Comparative study of the ability of four aminoglycoside assay techniques to detect the inactivation of aminoglycosides by beta-lactam antibiotics.

In vitro inactivation of aminoglycosides (tobramycin, gentamicin, and amikacin) by beta-lactams (cefazolin, cefotaxime, moxalactam, carbenicillin, piperacillin, mezlocillin, and azlocillin) was measured using the enzyme-mediated immunoassay (EMIT), fluorescence polarization immunoassay ( TDX ), radioimmunoassay (RIA), and bioassay. No significant inactivation of aminoglycosides was produced by high levels of the three cephalosporins as measured by EMIT, RIA, or bioassay. Inactivation of tobramycin and gentamicin by mezlocillin and azlocillin was comparable to that seen with piperacillin but less than that with carbenicillin. In general, the bioassay detected the greatest degree of aminoglycoside inactivation and the EMIT assay detected the least for all drug combinations. The TDX and RIA techniques were equivalent in their ability to detect aminoglycoside inactivation by beta-lactam antibiotics.

Heparin interferes with the radioenzymatic and homogeneous enzyme immunoassays for aminoglycosides.

Heparin interferes with measurement of aminoglycosides in serum by biological, radioenzymatic, and homogeneous enzyme immunoassay techniques, but not with radioimmunoassay. At concentrations greater than or equal to 10(5) and greater than or equal to 3 X 10(6) USP units/L, respectively, it interferes with the radioenzymatic assay by inhibiting the gentamicin 3-acetyltransferase (EC 2.3.1.60) and kanamycin 6'-acetyltransferase (EC 2.3.1.55) enzymes used in the assay. It interferes with the homogeneous enzyme immunoassays for gentamicin and tobramycin (at concentrations greater than or equal to 10(5) and greater than or equal to 10(4) USP units/L, respectively), but not with the commercially available homogeneous enzyme immunoassays for other drugs. Heparin interference with the homogeneous enzyme immunoassay for aminoglycosides requires both the heparin polyanion and glucose-6-phosphate dehydrogenase (EC 1.1.1.49) bound to a cationic aminoglycoside. This interference can be reproduced with dextran sulfate (but not dextran), and does not occur with free enzyme (glucose-6-phosphate dehydrogenase) alone. We describe how heparin interferes with these two assays and demonstrate interference at heparin concentrations that may be present in intravenous infusions or in seriously underfilled blood-collection tubes.