The CD4 count test, also known as the T-cell count test, gives an indication of the number of CD4 cells in your bloodstream. These cells help fight disease, so the more CD4 cells you have, the stronger your immune system is. After living with HIV for a while (if you don't take medications), the number of CD4 cells you have will usually fall. This is a sign that your immune system is being weakened.
A normal CD4 count for someone without HIV is usually between 500 and 1,600. Experts generally agree that it's best for your health to begin taking HIV medications before your CD4 count hits 500.