Assessment of a latex-agglutination-inhibition card test for serum gentamicin, with a study of the effects of potential interfering factors.

A latex-agglutination-inhibition test for serum gentamicin, based on inhibition by gentamicin of antibody-induced agglutination of gentamicin-coated latex particles, was found to be sufficiently reliable for use in therapeutic monitoring and to correlate well with a variety of established immunoassays. The test (a nonseparation, nonisotopic immunoassay) is performed on cards and has a simple visual end point by inspection for the presence or absence of agglutination. Severely elevated bilirubin or lipid levels, or gross hemolysis (which may cause interference with other nonseparation immunoassays) had no effect on the card test. With raised rheumatoid factor or complement, the cards gave accurate recovery of added gentamicin at 5 and 10 mg/L but low recovery at 2 mg/L. Of 67 patients' sera, three from one individual caused nonspecific agglutination of the latex and could not be assayed. The card test can be recommended for laboratories handling infrequent or small numbers of samples and for those without access to instrumentation.