opportunistic infections and cancers. Shortly thereafter a novel retrovirus, now called HIV, was discovered as the cause of AIDS. Laboratory measures were later determined to correspond with severity of disease and the rate of clinical progression. The two measures most commonly used to stage HIV infection are the absolute number of CD4 cells per cubic millimeter (mm3) of blood and the quantity of HIV RNA molecules per milliliter (mL) of plasma, also known as viral load.
The CD4 count indicates the degree of immune depletion or immunodeficiency. The remaining immunologic reserve, reflected in the CD4 count, is highly predictive of near-term risk of opportunistic diseases and mortality. The viral load, on the other hand, indicates the production rate of HIV virions and expected rate of subsequent CD4 cell destruction. Although CD4 count is the central means of staging the disease in HIV infection, the viral load is the most accurate means of following the success or failure of antiretroviral therapy. Researchers and clinicians have used both of these tests to determine prognosis. In the absence of combination antiretroviral therapy, Mellors et al. (1996) demonstrated that the combination of the viral load and CD4 count could be used together to determine prognosis and survival and to some extent as indicators guiding the optimal point to recommend the initiation of antiretroviral therapy (see Figure 3-1).