.
If you are having antiretroviral therapy, the viral load test is usually done every three months.
The viral load is extremely useful for monitoring the success or failure of treatment.
Successful treatment would usually cause at least a 10-fold fall in viral load.
If treatment is failing because the drugs are not been taken correctly or because the virus has become resistant to the drugs, the viral load will rise.
If you are on highly active antiretroviral treatment, the virus level may fall below 50 copies/ml and the result is reported as “lower than detectable”.
This does not mean, however, that there is no HIV in the blood or that the disease has been cured.
Approximate cost of HIV tests (South Africa, August 2002)
||Cost to government
||Cost in private health care
|HIV antibody ELISA test
|HIV rapid antibody test
|HIV p24 antigen test
|HIV viral load