Optimal tobramycin dosage in patients with cystic fibrosis : evidence for predictability based on previous drug monitoring

A retrospective analysis of files of patients with cystic fibrosis and pulmonary exacerbations was performed to investigate whether an individual dosage of tobramycin once established by serum level determination allows a reliable prediction of the adequate dosage in a consecutive exacerbation. All patients hospitalized < or = 2 times between May 1997 and September 1998 with pulmonary exacerbation due to Pseudomonas aeruginosa infection susceptible to tobramycin were included. The initial dosage to tobramycin was 5 mg/kg body weight every 12 h followed by drug level determinations to establish the optimal dose. In a consecutive exacerbation the same dosage per kg body weight was used again and drug level determinations were repeated. Sixteen patients (six female = 38%) with a mean age of 24 years (median: 26 years, range: 9-33) were hospitalized for 49 pulmonary exacerbations (2-6 per patient, mean: 3, median: 2.5). During the first episode of tobramycin treatment in the study period all trough levels were > 2 microg/ml (median: 0.6) and the peak levels were 7.1-16.9 microg/ml (median: 11.9). In four patients the peak level was < 12 microg/ml. In 28 consecutive episodes the dosage of tobra myci n was chosen based on optimal results of previous drug level monitoring and in 27 instances (96%) the previously established optimal dose was confirmed. In five consecutive episodes the tobramycin dosage had been increased erroneously and this resulted in abnormally high peak levels in three cases. These findings suggest that a safe and therapeutic tobramycin dosage in an individual patient with cystic fibrosis is predictable based on a previously established optimal dosage.