Heparin interferes with the radioenzymatic and homogeneous enzyme immunoassays for aminoglycosides

Abstract

Heparin interferes with measurement of aminoglycosides in serum by biological, radioenzymatic, and homogeneous enzyme immunoassay techniques, but not with radioimmunoassay. At concentrations ≥ 105 and ≥ 3 X 106 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 ≥ 105 and ≥ 104 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.