Contains Non-Binding Recommendations 
205 
 
Guidance for Industry 
 
Studies to Evaluate the Metabolism and Residue Kinetics of 
Veterinary Drugs in Food-producing Animals: Metabolism 
Study to Determine the Quantity and Identify the Nature of 
Residues (MRK) 
 
VICH GL46 
 
 
Submit comments on this guidance at any time.  Submit written comments to the Division of 
Dockets Management (HFA-305), Food and Drug Administration, 5630 Fishers Lane, Room 
1061,  Rockville,  MD  20852.    Submit  electronic  comments  on  the  guidance  at 
http://www.regulations.gov  All written comments should be identified with the Docket No. FDA-
2010-D-8228.  
For  further  information  regarding  this  document, contact Julia Oriani, Center for Veterinary 
Medicine,  (HFV-151),  Food  and  Drug  Administration,  7500  Standish  Place,  Rockville,  MD 
20855, 240-276-8204, e-mail:julia.oriani@fda.hhs.gov 
Additional copies of this guidance document may be requested from the Communications Staff 
(HFV-12), Center for Veterinary Medicine, Food and Drug Administration, 7519 Standish Place, 
Rockville,  MD  20855,  and  may  be  viewed  on  the  Internet  at  either 
http://www.fda.gov/AnimalVeterinary/default.htm or http://www.regulations.gov.   
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Veterinary Medicine 
September 15, 2011 
  Contains Non-Binding Recommendations 
          VICH GL 46 (MRK)  METABOLISM AND RESIDUE KINETICS 
February 2011 
For Implementation at Step 7 - Final 
 
 
S M
TUDIES TO EVALUATE THE  ETABOLISM 
R K V
AND  ESIDUE  INETICS OF  ETERINARY 
D F - A :
RUGS IN  OOD PRODUCING  NIMALS  
M S
ETABOLISM  TUDY TO DETERMINE THE 
Q I N
UANTITY AND  DENTIFY THE  ATURE OF 
R  
ESIDUES
 
 
Adopted at Step 7 of the VICH Process 
by the VICH Steering Committee 
in February 2011 
for implementation in February 2012 
 
 
This Guidance has been developed by the appropriate VICH Expert Working Group and is subject to 
consultation by the parties, in accordance with the VICH Process.  At Step 7 of the Process the final draft   
is recommended for adoption to the regulatory bodies of the European Union, Japan and the USA. 
 
 
 
 
 
 
 
  GL46 - 2 Contains Non-Binding Recommendations 
Table of Contents 
 
1.  INTRODUCTION 
 
1.1.  Objective of the guidance 
1.2.  Background  
 
2.  GUIDANCE 
 
2.1.  Purpose 
2.2.  Scope 
2.3.  Study to Determine the Quantity and Identify the Nature of Residues 
2.3.1.Test Materials 
        2.3.1.1.  Drug  
        2.3.1.2.  Radiolabeled Drug 
          2.3.1.2.1. Nature and Site of Label 
          2.3.1.2.2. Purity of Radiolabeled Drug 
          2.3.1.2.3. Specific Activity 
        2.3.1.3.  Analytical Standards 
2.3.2.  Test Systems 
        2.3.2.1.  Animals  
        2.3.2.2.  Animal Handling 
2.3.3. Test Procedures 
        2.3.3.1.  Drug Formulation 
        2.3.3.2.  Route of Administration 
        2.3.3.3.  Dosing 
2.3.3.4.  Number of Animals and Number of Euthanasia Intervals 
        2.3.3.5.  Animal Euthanasia 
        2.3.3.6.  Sample Collection  Edible Tissues 
        2.3.3.7.  Sample Collection  Excreta and Blood 
        2.3.3.8.  Determination of Total Radioactivity 
2.3.4.  Separation and Identification of Metabolites 
        2.3.4.1.  Analytical Method 
        2.3.4.2.  Extent of Characterization/Major Metabolites 
        2.3.4.3.  Characterization of Bound and Nonextractable Residues 
             2.3.4.3.1.  General Comments 
               2.3.4.3.2.  Characterization of the Bound Residue 
3.  REPORTING OF DATA 
 
