CD4 cell counts are used to assess the impact of treatment on the immune system. A rise in CD4 cell count indicates that the immune system is rebuilding. As this occurs, health will also slowly improve.

If the CD4 cell count falls while on treatment, this suggests that HIV is continuing to damage the immune system. Monitoring for drug resistance or drug levels in the blood, and alterations to the anti-HIV drug combination may help.

The CD4 cell count is seen as a good marker of a person's response to treatment. Initial rises in CD4 cell counts may be accompanied by transient 'immune reconstitution illnesses' as the body's ability to fight infection improves. It is now recognised that a sustained rise in CD4 cell count signals that the body is better able to fight infection.

Doubts over the CD4 cell count as a marker

During the early days of anti-HIV treatment, there was some doubt about the ability of the CD4 cell count to indicate the effectiveness of treatment. The most famous evidence against CD4 cell counts as a surrogate marker for HIV drugs came from the Concorde trial. This showed an early rise in CD4 cell counts of about 30 cells/mm³ and a consistent difference of about 30 cells/mm³ for people taking AZT (zidovudine, Retrovir) compared to those not taking AZT from the beginning of the trial. However, over three years there was no difference in the rates of progression to AIDS or survival between the two treatment groups.

Analyses of various trials have, however, suggested that sustained changes in CD4 cell counts predict the eventual health improvements. Now that potent treatments produce much greater CD4 cell count increases that those seen in the Concorde study, the CD4 cell count is probably a more reliable way to predict clinical changes. See Anti-HIV therapy: Effectiveness of HIV therapy for more details.