This calculator estimates the 12-month risks of progression to AIDS and death, based on the patient’s age and one of the following markers: CD4 percent, CD4 count, total lymphocyte count, or viral load. The estimates should only be used for guidance on when to initiate ART, and not as a hard and fast rule. Such decisions should take account of other information, including the child’s clinical status, the family’s preparedness to start therapy, adherence, etc. See www.pentatrials.org/guidelines.htm and www.aidsinfo.nih.gov/guidelines for further information.

Other important points to bear in mind when interpreting these estimates are:

• Predictions are unreliable after about age 12 years because of small numbers of older children. Predictions may also be inaccurate in very young infants as markers are highly variable at this age.
• The generalisability of these estimates is an issue since they are based on data going back to the mid-1980s from a diverse population. In particular, their accuracy has not been verified in resource-limited settings.
• The different markers may give very different estimates of risk for an individual child. Preliminary analyses indicate that the strongest individual predictor is CD4 count, followed by CD4 percent, followed by total lymphocyte count, followed by viral load. Alternatively, it may be prudent to base decisions on the highest of the estimates.
• All the markers are subject to high within-patient variability and measurement error. It is strongly recommended to repeat a test to confirm a single abnormal value.