EN ISO 22117:2019

SLOVENSKI STANDARD
01-maj-2019
1DGRPHãþD
SIST-TS CEN ISO/TS 22117:2011
Mikrobiologija v prehranski verigi - Posebne zahteve in napotki za preskušanje
strokovnosti z medlaboratorijsko primerjavo (ISO 22117:2019)
Microbiology of the food chain - Specific requirements and guidance for proficiency
testing by interlaboratory comparison (ISO 22117:2019)
Mikrobiologie der Lebensmittelkette - Spezielle Anforderungen und Anleitungen an die
Eignungsprüfung durch Ringversuche (ISO 22117:2019)
Microbiologie de la chaîne alimentaire - Exigences spécifiques et recommandations
relatives aux essais d'aptitude par comparaison interlaboratoires (ISO 22117:2019)
Ta slovenski standard je istoveten z: EN ISO 22117:2019
ICS:
07.100.30 Mikrobiologija živil Food microbiology
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 22117
EUROPEAN STANDARD
NORME EUROPÉENNE
March 2019
EUROPÄISCHE NORM
ICS 07.100.30 Supersedes CEN ISO/TS 22117:2010
English Version
Microbiology of the food chain - Specific requirements and
guidance for proficiency testing by interlaboratory
comparison (ISO 22117:2019)
Microbiologie de la chaîne alimentaire - Exigences Mikrobiologie der Lebensmittelkette - Spezielle
spécifiques et recommandations relatives aux essais Anforderungen und Anleitungen an die
d'aptitude par comparaison interlaboratoires (ISO Eignungsprüfung durch Ringversuche (ISO
22117:2019) 22117:2019)
This European Standard was approved by CEN on 19 February 2019.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 22117:2019 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 22117:2019) has been prepared by Technical Committee ISO/TC 34 "Food
products" in collaboration with Technical Committee CEN/TC 275 “Food analysis - Horizontal methods”
the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by September 2019, and conflicting national standards
shall be withdrawn at the latest by September 2019.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes CEN ISO/TS 22117:2010.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 22117:2019 has been approved by CEN as EN ISO 22117:2019 without any modification.

INTERNATIONAL ISO
STANDARD 22117
First edition
2019-02
Microbiology of the food chain —
Specific requirements and
guidance for proficiency testing by
interlaboratory comparison
Microbiologie de la chaîne alimentaire — Exigences spécifiques et
recommandations relatives aux essais d'aptitude par comparaison
interlaboratoires
Reference number
ISO 22117:2019(E)
©
ISO 2019
ISO 22117:2019(E)
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO 22117:2019(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Scheme design and purpose . 2
4.1 General . 2
4.2 Scheme objectives . 2
4.3 Laboratory requirements for schemes . 3
4.4 Choice of test matrices . 3
4.5 Information on test methods used by the PT provider . 3
4.6 Statistical design . 3
5 Technical requirements and guidance for sample design and content .4
5.1 Sources, characterization and traceability of organisms . 4
5.2 Target organisms level . 4
5.3 Non-target organisms and interferences . 5
5.4 Matrix selection and effects . 5
6 Sample verification by the provider . 6
6.1 General . 6
6.2 Sample homogeneity testing — General considerations . 6
6.3 Homogeneity testing for quantitative (enumeration) samples . 6
6.4 Homogeneity testing for qualitative methods . 7
6.5 Stability testing by the provider . 8
6.5.1 General. 8
6.5.2 Stability during storage conditions . . 8
6.5.3 Stability during transport conditions . 8
7 Sample handling . 9
7.1 General . 9
7.2 Instructions to participants . 9
8 Performance evaluations . 9
8.1 General . 9
8.2 Preliminary considerations . 9
8.3 Assessment of quantitati ve methods .10
8.3.1 General.10
8.3.2 Distribution of data .11
8.3.3 Determining the assigned value .12
8.3.4 Uncertainty of the assigned value .12
8.3.5 Methods of assessing performance .12
8.3.6 Using z-scores .12
8.3.7 Other methods of performance evaluation .14
8.3.8 Long-term performance assessment .16
8.4 Assessment of qualitati ve methods.17
8.4.1 General.17
8.4.2 Performance of individual laboratories .17
8.4.3 Scheme comparisons of laboratory performance .19
Annex A (informative) Example of details to be included in a PT scheme plan .21
Annex B (informative) Preparation of fungal spore suspensions .23
Annex C (informative) Methods of testing for variation between portions of test materials .24
Annex D (informative) Example of a safety data sheet .28
ISO 22117:2019(E)
Annex E (informative) A practical method to assess long-term performance of participants
in PT schemes using enumeration methods .30
Bibliography .32
iv © ISO 2019 – All rights reserved

