Effect of storage time at -20 degrees C on the determination of serum concentrations of tobramycin in the presence of 6 beta-lactams
Forey, F.; Coulet, M.; Freney, J.; Brun, Y.; Fleurette, J.
Pathologie-Biologie 31(5): 404-412
1983
ISSN/ISBN: 0369-8114 PMID: 6353332 Document Number: 202456
We assessed the role of six beta-lactam antibiotics and of storage time at - 20 degrees C, in inactivation of tobramycin in patients' sera. Several ranges of concentrations were used for the combined antibiotics. The tobramycin concentrations were measured both by microbiological assay (plate diffusion) and an enzyme mediated immunoassay technique (EMIT). Using the bioassay method, the results after 8 days of frozen storage were as follows: carbenicillin and ticarcillin (256 mg/l) induced a 40-50% reduction of tobramycin activity, whereas mezlocillin (256 mg/l) had less effect: a 15-20% reduction. After 15 days at -20 degrees C the results were nearly the same except for azlocillin (256 mg/l), increasing its percentage reduction to 20%. The results obtained by EMIT procedure were significantly better after 8 days of frozen storage: only mezlocillin (5% reduction) and azlocillin (10%) were effective. Nevertheless after 15 days at -20 degrees C, the inactivating action of the pre-cited antibiotics were similar to those obtained at the same time with the bioassay method. Piperacillin and cefsulodin never induced any reduction of tobramycin levels, whatever the time of storage or quantification procedure used. So, our opinion is that patients' sera containing a beta-lactam antibiotic in combination with tobramycin should be assayed immediately upon receipt. Keeping it at -20 degrees C is not sufficient to prevent in vitro tobramycin inactivation.