The ratio of CD4+ to CD8+ cells is often clinically reported. This is calculated by dividing the CD4+ value by the CD8+ value. In healthy people, this ratio is between 0.9-1.9, meaning that there are about 1-2 CD4+ cells for every CD8+ cell. In people with HIV infection, this ratio drops dramatically, meaning that there are many times more CD8+ cells than CD4+ cells.
The CD4+ cell count is a key measure of the health of the immune system. The lower the count, the greater damage HIV has done. Anyone who has less than 200 CD4+ cells or a CD4+ percentage less than 14 % is considered to have AIDS.

CD4+ counts are used together with the viral load to estimate how long someone will stay healthy. Thus CD4+ counts are used to indicate when to start certain types of antiviral drug therapy.