Development of a one step strip test for the detection of sulfadimidine residues

The one step strip test described is a competitive immunoassay in which the detector reagent consists of colloidal gold particles coated with affinity purified polyclonal anti-sulfadimidine (SDD) antibodies. The capture reage nt in the assay is an SDD-ovalbumin conjugate which is immobilised on the l ateral flow membrane of the test device. In the test procedure, 150 mu l (f our drops) of a liquid sample (buffer, urine or milk) are brought into the sample well of the test device and allowed to migrate over the membrane. Th e more analyte present in the sample, the more effectively it will compete with the SDD immobilised on the membrane for binding to the limited amount of antibodies of the detector reagent. A sufficient amount of SDD in the sa mple will therefore prevent the binding of the detector reagent to the SDD immobilised on the membrane. Therefore, a positive sample will not show a t est line in the read-out zone. With spiked buffer or calf urine this was ob tained at a level of > 10 ng ml(-1) of SDD and with spiked (diluted) fresh cow milk at a level > 20 ng ml(-1) of SDD. At these levels, the test is app licable only as a qualitative assay. The presence or absence of a test line indicates lower or higher levels of SDD, respectively. The major advantage s of the one step strip test an that results can be obtained within 10 min and that all reagents are included in the test device.