Application of a modified bioassay for monitoring serum teicoplanin and vancomycin in febrile neutropenic patients. Teicoplanin is a glycopeptide antibiotic with a mode of action and spectrum of activity similar to those of vancomycin. Its efficacy and tolerability as empiric therapy and its pharmacokinetic properties in neutropenic patients are being studied in a double-blinded, randomized trial in comparison with those of vancomycin. We report here a modified agar diffusion bioassay which is suitable for monitoring levels of either teicoplanin or vancomycin in serum during combination therapy with beta-lactams, aminoglycosides, and amphotericin B. Serum samples spiked with either teicoplanin or vancomycin gave reproducible results (mean coefficient of variation, 8.8%) regardless of the presence of tobramycin, amikacin, piperacillin, ceftazidime, amphotericin B, or their combinations. Among 25 patients who received teicoplanin at a dosing schedule of 6 mg/kg every 24 h intravenously, steady state was reached after 14.2 +/- 4.0 days, and 1-h peak and trough concentrations of teicoplanin in serum at steady state were 40.8 +/- 15.0 and 12.5 +/- 3.2 mg/liter, respectively. In contrast, among 25 patients who received vancomycin at a dosing schedule of 15 mg/kg every 12 h intravenously, steady state was reached by 24 h, and the 1-h peak and trough concentrations in serum were 37.5 +/- 15.6 and 8.3 +/- 3.8 mg/liter, respectively. The elimination half-lives for teicoplanin estimated by two separate approaches agreed closely with each other: 80.5 +/- 21.5 h by an accumulation model (M. Gilbaldi and D. Perrier, Pharmacokinetics, 2nd ed., p. 121, 1982) and 87.3 +/- 19.3 h as predicted from the degree of renal function (M. Rowland, Clin. Pharmacokinetic 18:184-209, 1990). These values were 14- to 15-fold higher than that for vancomycin (5.6 +/- 1.8 h). 