4.  GLOSSARY  
 
  GL46 - 3 Contains Non-Binding Recommendations 
Studies To Evaluate The Metabolism And Residue Kinetics Of 
Veterinary Drugs In Food-Producing Animals: Metabolism Study To 
Determine The Quantity And Identify The Nature Of Residues 
 
This guidance represents the Food and Drug Administrations (FDAs) current thinking on this topic.  It 
does not create or confer any rights for or on any person and does not operate to bind FDA or the public.  
You can use an alternative approach if the approach satisfies the requirements of the applicable statutes 
and regulations.  If you want to discuss an alternative approach, contact the appropriate FDA staff.  If you 
cannot identify the appropriate FDA staff, call the appropriate number listed on the title page of this 
guidance. 
 
1.  INTRODUCTION 
 
1.1.  Objective of the guidance 
Drug  sponsors  should  perform  a  comprehensive  set  of  metabolism,  residue  depletion  and 
pharmacokinetic studies to establish the safety of veterinary drugs in food.  The objective of this 
guidance is to provide recommendations for internationally-harmonized test procedures to study the 
quantity and nature of residues of veterinary drugs in food-producing animals.  
1.2.  Background 
This guidance is one of a series developed to facilitate the mutual acceptance by national/regional 
regulators of residue chemistry data for veterinary drugs used in food-producing animals.  This 
guidance  was  prepared  after  consideration  of  the  current  national/regional  requirements  and 
recommendations for evaluating veterinary drug residues in the European Union, Japan, United 
States, Australia, New Zealand and Canada. 
Although this guidance recommends a framework for metabolism testing, it is important that the 
design of the studies remain flexible.  It is recommended that studies be tailored to sufficiently 
characterize the components of the residue of concern.   
FDAs  guidance  documents,  including  this  guidance,  do  not  establish  legally  enforceable 
responsibilities.  Instead, guidances describe the Agencys current thinking on a topic and should be 
viewed only as recommendations, unless specific regulatory or statutory requirements are cited.  The 
use of the word should in Agencys guidances means that something is suggested or recommended, 
but not required. 
2.  GUIDANCE 
 
2.1.  Purpose 
The human food safety evaluation of veterinary drug residues helps ensure that food derived from 
treated food-producing animals is safe for human consumption.  As part of the data collection 
process, studies should be conducted to permit an assessment of the quantity and nature of residues 
in food derived from animals treated with a veterinary drug.  These metabolism studies provide data 
on (1) the depletion of residues of concern from edible tissues of treated animals at varying times 
  GL46 - 4 Contains Non-Binding Recommendations 
after drug administration, (2) the individual components, or residues, that comprise the residue of 
concern in edible tissues, (3) the residue(s) that can serve as a marker for analytical methods intended 
for compliance purposes (i.e., monitoring of appropriate drug use), and (4) the identification of a 
target tissue or tissues, as applicable to national or regional programs. 
2.2.  Scope 
Metabolism  studies  in  food-producing  animals  most  often  are  accomplished  using  radiolabeled 
drugs.  These studies are sometimes referred to as total residue studies because they are capable of 
monitoring all (i.e., total) drug-derived residues resulting from the administration of test material. 
This guidance recommends procedures for metabolism studies conducted with radiolabeled drugs, 
when such studies are performed.  
 
