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.