Assay of beta-lactam antibiotics when used therapeutically in combination with aminoglycosides.

Assay of f3-lactam antibiotics when used therapeutically in combination with aminoglycosides In 1977, Stevens and Young' reported the use of the cation-exchanger cellulose phosphate (CP) as an inactivator of aminoglycosides in serum samples which also contained a f8-lactam antibiotic. After inactivation of the aminoglycoside the /8-lactam antibiotic was subsequently determined in a mic-robiological assay. This method has advantages over the use of aminoglycoside-resistant test strains, in which the specificity of the assay may become doubt-787 ful.2 Our initial experiences with the CP method were disappointing. We attempted to find the cause of this and succeeded in obtaining reliable results when two simple modifications were introduced. This paper discusses the effect of these modifications. Material and methods The original method as described by Stevens and Young' involved adding 100 mg cellulose phosphate powder to 1 ml of patient's serum. After thorough mixing the sample was centrifuged and the super-natant was taken for microbiological assay. This technique was modified in two respects: (i) serum was diluted 1/3 with 0.1 M phosphate-buffered saline (PBS) pH 7-0 and (ii) 20 mg of CP instead of 100 mg/ml was used. Discs for obtaining standard curves were prepared using serial twofold dilutions of /8-lactam antibiotics in normal human serum-PBS diluent. An aliquot (1 ml) of each dilution was transferred to a tube containing 20 mg CP, shaken for a few seconds on a whirl mixer, left at room temperature for 30 min and then centrifuged at 3000 rpm for 10 min. Discs for standard curves were prepared using this supemat-ant as well as the original working dilutions.