The CD4 counting test is done from a blood sample of a person infected by the HIV. In this test the number of CD4 cells is counted. If that number (concentration) is abnormally low it means immunodeficiency and the patient usually must be treated with antiretroviral drugs. The other test used to follow up the infection is the viral load test. In this test the number of HIV virus is estimated from a blood sample indicating whether the treatment (control) of the disease has been successful or not.