Understanding Absolute CD4 Count and CD4 Percentage

Measuring CD4 T-cells is important in helping healthcare providers determine the stage and likely outcome of HIV infection. While they are no longer used to determine when HIV therapy should be started—therapy should always be started at the time diagnosis—they can provide practitioners a clear idea of how the person is and what can be done to improve their long-term health.

Understanding CD4 and CD8 T-Cells

To begin, lymphocytes are a type of white blood cell that helps the body fight infection. There are two main types of lymphocytes: B-lymphocytes and T-lymphocytes. Of these, there are two types of T-cells doctors monitor during the course of an HIV infection:

• CD4 T-cells are T-lymphocytes that have molecules called CD4 on their surface. They start the immune response by signaling other defensive immune cells to infectious pathogens such as bacteria and viruses.
• CD8 T-cells which have molecules on their surface called CD8. These are the "killer" T-lymphocytes tasked with destroying infected cells and producing antiviral substances that fight off infectious organisms.

The Value of the Absolute CD4 Count

The absolute CD4 count is a measurement of how many functional CD4 T-cells are circulating in your blood. The lower the absolute CD4 count, the weaker the immune response.

The absolute CD4 count is measured by a simple blood test, the results of which are reported as the number of CD4 cells per cubic millimeter of blood.

HIV-negative people typically have absolute CD4 counts between 600 and 1200 CD4 cells per cubic millimeter. By contrast, immune-suppressed individuals with HIV have counts that are typically less than 500, depending on the stage of infection, while people with advanced HIV can have 200 or fewer CD4 T-cells per cubic millimeter.

CD4 cell counts as well as viral load tests offer a more accurate representation of HIV activity and disease progression.

What the CD4 Percentage Tells Us

The CD4 percentage represents the percentage of total lymphocytes that are CD4 cells and is measured using the same blood test as that for the absolute CD4 count.

Typically, HIV-negative people will have a CD4 percentage of about 40%, while HIV-infected people's CD4 percentage can be as low as 25% or less. Clearly, the higher the percentage, the more robust the immune response.

If your CD4 count is lower than you expect, the CD4 percentage can put it into better perspective by telling this whether this is an actual change or just a fluctuation.

CD4/CD8 Ratio as a Snapshot of Immune Health

One way to gain better insights into a patient's immune function is to additionally examine the patient's CD4/CD8 percentage, which assesses the number of CD4 T-cells compared to the number of CD8 T-cells. With the test, your doctor can see if the disease is progressing by tracking the depletion of "killer" T-cells in blood samples.

Typically, in advancing disease, as the immune system becomes exhausted, it is less able to produce T-cells to defend itself. The CD4/CD8 percentage helps healthcare practitioners see this.

In recent years, greater emphasis has been placed on the CD4/CD8 dynamic in the aging HIV population.

Why These Tests Are Important

The absolute CD4 count and CD4 percentage give your healthcare provider a snapshot of the health of your immune system, as well as the prognosis of your disease moving forward.

If, for example, the CD4 count is higher only because the total lymphocyte count is higher, your healthcare practitioner would still have a cause of concern about your health. If, on the other hand, the CD4 is lower due to a lower lymphocyte count, your doctor could interpret outcomes differently.