Alternative approaches (i.e., not using radiolabeled drug) to characterize the components of the 
residue in food derived from treated animals might be suitable.   
In any case, testing should be conducted in compliance with applicable Good Laboratory Practice 
(GLP). 
2.3.  Study to Determine the Quantity and Identify the Nature of Residues  
2.3.1.  Test Materials 
 
2.3.1.1.  Drug 
The chemical identity (including, for example, the common name, chemical name, CAS-number, 
structure and molecular weight) and purity of the drug substance should be described.  The 
chemical name and structure should indicate the position(s) of the radiolabel.  Handling and 
disposal of radiolabeled materials should be in compliance with applicable laws and regulations.  
2.3.1.2.  Radiolabeled Drug 
2.3.1.2.1.  Nature and Site of Label 
Carbon-14 (14C) is the label of choice because intermolecular exchange is not an issue.  Other 
isotopes, such as 3H, 32P, 15N or 35S, might be appropriate.  Tritium (3H) might be considered 
suitable if a rigorous demonstration of the stability of the tritium label is provided; for example, 
the extent of exchange with water is assessed and found to be 5%.   
The drug should be radiolabeled in a site, or in multiple sites, to assure that the portions of the 
parent drug that are likely to be of concern are suitably labeled.  The radiolabel should be placed 
in a metabolically stable position(s).   
2.3.1.2.2.  Purity of Radiolabeled Drug 
Radiolabeled drugs should have a high level of purity, preferably of approximately 95%, in order 
to minimize artifactual results.  Radiochemical purity should be demonstrated via appropriate 
analytical techniques (e.g., using two chromatographic systems). 
2.3.1.2.3.  Specific Activity 
The specific activity of the synthesized radiolabeled drug should be stated in the study report.  The 
specific activity should be high enough to permit tracking of the residue of concern in edible 
tissues.  The sensitivity should be determined by the potency of the drug. 
  GL46 - 5 Contains Non-Binding Recommendations 
The specific activity can be adjusted by mixing radiolabeled drug with unlabeled drug.  To 
facilitate analytical measurements and conserve radiolabeled drugs, animals to be euthanized at 
early withdrawal periods can be dosed with drug of lower specific activity, while animals to be 
euthanized at later withdrawal periods can be dosed with drug of higher specific activity.   
2.3.1.3.  Analytical Standards 
Analytical  standards  should  be  available  for  the  parent  drug  and,  if  possible,  for  putative 
metabolites for use in the chromatographic characterization of drug residues.   
2.3.2.  Test Systems 
There  are  some  national/regional  differences  regarding  the  designation  of  major  and  minor 
species, particularly for turkeys and sheep.  These differences can affect national/regional data 
collection  requirements  and  recommendations.    In  certain  instances,  the  total  residue  and 
metabolism data for a drugs use in a major species might be extrapolated to the minor species.  
When a national/regional authority calls for a total residue and metabolism study for a minor 
species or for a species considered to be major in one region but not another, the study design 
outlined in this guidance should be acceptable. 
2.3.2.1.  Animals 
Animals  used  in  the  metabolism  study  should  be  representative  of  commercial  breeds  and 
representative of the target population.  The source of the animals, their weights, health status, 
ages and sex should be provided. 
Ordinarily, a single study can be performed in swine (~40 to 80 kg), sheep (~40 to 60 kg) and 
poultry.  For cattle, a single study in beef cattle (~250 to 400 kg) could apply to dairy cattle, and 
vice  versa.    Generally,  the  results  of  a  metabolism  study  in  adult  cattle  and  sheep  can  be 
extrapolated to calves and lambs, respectively.  However, a second study might be appropriate for 
pre-ruminating animals if there is sufficient reason to believe the pre-ruminating animal will have 
significantly different metabolism than the adults.  A separate study should be performed to 
demonstrate the total residue in milk of dairy cows.   
If the study is intended to support a withdrawal period, the study parameters should address the 
worst-case study conditions (for example, animal weights and associated maximum injection 
volume). 
2.3.2.2.  Animal Handling 
Animals should be allowed time to acclimatize.  Normal husbandry practices should be applied to 
the  extent  possible.    It  is  recognized  that  these  studies  might  call  for  metabolism  cages,  a 
departure from normal practices; therefore metabolism cages should only be used if the study is 
intended to collect urine and excreta or other specification.  Animals should be healthy and, 
preferably, should not have been previously medicated.  The feed and water supplied to the 
