Radioimmunoassay for measurement of gentamicin in blood.

Gentamicin toxicity has been shown to be related to high concentrations in serum. Because there is a narrow range between its therapeutic and toxic levels, serial monitoring of gentamicin is the most reliable method of guiding therapy. Microbiological assays commonly in use do not afford the desired speed and accuracy, and results may be difficult to interpret in the presence of other antimicrobials. Hence, a rapid, sensitive, and highly specific radioimmunoassay for measurement of gentamicin in serum has been developed. Antibody to gentamicin was raised in rabbits by using a gentamicin-albumin conjugate. Tritiated gentamicin (specific activity 1.0 Ci/mM) competes with unlabeled gentamicin for binding sites on the antibody. Dextran-coated charcoal separates the unbound from antibody-bound gentamicin. Serum levels of gentamicin are determined by comparison with a standard curve. This method can detect concentrations as low as 0.01 mug/ml. Results of a 24-tube run can be obtained in 1 h, thus allowing modification of gentamicin dosage to advantage.