1 Immune system and CD4 count

1.9 Interpreting CD4 results: CD4 count and CD4 percentage

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A CD4 count is the number of CD4 cells in a cubic millimetre (cells/mm³) or microlitre (cells/µL) of blood.

In scientific papers this is sometimes written as cells x 10⁶/L.

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 can go up and down depending on the time of day, whether you have eaten a fatty meal, if you have just been running up and down stairs, whether you have other infections or even if there were just more or fewer cells in that sample of blood.

This is why 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 total lymphocytes that are CD4 cells.

CD4% is sometimes a more stable indication of whether there has been a change in the immune system.

• 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).

Table: CD% and CD4 count for babies and children