AIDS: Quantitation of CD4+ lymphocytes is important in the staging and monitoring of patients infected with HIV. Throughout the course of the disease, the total T cell levels remain fairly constant despite a fall in CD4+ cell count, due to a concomitant rise in CD8+ cells. Therefore, the ratio of CD4+ to CD8+ cells is an additional important measure of disease progression. Measurement of lymphocyte subsets is done by phenotypic identification, using fluorescent-labeled monoclonal antibodies against various groups of molecules with similar "clusters of differentiation", or CD on the surface of lymphocytes. For an adequate assessment of immune status, the percentage and absolute number of CD4 cells is necessary. All measurements use pairs of monoclonal antibodies, employing the replicate CD3 values to monitor variability between samples and to differentiate T cells from other cell types. A number of biological variables should be considered when interpreting T cell counts including: exercise, circadian variation, corticosteroid therapy and other medications, smoking, alcoholic beverages and most bacterial infections. The best assessment of immune function can be made by evaluating the change between the means of several measurements.