animals  should  be  free  from  other  drugs  and/or  contaminants  and  adequate  environmental 
conditions should be ensured to be consistent with animal welfare, in accordance with applicable 
national  and  regional  regulations.    However,  it  is  recognized  also  that  animals  might  have 
received biological vaccinations or other treatment, for example with anthelmintics.  In any case 
an appropriate wash-out time should be observed for the animals prior to their being put on study.  
Animals should have a known history of medication.    
  GL46 - 6 Contains Non-Binding Recommendations 
Handling and disposal of animals and tissues from animals treated with radiolabeled materials 
should be in compliance with applicable laws and regulations.       
2.3.3.  Test Procedures 
2.3.3.1.  Drug Formulation 
The drug formulation, method of dose preparation, and stability of the drug in the formulation 
during the dosing period should be described in the study report.  Although it is recognized that 
metabolism studies can be conducted well in advance of definitive formulation decisions, the drug 
should be administered to test animals via the intended final formulation whenever possible; 
otherwise, representative or prototype formulations can be considered appropriate. 
2.3.3.2.  Route of Administration 
The drug should be administered via the intended route of administration (e.g., orally, dermally, 
intramuscularly, subcutaneously).  For drugs that are intended for oral administration, especially 
via feed or drinking water, gavage or bolus dosing can be employed to ensure that animals receive 
the complete dose and to minimize environmental concerns.  For drugs that are intended for oral 
and  parenteral  administrations,  usually  separate  metabolism  studies  should  be  performed.  
Ordinarily, a single study with a parenteral route will be applicable to cover all parenteral routes 
including intramuscular, intramammary, subcutaneous and topical.  Similarly, a single study with 
an oral route ordinarily will be applicable to all potential oral formulations (e.g., drinking water, 
in-feed and quick release tablets). 
2.3.3.3.  Dosing  
The dose should be the highest intended treatment concentration and should be administered for 
the maximum intended duration or for the time required for steady state to be achieved in edible 
tissues.  Predosing of animals with unlabeled drug, followed by administration of radiolabeled 
drug, is not recommended. 
For continuously administered drugs, a separate study to determine the time for residues to reach 
steady state in edible tissues might be appropriate.  When a drug administered in a single dose is 
intended to have zero withdrawal, completion of the absorption phase should be demonstrated.    
When gavage dosing for the feed and water routes, the dose should be divided and given in the 
morning and afternoon to better approximate actual use conditions. 
2.3.3.4.  Number of Animals and Number of Euthanasia Intervals 
At least four groups of animals, evenly-mixed as to sex if the drug is intended for use in both 
males and females, should be euthanized at appropriately spaced time points. The following 
numbers of animals are recommended: 
Large animals (cattle, swine, sheep):  3 per euthanasia time 
Poultry:  3 per euthanasia time 
Lactating cattle for milk collection:  8 multiparous cattle representative of high and low milk 
production 
Laying birds for egg collection:  a number sufficient to collect 10 eggs  
  GL46 - 7 Contains Non-Binding Recommendations 
For drugs for which a withdrawal period is not anticipated, euthanasia timepoints should reflect 
geographical distances in the amount of time to transport animals to the abattoir. 
Typical timepoints for practical zero withdrawal would be: 
  0-2, 3-4, and 6 hours for poultry 
  0-3, 6-8, and up to 12 hours for large animals 
  Up to 12 hours for milk 
A sufficient amount of control tissues should be available to permit a determination of background 
concentrations and combustion efficiency, and to provide tissue for related analytical methods 
testing. 
2.3.3.5.  Animal Euthanasia 
Animals  should  be  euthanized  using  commercially  applicable  procedures,  making  certain  to 
observe  appropriate  exsanguination  times.    Chemical  euthanasia  can  be  used  unless  it  will 
interfere with analysis of metabolites of interest.  
2.3.3.6.  Sample Collection  Edible Tissues 
Following  euthanasia,  samples  of  sufficient  amounts  of  edible  tissues  should  be  collected, 
trimmed of extraneous tissue, weighed, and divided into aliquots.  If the analysis cannot be 
completed immediately, the samples should be stored under frozen conditions pending analysis.  
If samples are stored after collection, the sponsor should ensure that the radiolabeled compound 
remains intact throughout the storage period. 
  GL46 - 8 Contains Non-Binding Recommendations 
 
