Validation of SNAP ST

The SNAP ß-lactam ST (IDEXX Laboratories Inc. Westbrook, Maine, USA) is a competitive receptor test for the rapid screening of residues of ß-lactams (penicillins and cefalosporins) in raw milk. The test is an improvement of the SNAP ß-lactam test, with an incubation at the ambient temperature (15-30ºC) so an incubator is no longer needed. The time to result for the assay is approximately 7 minutes (an estimate 45 seconds of milk flow time and 6 minute of incubation at ambient temperature to allow for color development). In addition, compared to current SNAP ß-lactam test, the SNAP ß-lactam ST expands the detection capability of ß-lactams in milk: more antibiotic residues can be detected by SNAP ß-lactam ST at the MRL levels.

This new test was validated at ILVO-T&V (Technology & Food Science Unit of the Institute for Agricultural and Fisheries Research of the Flemish Community) according to Commission Decision 2002/657/EC and to the CRL guidelines for the validation of screening methods for residues of veterinary medicines (Anonymous, 2010). The following analytical parameters were checked: test specificity, detection capability, and test robustness (impact of deviation of the test protocol impact of the milk composition, and batch differences of reagents). Further the suitability of the SNAP ß-lactam ST to screen different milk types (UHT milk, sterilized milk, reconstituted milk powder, thawed milk) or milk from animal species (goat, ewe, and mare) other than the cow was also tested. Finally, the test was integrated in the monitoring of dairy samples to check the occurrence of false negative or false positive results, and the test was also included in two national ring trials and an international proficiency study.

The test was very specific; no interaction was noticed by compounds of antibiotic families or chemotherapeutics other than ß-lactams except for the interference by clavulanic acid. All ß-lactams with a Maximum Residue Limit (MRL) in milk were detected at their respective MRL except for cefalexin (>7500 µg/kg) and desacetylcephapirin (70 µg/kg).

The repeatability of the reader and the test were very good. The test result remained reliable even when performing the SNAP ß-lactam ST with the timing changes of different steps in the testing protocol including delay in activation of SNAP device, delay after dissolving the pellet in the milk and delay of reading. Also, the impact of milk parameters (somatic cells, total bacterial count, fat and protein content, pH) was tested as part of the robustness testing. No interference by these milk parameters was noticed except for milk of pH 6.0. No significant differences were noticed in testing different milk types (raw milk, UHT-milk, sterilized milk, reconstituted milk powder, or thawed milk) and other species animal milk (goats’, mares’, or ewes’ milk). Therefore, SNAP ß-lactam ST offers the flexibility of testing all different milks. No significant differences in testing capability were found between two batches of reagents of SNAP ß-lactam ST.

Out of the data of this validation study it can be concluded that the SNAP ß-lactam ST is a very fast, simple, and reliable test for the control of residues of ß-lactam antibiotics in tanker milk at the entrance. The test could also be used at farm level in order to prevent tanker milk contamination. The test is also giving reliable rest results for different milk types and milk of animal species different from the cow (goat, ewe, mare).