  The concentration of CD4 T-lymphocytes (CD4 count), in a person's plasma is
widely used to decide when to start HIV-positive people on anti-retroviral
therapy (ART) and to predict the impact of ART on the future course of HIV and
tuberculosis (TB). However, CD4 cell-counts vary widely within and among
populations and depend on many factors besides HIV-infection. The way in which
CD4 counts decline over the course of HIV infection is neither well understood
nor widely agreed. We review what is known about CD4 counts in relation to HIV
and TB and discuss areas in which more research is needed to build a consensus
on how to interpret and use CD4 counts in clinical practice and to develop a
better understanding of the dynamics and control of HIV and HIV-related TB.
