Evidence of excessive concentrations of 5-flucytosine in children aged below 12 years: a 12-year review of serum concentrations from a UK clinical assay reference laboratory.

5-flucytosine (5-FC) is an antifungal drug used for the treatment of serious infections caused by Candida or Cryptococcus spp. In the UK, the recommended pre- and post-dose serum therapeutic ranges are 30-40 mg/L and 70-80 mg/L, respectively. A 12-year retrospective review of serum concentrations of 5-FC in three groups of children aged 1-30 days (n=167), 31-60 days (n=102) and 91 days to 12 years (n=122) was conducted. In these three age groups, 65.1%, 44.4% and 21.3% of pre-dose samples and 39.3%, 29.2% and 19.7% of post-dose samples were above the recommended ranges. Both the mean concentration and the percentage of concentrations above the recommended ranges were significantly higher in the youngest age group (1-30 days old), suggesting that the standard dose of 100 mg/kg daily may not be an appropriate dose in this age group.