Treatment training for advocatesA CD4 count is the number of CD4 cells in a cubic millimetre (cells/mm3).
This is sometimes written as cells in a microlitre (cells/µL) of blood or as cells x 106/L. Information about units of measure for blood tests.
A CD4 count is sometimes called an absolute CD4 count, because it counts the actual number of cells in a blood sample.
A single CD4 count doesn’t tell you very much. You really need to get several results over time to see the trend.
CD4 counts fluctuate. They go up and down during the day. For example, CD4 counts are lower when you first wake up and higher later in the day. They are higher immediately after exercise – even running up and down stairs. They can also be affect by diet, whether you have other infections or even if there were just more or fewer cells in that sample of blood. None of these variations mean that your immune system is stronger or weaker.
This is why the trend of CD4 results is more important than any single count. The trend looks at the average of several results.
When you have several results you can see whether the trend is going up or down and how quickly it is changing – or whether it is generally stable.
Looking at a CD4 count trend
Each point on the dotted line shows an individual (absolute) CD4 count.
The solid line shows the average of these results. In this example it shows that the trend is for the CD4 count to fall over time.
Because CD4 counts vary, an unexpectedly high or low count should be checked with a second test, if possible.
CD4 percentage (CD4%)
The CD4 percentage (CD4%) is the percentage of white blood cells (lymphocytes) that are CD4 cells.
The immune system contains lots of different cells. The two main types of lymphocytes are T cells and B cells. CD4 cells are a type of T cell. So the CD4% looks at the CD4 count in relation to other immune cells.
CD4% is sometimes a more stable indication of whether there has been a change in the immune system. An unexpected drop in CD4 count when the percentage hasn’t changed indicates this drop is not clinically significant.
- A CD4% of 12-15% is about the same as a count of under 200 cells/mm3.
- A CD4% of 29% is about the same as a count of over 500 cells/mm3, but there is a wider range for higher values.
- An HIV-negative person has a CD4 percentage of about 40%.
CD4 counts are not used for children under 12 years old, who are monitored by CD4 percentage. This is because you are born with very high CD4 levels (several thousand cells/mm3 in babies).