A test that compares the number of CD4 cells to the number of CD8 cells. It is used to help diagnose primary (recent, early) HIV infection and guide decisions about treatment. Normally the CD4 cell count is higher than the CD8 cell count; normal values are approximately 30-60% CD4 and 10-30% CD8 depending on age. In early HIV infection the CD4 cell count decreases and the CD8 cell count increases.
The CD4:CD8 ratio is rarely less than 1.0 in HIV negative individuals, but may drop as low as 0.1 in patients with recent HIV infection or very advanced disease. There is almost always substantial recovery of this ratio, even without antiretroviral therapy, during the 2-3rd month of HIV infection, which then persists for some time. The CD4:CD8 ratio will generally gradually decline over years of HIV infection in the absence of HIV antiretroviral therapy (ART). When HIV ART is started fairly early after infection, this ratio may again rise to above 1.0 – a recovery rarely seen in patients with more advanced HIV infection who start on treatment.