The CD4 T-cell count is not an HIV test, but rather a procedure where the number of CD4 T-cells in the blood is determined.
A CD4 count does not check for the presence of HIV. It is used to monitor immune system function in HIV-positive people. Declining CD4 T-cell counts are considered to be a marker of progression of HIV infection. In HIV-positive people, AIDS is officially diagnosed when the count drops below 200 cells/μL or when certain opportunistic infections occur. This use of a CD4 count as an AIDS criterion was introduced in 1992; the value of 200 was chosen because it corresponded with a greatly increased likelihood of opportunistic infection. Lower CD4 counts in people with AIDS are indicators that prophylaxis against certain types of opportunistic infections should be instituted.
Low CD4 T-cell counts are associated with a variety of conditions, including many viral infections, bacterial infections, parasitic infections, sepsis, tuberculosis, coccidioidomycosis, burns, trauma, intravenous injections of foreign proteins, malnutrition, over-exercising, pregnancy, normal daily variation, psychological stress, and social isolation.
This test is also used occasionally to estimate immune system function for people whose CD4 T cells are impaired for reasons other than HIV infection, which include several blood diseases, several genetic disorders, and the side effects of many chemotherapy drugs.
In general, the lower the number of T cells the lower the immune system's function will be. Normal CD4 counts are between 500 and 1500 CD4+ T cells/microliter, and the counts may fluctuate in healthy people, depending on recent infection status, nutrition, exercise, and other factors. Women tend to have somewhat lower counts than men.