Recommended samples are shown in Table 1. 
Table 1.  Recommended samples to be taken from animals in the metabolism study   
 
Edible  Species / Sample Description 
Tissue 
Type  Cattle/Sheep  Swine  Poultry 
Muscle  Loin  Loin  Breast 
Core of muscle tissue  Core of muscle tissue 
Collect sample from 
~ 500 g  ~ 500 g 
entire site of injection, 
Injection 
e.g., chicken: whole 
Site  10 cm diameter x 6 cm deep for  10 cm diameter x 6 cm 
neck, whole breast, or 
Muscle  IM;  deep for IM; 
whole leg. Larger birds: 
15 cm diameter x 2.5 cm deep  15 cm diameter x 2.5 cm  not to exceed 500 g  
for SC  deep for SC 
Liver  Cross-section of lobes  Cross-section of lobes  Entire 
Composite from combined  Composite from combined  Composite from 
Kidney 
kidneys  kidneys  combined kidneys 
Fat  Peri-renal  NA  NA 
Skin with fat in natural  Skin with fat in natural 
Skin/Fat  NA 
proportions  proportions 
Milk  Whole milk  NA  NA 
Composite from 
Eggs  NA  NA  combined white and 
yolk  
NA: Not Applicable 
The tissues shown in Table 1 should be analyzed.  Additional tissues should be collected and 
analyzed to provide information on the one additional tissue to be analyzed in the marker residue 
depletion study. (see VICH GL 48 Studies to Evaluate the Metabolism and Residue Kinetics of 
Veterinary Drugs in Food-producing Animals: Marker Residue Depletion Studies to Establish 
Product Withdrawal Periods, section 2.2.6.1) As appropriate to species, the additional tissues 
might include heart (cattle, swine, sheep, poultry), small intestines (cattle and swine) and gizzard 
(poultry); furthermore, it might be appropriate to collect and analyze other edible offal from the 
various species if it is deemed important for a safety assessment (e.g., offal with expected high 
residue concentrations or with residues having slow depletion rates). 
  GL46 - 9 Contains Non-Binding Recommendations 
 
