Two alternative procedures (A and B) are given. Procedure A is the simpler as it does not include a clarification treatment and is especially suited for milk. Procedure B includes a clarification step which allows a more sensitive detection through which better quantitative results can be obtained, if desired.

If either method is used for checking the proper pasteurization of these products or their raw materials, then further tests are necessary to ensure that the phosphatase activity is not due to either heat-stable microbial phosphatase or reactivated phosphatase.

The method is suitable for measuring low levels of hen's egg white lysozyme with a quantification limit of 5 mg/kg.

The method is applicable to raw milk, heat-treated milk and reconstituted dried milk.

This document is intended to be useful for manufacturers of screening test kits, laboratories validating screening methods or tests, competent authorities and dairies or end users of reagents or tests for the detection of veterinary drug residues in milk products. This document facilitates and improves the validation and verification of screening methods. The goals of this document are a harmonization in validation of methods or test kits in order for all stakeholders to have full trust in the result of residue screening and to limit the overlap and multiplication of validation work in different laboratories by sharing the validation results generated by an independent laboratory. Furthermore, a harmonized validation and verification procedure allows for comparison of the performance of different screening methods.

This document does not imply that all end users are bound to perform all verification work proposed.

The verification of the correct use of reagents/kits for the detection of antimicrobials is not part of the scope of this document.

The method is also applicable to environmental samples collected from milk powder or infant formula factories.