Gentamicin/heparin interactions: effects on two immunoassays and on protein binding.

Heparin has been reported to have two different effects on gentamicin. One is the interference by heparin with the quantitation of gentamicin; the other is an increase in the bound fraction of gentamicin in the presence of serum. The reports conflict regarding what concentration of heparin causes these effects. The main objective of this study was to determine what effect heparin had on the determination of gentamicin using an enzyme immunoassay (EIA) or a fluorescence polarization immunoassay (FPIA). Heparin concentrations studied simulated the amounts that would be found in plasma when blood samples are collected with an evacuated heparinized tube (less than 30 U/ml), or the greater amounts that might be present when blood is collected from indwelling lines (100-200 U/ml). Heparin had no effect on the quantitation of gentamicin by FPIA. In measurements done by EIA, heparin had no effect at 28 U/ml but did significantly inhibit the reaction by 10-20% when its concentration was 100-200 U/ml. We studied the effects of heparin on the distribution equilibrium of gentamicin by measuring free gentamicin produced in an ultrafiltrate. It was found that the bound fraction increased as the heparin concentration increased. Our results with aqueous and serum-based samples suggest that the increased binding was the result of a direct binding of gentamicin to heparin, as well as a more complex interaction involving heparin, gentamicin, and proteins.