Heparin interferes with tobramycin serum concentration determinations by Emit

Area of Science:

• Clinical Chemistry
• Pharmacokinetics
• Analytical Toxicology

Background:

• Enzyme multiplied immunoassay (Emit) is a common method for determining aminoglycoside concentrations, often comparable to radioimmunoassay (RIA).
• Previous reports suggest potential issues with Emit assay reproducibility and quantitation in specific sample types, including heparinized specimens.
• The clinical significance of these interferences, particularly with heparin, remains poorly documented.

Observation:

• A case involving an end-stage renal disease (ESRD) patient highlighted a significant underestimation of tobramycin concentration using Emit.
• The patient's plasma sample, collected in a heparinized tube (41 U/ml), showed an 82% lower tobramycin level compared to RIA.
• This discrepancy occurred despite other samples showing no clinically significant differences between Emit and RIA.

Findings:

• Heparin interference in the Emit tobramycin assay can lead to dramatic reductions in measured drug concentrations.
• The observed underestimation in a heparinized sample could have led to excessive tobramycin administration and potential toxicity.
• This case demonstrates a critical limitation of the Emit assay in clinical settings with heparinized samples.

Implications:

• Clinicians must be aware of potential heparin interference with Emit tobramycin assays.
• Careful consideration of sample collection tubes is crucial to ensure accurate tobramycin quantitation.
• Further research is warranted to fully understand and mitigate heparin's impact on Emit assays for aminoglycosides.