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healthy infants who are not infected with HIV are considerably higher than those observed in uninfected adults and slowly decline to adult values by the age of about six years. Age must therefore be taken into account as a variable in considering absolute CD4 counts or %CD4+ . Among children younger than five years of age, the absolute CD4 count tends to vary within an individual child more than the %CD4+. Currently, therefore, the measurement of the %CD4+ is thought to be more valuable in younger childreni . Absolute CD4 counts (and less so %CD4+) fluctuate within an individual and depend on intercurrent illness, physiological changes or test variability. Measuring the trend over two or three repeated measurements is therefore more informative than an individual value. Not all the equipment in use in resource-constrained settings can accurately estimate the %CD4+. The dedicated cytometers are designed to exclusively perform absolute CD4 measurements without the need for a haematology analyser and therefore do not provide %CD4+ii . Any classification of immune status has to consider age. The 1994 immunological classification of the United States CDC has previously been used [60]. WHO has proposed a modified immunological classification based on more recent analysis of the prognosis. Analysis of prognosis from 17 studies of children including 3941 children living with HIV from United States and European settings provide estimations of CD4 and age-related risk of progression to AIDS or death. A %CD4+ of 35 is associated with a 15% risk of progression to AIDS in the next 12 months among children aged three months and an 11% risk among those six months old. The revised WHO classification attempts to better reflect this increased risk in these younger children. Based on reanalysis of the data, the thresholds for severe immunodeficiency in children have been revised.
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