Determination of Sulphadimidine (Sulfamethazine) Residues in Milk, Plasma, Urine and Edible Tissues by Sensitive ELISA

A highly sensitive immunoassay for sulphadimidine (SDM) is described, demonstrating the benefit of using spacer arms with different chemical compositions in the hapten immunogen and peroxidase tracer. The antiserum used in the optimized SDM assay exhibited 50% binding inhibition in buffer at approximately 0.15 microg l^-1, while the developed enzyme-linked immunosorbent assay (ELISA) exhibited approximately 100% cross-reactivity between SDM and its major metabolite N⁴-acetyl-SDM, relatively slight reaction with sulphamerazine (5.2%) and negligible reaction with 25 other related antimicrobials. The assay was evaluated using control and SDM-spiked milk, plasma, urine and edible tissue samples. The recovery of added SDM at the concentration of regulatory interest varied around 100%. The precision of the ELISA was below 5 and 15% for inter-and intra-assay variability, respectively. The highly sensitive method has been demonstrated to be an extremely effective way of overcoming problems caused by matrix interference, although care must be taken when using extremely high sample dilution.