Therapeutic drug monitoring of pharmacologically active ampicillin, cefuroxime, meropenem, tazobactam and piperacillin - method validation and results from one-year experience.

Therapeutic drug monitoring (TDM) is an important tool for securing optimal treatment of drugs like beta-lactam antibiotics in hospitalized patients. Developing accurate and robust methods for quantification of the pharmacologically active fraction is needed. A LC-MS/MS assay, using ultrafiltration for selection of the non-protein bound (i.e. pharmacologically active) fraction followed by a 5.5 min chromatographic separation was developed and validated for simultaneous determination of Ampicillin (AMPI), Cefuroxime (CEFU), Meropenem (MERO) and Piperacillin (PIPE)/Tazobactam (TAZO). Inter-batch imprecision was <10%. Recovery was 88-115% from lowest level of quantification (LLOQ) to 2500 mg/L for MERO, PIPE and TAZO and LLOQ-500 mg/L for AMPI and CEFU. Storage temperature had great impact on measured concentrations, with PIPE being most instable. No effect of temperature during ultrafiltration was found for the measured beta-lactams. When evaluating one-year routine clinical use of the analysis, representing 581 (mainly ICU) patient samples, >50% of patients treated with PIPE (and AMPI), and about 30% treated with MERO and CEFU, exhibited beta-lactam levels higher than the local guidelines for maximum recommended levels. Potential toxic levels were found in several patients. We believe that the analytical method developed in this study represents an accurate measure of the pharmacologically active beta-lactam fraction. Evaluation of patient data after one-year use of the method shows higher than expected beta-lactam levels, highlighting the potential benefit of TDM. More studies are needed to determine the clinical significance of TDM in beta-lactam antibiotics.