An enzyme-linked immunosorbent assay (ELISA) and an enzyme-linked immunofiltration assay (ELIFA) were developed for the screening of sulfamethazine (SMZ) in porcine urine. Incurred urine samples were measured by ELISA with a working range of 0 to 10 ng of SMZ per ml. The assay showed good accuracy and precision, with recoveries above 99.8% and intra-and interassay coefficients of variation (CVs) ranging from 2.6 to 5.6% and from 5.9 to 12.7%, respectively. Good agreement was observed when the results of the immunoassay were compared with those of liquid chromatography/tandem mass spectrometry analysis. For the ELIFA, a nylon membrane is placed on top of an absorbent material and covalently coated with rabbit anti-rat immunoglobulins. Free binding sites are blocked, and monoclonal anti-SMZ antibodies, SMZ standard or urine, and SMZ–horse radish peroxidase conjugate are subsequently dropped onto the membrane. During the assay, the reactants are drawn through the membrane because of its close contact with the absorbent pad. Finally, a substrate solution is added for blue color development. The blue spot produced can be visually evaluated or instrumentally measured (numeric value), and the intensity of its color is inversely proportional to the analyte concentration. When a blue dot appears on the membrane, even if its color is less intense than that of the negative control, the sample is considered “negative,” i.e., it is thought to contain a concentration of SMZ that is below the visual detection limit. If no color appears on the test membrane, the sample is considered “positive,” i.e., it is thought to contain a concentration of SMZ that is equal to or above the visual detection limit. Validation of the assay showed good inter- and intra-assay precision (CV < 10%). Because samples can be analyzed after a simple dilution in <30 min with this assay format, it has strong potential for application in the field.

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