Appropriateness of sampling times for therapeutic drug monitoring

Serum sampling for drug concentration was evaluated for usefulness in pharmacokinetic calculations and dosage adjustment. The prospective study included serum samples for 11 drugs in hospital inpatients during a four-week period. Appropriateness of sampling regarding (1) time of drug administration and (2) steady state was determined. Both of these evaluations were used in the final rankings of appropriateness of the samples with regard to ease of interpretation and clinical usefulness. Digoxin, theophylline, tobramycin, and phenytoin accounted for 81% of the 466 samples evaluated. Sampling time was recorded for 56% of all samples. Based on time of dose administration, 54% of samples were obtained at the recommended time; 38% of samples were obtained at steady state. Basic pharmacokinetic methods could be used to interpret 15% of samples, while 44% required complex pharmacokinetic methods, 34% required assumptions about sampling time versus dosing history, and 7% should not have been used in dosage adjustment. The majority of plasma drug samples required more than basic pharmacokinetic methods for evaluation. For assessment of therapeutic drug regimens by monitoring serum drug concentrations, sophisticated pharmacokinetic consultation services are needed.