2.3.3.7.  Sample Collection  Excreta and Blood 
Excreta and blood are not typically collected.  However, analyses of these samples can be useful 
from several perspectives:  first, analyses of the excreta and blood allow an estimate of the mass-
balance, a valuable tool in assessing the quality of the study; second, the samples of excreta can be 
a good source of metabolites; and third, the samples can be of use in conducting an Environmental 
Risk Assessment.  If the decision is made to collect such data, it is recommended that urine and 
excreta be collected from selected animals on a daily basis.   
Blood samples can be taken from selected animals at various time points and at euthanasia.  Data 
on the total residue in blood can provide valuable pharmacokinetic information. 
2.3.3.8.  Determination of Total Radioactivity 
Total  radioactivity  in  samples  should  be  determined  by  established  procedures  which  might 
include, for example, combustion followed by liquid scintillation counting, solubilization and 
counting,  or  direct  counting,  depending  on  the  nature  of  the  sample.    The  details  of  the 
radioassays, including preparation of analytical samples, instrumentation, and data from standards, 
control tissues, fortified tissues, and incurred tissues, should be completely described.  The ability 
of the procedure to recover radioactivity added to control tissues should be demonstrated.  
The results of analyses of samples for radioactivity should be reported on a wet weight basis and 
on a weight/weight basis, with g/kg the preferred units.  Sample calculations showing conversion 
from cpm/weight or dpm/weight to the weight/weight basis should be described in the study 
report.  
2.3.4.  Separation and Identification of Metabolites  
Commonly available analytical technology (including, for example, thin layer chromatography, 
high performance liquid chromatography, gas chromatography, and mass spectrometry) enable the 
separation of the total residue into its components and the identification of the drug-derived 
residues. 
2.3.4.1.  Analytical Method 
A complete description of the analytical method should be provided in the study report.  The 
description should include the preparation of standards, reagents, solutions, analytical samples; the 
extraction, fractionation, separation and isolation of the residues; the instrumentation; and the data 
derived  from  standards,  control  tissues,  fortified  tissues  and  incurred  tissues.    The  analytical 
method should be validated at least to demonstrate the recovery, the limit of detection and the 
variability. 
2.3.4.2.  Extent of Characterization/Major Metabolites 
The  degree  of  characterization  and  structural  identification  depends  on  several  factors  which 
include the amount of residue present, the concern for the compound or for the class of compounds 
to which it belongs, and the suspected significance of the residue based on prior knowledge or 
experience. 
In  general,  characterization  and  structural  identification  of  major  metabolites  should  be 
accomplished using a combination of techniques and might include chromatographic comparison 
to standards or mass spectrometry.  As a point of reference, major metabolites are those comprising  
  GL46 - 10 Contains Non-Binding Recommendations 
100 g/kg or 10% of the total residue in a sample collected at the earliest euthanasia interval (or 
following attainment of steady-state or at or near the end of treatment for continuous-use drug 
products).    In  some  cases,  chemical  characterization  rather  than  unequivocal  structural 
identification for a major metabolite will be appropriate (e.g. when a conjugate is present or if mass 
spectrometry information indicates the likely biotransformation pathway (for example M+16 for 
hydroxylation)).  Ordinarily, no differentiation of the radioactivity below these levels (i.e., of the 
minor metabolites) would be recommended unless there are concerns over residues occurring at the 
lower levels. 
2.3.4.3.  Characterization of Bound and Nonextractable Residues 
An investigation into the nature of bound residues is usually optional.  However, the information 
obtained from such an investigation might warrant the discount of some of the residues from the 
total residue of concern. 
2.3.4.3.1.  General Comments 
The use of veterinary drugs in food-producing animals can result in residues that are neither 
extractable  from  tissues  using  mild  aqueous  or  organic  extraction  conditions  nor  easily 
characterized.  These residues arise from (a) incorporation of residues of the drug into endogenous 
compounds, (b) chemical reaction of the parent drug or its metabolites with macromolecules 
(bound residues), or (c) physical encapsulation or integration of radioactive residues into tissue 
matrices.   
Those nonextractable residues shown to result from incorporation of small fragments of the drug 
(usually one or two carbon units) into naturally occurring molecules are not of significance. 
Characterization of the bound residues of a veterinary drug is usually prompted when the bound 
residue comprises a significant portion of the total residue or when the concentration of bound 
residue is so high as to preclude the assignment of a practicable withdrawal period for the drug 
(i.e., the total residue does not deplete below the residue of concern because of the amount of 
bound residue).  The extent of data collection on the bound residue depends on a number of factors, 
including the amount of bound residue, the nature of the bound residue and the potency of the 
parent drug or metabolite on which the Acceptable Daily Intake (ADI) is based. 
2.3.4.3.2.  Characterization of the Bound Residue 
The characterization of bound residues is usually difficult, involving vigorous extraction conditions 
or enzymic preparations that can lead to residue destruction or artifact formation. 
However, the biological significance of residues of veterinary drugs in foods usually depends on 
the  degree  to  which  those  residues  are  absorbed  when  the  food  is  ingested.    Therefore,  the 
determination of the bioavailable residues that result when tissue containing bound residue is fed to 
test  animals  can  be  a  useful  characterization  tool  (the  method  of  Gallo-Torres  (Journal  of 
Toxicology and Environmental Health, 2: 827-845 (1977)) might be an appropriate procedure for 
demonstrating bioavailability). 
  GL46 - 11 Contains Non-Binding Recommendations 
3.  REPORTING OF DATA   
The data should be presented so that it is possible to determine the marker residue to total residue 
ratio, the marker residue, and the target tissue if these concepts are called for by national/regional 
regulators.  The total residue concentration for each tissue should be reported for each collection 
timepoint.  The amounts of total residue radioactivity extracted (percentage extractable) using 
various treatments (enzyme, acid) should be provided.  The target tissue is the edible tissue selected 
to monitor for the total residue in the target animal. The target tissue is usually, but not necessarily, 
the tissue with the slowest depletion rate of the residues. 
The  components  of  the  total  residues  should  be  reported  for  each  collection  timepoint  for 
comparison to the total residue concentrations.  The components of the total residues (parent drug 
plus metabolite(s)) should be examined to select the marker residue.  The marker residue might be 
the parent compound.  However, the marker residue might also be defined as a combination of 
parent compound plus a metabolite(s) or as a sum of residues that can be chemically converted to a 
single derivative or fragment molecule. 
 
