Quantitation of serum gentamicin concentration by a solid-phase immunofluorescence method.

We describe the use of a solid-phase immunofluorescence method (FIAX system) for measuring drug concentrations, specifically gentamicin, in serum or plasma. Antibody to gentamicin is immobilized on a polymeric surface. This Sampler is immersed in a mixture of serum standard or unknown and a buffered solution of fluorescent-labeled gentamicin. After 30 min, the fluorescence of the specifically bound labeled gentamicin is measured with a fluorometer. The assay detects gentamicin in concentrations as low as 50 mug/L. It is specific for gentamicin, cross reactivity to some other therapeutic drugs, except netilmicin and sisomicin, being low. Results by this assay compare well with those by a radioimmunoassay (r = 0.94) and a radioenzymic assay (r = 0.95). The assay is specific, economical, simply performed, requires little pipetting, and does not involve the use of radioisotopes.