Measurement of the CD4+ T cell count and level of plasma HIV RNA are important parts of the routine evaluation and monitoring of HIV-infected individuals. The CD4+ T cell count is a generally accepted indicator of the immunologic competence of the pt with HIV infection, and there is a close relationship between the CD4+ T cell count and the clinical manifestations of AIDS (Fig. 226-31, p. 1248, in HPIM-19). Pts with CD4+ T cell counts <200/µL are at higher risk of infection with Pneumocystis jiroveci. Once the count declines to <50/µL, pts are also at higher risk for developing CMV disease and infection with Mycobacterium avium intracellulare. Pts should have their CD4+ T cell count measured at the time of diagnosis and every 3-6 months thereafter. (Measurements may be done more frequently in pts with declining counts.) While the CD4+ T cell count provides information on the current immunologic status of the pt, the HIV RNA level predicts what will happen to the CD4+ T cell count in the near future. Measurements of plasma HIV RNA levels should be made at the time of HIV diagnosis and every 3-4 months thereafter in the untreated pt. Measurement of plasma HIV RNA is also useful in making therapeutic decisions about antiretroviral therapy (see below). Following the initiation of therapy or any change in therapy, HIV RNA levels should be monitored approximately every 4 weeks until the effectiveness of the therapeutic regimen is determined by the development of a new steady-state level of HIV RNA. During therapy, levels of HIV RNA should be monitored every 3-6 months to evaluate the continuing effectiveness of therapy.

The sensitivity of an individual's HIV virus(es) to different antiretroviral agents can be tested by either genotypic or phenotypic assays. In the hands of experts, the use of resistance testing to select a new antiretroviral regimen in pts failing their current regimen leads to a ~0.5-log greater decline in viral load compared with the efficacy of regimens selected solely on the basis of drug history. HIV resistance testing may also be of value in selecting an initial treatment regimen in geographic areas with a high prevalence of baseline resistance.