Understanding Absolute CD4 Count and CD4 Percentage

The Relationship of Immune Cells Helps Doctor Predict Disease Outcome:

Measuring CD4 T-cells are important in helping doctors determine the stage and likely outcome of HIV infection. While they are no longer used to determine when HIV therapy should be started—therapy shouldalways be started at the time diagnosis—they can provide treaters a strong picture of how the patient is and what can be done to improve his or her long-term outcomes. 

What are 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. There are two types of T-cells we commonly monitor during the course of an HIV infection are:  CD4 T-cells which are T-lymphocytes that have molecules called CD4 on their surface. They start the immune response by signalling other defensive immune cells to infectious pathogens such as bacteria and 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. What is 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 stage of infection, while people with AIDS can have 200 or less CD4 T-cells per cubic millimeter.

What is the CD4 Percentage?
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.

Why These Tests Are Important:
The absolute CD4 count and CD4 percentage give your doctor a snapshot of the health of your immune system, as well as the prognosis of your disease moving forward. In its simplest form, we know that CD4 counts of less than 200 place a person with HIV at risk of opportunistic infections. consideration the total number of lymphocytes and is somewhat more predictive of a person's immediate and long-term health. If, for example, the CD4 count is higher only because the total lymphocyte count is higher, we would still have cause of concern about the patient's health. If, on the other hand, the CD4 is lower as a result of a lower lymphocyte count, we could interpret outcomes differently.  Other Important CD4 Tests One way to gain better insights of a patient's immune function is to additionally examine the patient'sCD4/CD8 percentage, which assesses the number of CD4 T-cells compared to the number of CD8 T-cells. With the test we 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 us see this.

Sources:
Gebo, K. "Absolute CD4 versus CD4 Percentage for Predicting the Risk of Opportunistic Illness in HIV Infection." Journal of Acquired Immune Deficiency Syndromes. August 24, 2004. Moore, D.M. "CD4 Percentage is an Independent Predictor of Survival in Patients Starting Antiretroviral Therapy with Absolute Cell Counts between 200 and 350 cells/cubic millimeter."HIV Medicine. September 13, 2006.

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