As in adults, the two key tests used to monitor HIV infection in children are CD4 cell counts and HIV viral load tests. However, because children don’t have fully developed immune systems, their CD4 cell counts and viral load are different to those seen in adults.

Normal CD4 cell counts tend to be much higher in very young children than adults. On average the CD4 cell count in a six-month old baby is about 3,000 and is about 1,500 in one year olds and often over 1,000 in children under six. However, CD4 cell counts stabilise at similar levels to those seen in adults when a child is aged between about six and twelve.

In HIV-positive babies, HIV viral load can rise to very high levels (over 1 million copies/ml) within a few weeks and gradually decline over the first few years of life. The exact reason for this is not known. This contrasts to adults, where viral load normally falls back to relatively low levels (about 20,000 copies/ml) a few months after infection with HIV.

Using CD4 cell counts and viral load to judge the risk of disease progression in children is therefore much harder than in adults. Doctors calculate the risk of a child becoming ill by taking into account their age, CD4 cell count, and viral load. A ‘calculator’ is available on-line at this website address: http://www.ctu.mrc.ac.uk/penta/hppmcs/calcProb.htm

To find out more about CD4 and viral load tests, read the booklet, Viral load and CD4 in this series.