Application of an enzyme immunoassay for the determination of sulphamethazine (sulphadimidine) residues in swine urine and plasma and their use as predictors of the level in edible tissue.

The potential of an enzyme immunoassay (EIA) with high cross-reactivity towards the major metabolite (N4-acetyl-sulphamethazine) of sulphamethazine was tested for screening fluids and tissues. Healthy pigs were given 20 mg sulphamethazine per kg body weight per day in their drinking water for 2 days. Groups of four pigs were slaughtered after 3, 4 and 7 days withdrawal. The results were compared with liquid chromatographic analysis for urine, plasma, kidney, liver, gluteal muscle and diaphragm. In general, concentrations found by the EIA were higher than those found by liquid chromatography (LC) because sulphamethazine metabolites were detected by the EIA and not by LC. Using the EIA for the detection of sulphamethazine and the major metabolite in urine and plasma, predictive relationships (tissue-fluid ratios) for the concentration of the parent drug in tissue, determined by LC, were calculated. The tissue-plasma ratios for muscle, liver and kidney were 0.1, 0.2 and 0.1, respectively. The tissue-urine ratios for muscle, liver and kidney were 0.02, 0.03 and 0.03, respectively. Owing to the higher concentration of the parent drug in both fluids, the presence of the major metabolite in urine and the sensitivity of the EIA, tissue can be screened for low concentrations of sulphamethazine.