count below 200/mL is a criterion for AIDS.

T-cell Test (CD4+ Count)

The T-cell test is usually reported as the number of T-cells, also known as CD4+ cells, in one milliliter (mL) of blood. Healthy, uninfected people have between 500 and 1600 CD4+ cells per mL of blood. According to the Centers for Disease Control and Prevention (CDC), a CD4+ cell count below 200/mL is a criterion for AIDS.

T-cell tests also are used to measure of a different kind of immune system cell, known as a CD8+ cell. Sometimes, the ratio of CD4+ to CD8+ cells is used to monitor HIV infection, since the ratio drops so dramatically in patients with HIV/AIDS. In healthy people, there are normally about 1 to 2 CD4+ cells for every CD8+ cell. In patients with HIV/AIDS, the ratio is reversed and there are many CD8+ cells for every CD4+ cell.

T-cell levels depend on a variety of factors (e.g., stress, fatigue) and can change considerably throughout the day. If possible, patients should have the test at the same time of day each time.

The number of CD4+ and CD8+ cells increases during an infection. So the results of T-cell tests can vary depending on whether the patient has had any recent infections.