Stable competitive enzyme-linked immunosorbent assay kit for rapid measurement of 11 active beta-lactams in milk, tissue, urine, and serum.

Beta-lactams are used as veterinary drugs for the treatment of food-producing animals. For consumer protection, legislation is in place to set limits for their residues. An enzyme-linked immunosorbent assay (ELISA) was developed which allowed, in a single reaction, the class-specific measurement of 11 beta-lactams, with limits of detection below European maximum residue limits. Determinations were feasible in milk, tissue, urine, and serum with simple and rapid sample preparation. In this format, the specific capture antibodies were precoated on the microtiter plate and horseradish peroxidase-labeled conjugate was used to compete with free beta-lactams. The stability of the precoated microtiter plate and conjugate was at least 1 year when stored at 2 to 8 degrees C; upon reconstitution, the conjugate was stable for 6 days at 2 to 8 degrees C. The stability of lyophilized ampicillin standards was at least 6 months when stored at 2 to 8 degrees C and at least 1 year when stored at -20 degrees C. A low cross-reactivity, 3.6%, was observed with ampicillin with open beta-lactam ring relative to 100% for intact ampicillin. Generic recognition was shown by relative cross-reactivity values ranging from 22 (penicillin V) to 144% (nafcillin). Cross-reactivity for cephalosporins was <0.1%. Intra- and interassay precisions expressed as coefficient of variation were typically 2-8%. The inhibitory concentration with 50% binding for ampicillin was typically 2 ppb. Recovery for different spiked levels was >70% with all the matrixes.