The CD4 count is a measurement of how many functional (HIV-uninfected) CD4 T-cells are circulating in the blood. The lower the CD4 count, the weaker the immune system. As HIV mainly targets these cells their number declines as the infection progresses (if HIV ARV treatment is not taken). The effect of HIV on the immune system is assessed by monitoring the CD4 cell count over time. The CD4 cell count is used to anticipate the onset of opportunistic infections and is used to decide when to start treatment.
The rate of CD4 decline is proportionate to the HIV viral load. In other words, as viral load falls (with effective HIV ARV treatment) gradually CD4 count returns to baseline (i.e. increases to about the same as what it was at the time of infection). HIV Treatment Guidelines now recommend early HIV (ARV) treatment regardless of CD4 count loss. HIV (ARV) treatment keeps the virus levels low (see viral load), slowing or stopping the invasion of CD4 cells, then the CD4 count numbers can grow and survive again, making the immune system protected from further HIV replication in the body.
The CD4 count tends to be lower in the morning and higher in the evening (natural fluctuation).
The CD4 count is measured by a simple blood test and is reported as the number of CD4 cells per cubic millilitre of blood. HIV-negative people typically have CD4 counts between 600 and 1200 (range). HIV-positive people have counts that are typically less than 500 (but can be higher when taking effective ARV treatment). HIV-positive people with AIDS can have 200 CD4 cells or fewer (although AIDS-related conditions can also occur at higher CD4 counts than this as well). The CD4 count provides a general guide on level of robustness of the immune system to keep AIDS opportunistic infections away.