The same study highlighted the relationship between drug levels and risk of side effects. The relative risk in this group was 3.0. However virological failure is multi-factorial and there are other important factors such as adherence to consider. Table 3 shows the relationship of dose to concentration in children using indinavir and interpatient variability in pharmacokinetics with PIs. This variability in indinavir pharmacokinetics can be partly explained by the age of the children.
Drug clearance reduces with age.
It therefore appears that you have to increase the dose even more in the youngest children even when correcting for metabolic weight. There are several important aspects for TDM that I would like to raise. Although it is reasonable to look at trough levels if you are looking at virological efficacy, it is important to realise that the trough level is not always the C min. After taking medication the drug level continues to decrease for the next 1-2 hours. There may also be some variation between trough levels in the evening and morning.