An appropriate marker residue has the following properties: 
1) there is a known relationship established between the marker residue and the total residue 
concentration in the tissue of interest; 
2) the marker residue should be appropriate to test for the presence of residues at the time point of 
interest, i.e. adherence to the withdrawal period; and 
3) there should be a practicable analytical method to measure the marker residue at the level of the 
MRL. 
  GL46 - 12 Contains Non-Binding Recommendations 
4.  GLOSSARY 
The following definitions apply for purposes of this document. 
Acceptable daily intake (ADI) of a chemical is the daily intake which, during an entire lifetime, 
appears to be without appreciable risk to the health of the consumer. The ADI most often will be 
set on the basis of the drugs toxicological, microbiological or pharmacological properties. It is 
usually expressed in micrograms or milligrams of the chemical per kilogram of body weight.  
Bound residues are residues formed by covalent binding of the parent drug or its metabolites with 
macromolecules in food-producing animals. 
Edible tissues are tissues of animal origin that can enter the food chain and include, but are not 
limited to muscle, injection site muscle, liver, kidney, fat, skin with fat in natural proportions, 
whole eggs and whole milk. 
Good laboratory practice (GLP) is the formalized process and conditions under which laboratory 
studies on veterinary drugs are planned, performed, monitored, recorded, reported and audited.   
Studies performed under GLP are based on national or regional requirements and are designed to 
assure the reliability and integrity of the studies and associated data. 
Major metabolites are those comprising 100 g/kg or 10% of the total residue in a sample 
collected from the target animal species in the metabolism study (VICH GL 46). 
Marker residue is that residue whose concentration is in a known relationship to the concentration 
of total residue in an edible tissue. 
Maximum residue limit (MRL) is the maximum concentration of a veterinary drug residue that is 
legally permitted or recognized as acceptable in or on a food as set by a national or regional 
regulatory authority.  The term tolerance, used in some countries, can be, in many instances, 
synonymous with MRL. 
Metabolism, for purposes of this guidance, is the sum total of all physical and chemical processes 
that occur within an organism in response to a veterinary drug.  It includes uptake and distribution 
of the drug within the body, changes to the drug (biodegradation), and elimination of drugs and 
their metabolites. 
Practical zero withdrawal is representative of the shortest time interval between administration of 
the last dose of the drug (e.g. at the farm) and slaughter (including transport from the farm).  
Residue means the veterinary drug (parent) and/or its metabolites. 
Residue  of  concern  refers  to  the  total  amount  of  residues  that  have  relevance  to  the  ADI 
established for the veterinary drug.   
Total  residue  of  a  drug  in  edible  tissues  is  the  sum  of  the  veterinary  drug  (parent)  and  all 
metabolites as determined in radiolabeled studies or other equivalent studies. 
Wet weight basis means that samples are analyzed fresh, with no allowance made for the moisture 
content. 
  GL46 - 13