Heparin interferes with tobramycin serum concentration determinations by Emit

Enzyme multiplied immunoassay (Emit) commonly is used to determine aminoglycoside concentrations. Its accuracy generally is comparable to that of radioimmunoassay (RIA). Poor reproducibility and questionable quantitation by the Emit assay have been reported when heparinized, severely lipemic, or icteric samples have been used. However, the significance of these interferences is documented poorly. We observed a ESRD patient in whom the underestimation by Emit of the tobramycin concentration in a plasma sample (heparin concentration of 41 U/ml) could have resulted in excessive drug administration and potential toxicity. Tobramycin therapy was initiated with a loading dose of 1.6 mg/kg and three tobramycin concentrations were obtained (2, 12, and 36 hours post-infusion) to define the patient's pharmacokinetic parameters. The first and third samples were collected in serum specimen tubes while the second sample was collected in a plasma tube. The tobramycin concentration in the plasma sample measured by Emit was 82 percent lower than the RIA value. Analyses of other samples by both procedures revealed no clinically significant differences. This case demonstrates that the presence of heparin may interfere with the Emit tobramycin assay in the clinical setting. The degree of reduction in tobramycin concentrations may be dramatic and potentially can alter a patients apparent tobramycin dosing requirements. Further investigation is warranted.