Matrix ranking for prioritising testing of veterinary medicine residues

Farmed animals including fish, game and bees can suffer from diseases.  Farmers have a duty to protect the health and welfare of their animals, so may use medicines to treat or prevent disease.  A withdrawal period is set for an authorised veterinary medicine, which is the period that must pass before the treated animal can be slaughtered, or an animal product is taken, for human consumption after the end of the treatment.  Unauthorised or prohibited veterinary medicines cannot be used in the EU, and should not be present in imported foodstuffs entering the EU.

In response to the discovery of unauthorised veterinary medicines in imported foodstuffs 10 years ago the VRC introduced Matrix Ranking with the objective of providing the Committee with a way of systematically assessing substances in order to inform the surveillance recommendations that the Committee makes to the Veterinary Medicines Directorate (VMD).  Whilst this is of greater use for the Non-Statutory Surveillance (NSS) programme to ensure the most efficient use of reduced Defra resources, it is also very useful for the larger, statutory, National Residues Control Plan which is funded by industry.

The process was presented at the British Toxicology Society and the United Kingdom Environmental Mutagen Society in 2012.  It has since been further tested, refined and updated by a subgroup of VRC members with valued assistance from representatives from the Food Standards Agency and the VMD.  You are invited to view the updated version.  There is also a list of substance groups included in residues surveillance with an explanation of their status and their function.


The prioritisation score is a combination of two factors

            Total Score = Toxicity score  X  Exposure score

The Toxicity score comprises separate scores for

  • Hazard (A)
  • Potency (B)


The Exposure score is comprised of four separate scores

  • Proportion of the UK diet coming from animals which may be treated (C)
  • Frequency of dosing with a particular substance (D)
  • Evidence of high exposure groups (E)
  • Evidence of detectable residues (F).

Substances are assessed under each category, each category being sub-divided into criteria. Criteria within each category are assigned numerical values.  For instance if there are no adverse effects reported after exposure to a substance, the score for A would be 0.  Alternatively, if there was evidence that it was a potential mutagen/carcinogen in humans the score for A would be 6.  For more details of the numerical values, see Summary Table of Matrix Ranking Assessments

Each substance is assessed against specific criteria to arrive at a score using the following formula: 

Substance total score = (A+B) x (C+D+E+F)

The higher the score, the greater the level of concern, and the more likely a substance is to be included in the plans for surveillance.      

The success of the scheme depends on:

  • The assumptions, values and judgements inherent in the procedure. 
  • The quality and quantity of the data.  For veterinary medicines various regulatory bodies such as EFSA (European Food Standards Authorities) and EMA (European Medicines Agency) also provide expert advice.
  • Effective horizon scanning to monitor forthcoming trends and changes in veterinary medicine use, especially from third countries.
  • Use of diverse sources of intelligence/information and expertise.
  • Frequency of review and updating the Matrix Ranking Scheme.   

There are several advantages of using such a system:

  • The Matrix Ranking Scheme  provides a clear systematic approach
  • The process is relatively simple and transparent. 
  • Diverse information and intelligence is used for making choices as to which substances to include in NSS surveillance plans.
  • It is capable of dealing with an absence of information. 
  • The use of ADI to define potency as well as defining the nature of the hazard is considered to provide a reasonable characterisation of hazard.  
  • Although the list represents only a fraction of veterinary medicines, it encompasses most therapeutic classes and the substances selected represent the ‘worst case’ in terms of overall risk in each class. 



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