ISO 22117:2019(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 9,
Microbiology.
This first edition cancels and replaces ISO/TS 22117:2010, which has been technically revised. The
following changes have been made:
— updates have been made to align the document with ISO 13528:2015.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
ISO 22117:2019(E)
Introduction
General requirements for organization of proficiency testing (PT) schemes of all types are given
through ISO/CASCO (Committee on Conformity Assessment) in ISO/IEC 17043. Additionally, general
guidance is available from the International Union of Pure and Applied Chemistry (IUPAC), see
Reference [12]. However, these recommendations may not be directly applicable to all cases and should
be interpreted specifically for different laboratory sectors where PT schemes are organized. For this
reason, a document is needed to establish the criteria for a provider (and associated collaborators) of
PT schemes for microbiological examinations to meet and be recognized as competent. This applies
particularly to the specific technical requirements necessary to deal with microorganisms, such as
sample homogeneity and stability, as well as with the interpretation of detection tests which is not
covered by an existing document.
PT schemes for microbiology laboratories are mainly used to evaluate performance, particularly
trueness (bias) and in some cases precision, of food microbiological examinations in specific
laboratories.
Additionally, data from such PT schemes can be used:
a) to provide information to the organizations responsible for laboratory acceptance within an official
control framework and to allow continuous monitoring;
b) to aid laboratory accreditation in a general framework of quality management;
c) to inform those responsible for quality in the participating laboratories as part of the educative
elements of external quality assessment of trueness (bias).
Information from PT schemes may also be used for:
— identification of the possible sources of errors, particularly the bias component of uncertainty, to
improve performance;
— estimation of uncertainty of test results, in conjunction with routine results, for quantitative
(enumeration) methods (see ISO/TS 19036) and levels of detection for qualitative (detection)
methods;
— demonstration of staff competence to perform a specific microbiological examination;
— evaluation or validation of a given method by the study of trueness, precision and robustness;
— identification of variability in test results between individual laboratories;
— assignment of a “target” value for a microorganism in a material in order to establish a reference
material (see ISO 17034).
However, these aspects are not specifically covered in this document.
PT schemes are therefore designed to meet certain criteria and the testing programme (frequency,
number of samples, number of repeats, etc.) to meet the requirements of the type of method used and
commodity tested, to achieve the level of control required by all parties.
vi © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 22117:2019(E)
Microbiology of the food chain — Specific requirements
and guidance for proficiency testing by interlaboratory
comparison
1 Scope
This document specifies requirements and gives guidelines for the organization of proficiency testing
(PT) schemes for microbiological examinations of
a) foods and beverages,
b) feeding animals,
c) environmental samples from food and feed production and handling, and
d) primary production stages.
This document is also applicable to the microbiological examination of water where water is either used
in food production or is regarded as a food in national legislation.
This document relates to the technical organization and implementation of PT schemes, as well as the
statistical treatment of results of microbiological examinations.
This document is designed for use with ISO/IEC 17043 and ISO 13528, and deals only with areas where
specific or additional details are necessary for PT schemes dealing with microbiological examinations
for the areas specified in the first paragraph.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3534-1, Statistics — Vocabulary and symbols — Part 1: General statistical terms and terms used in
probability
ISO 3534-2, Statistics — Vocabulary and symbols — Part 2: Applied statistics
ISO 5725-1, Accuracy (trueness and precision) of measurement methods and results — Part 1: General
principles and definitions
ISO 13528:2015, Statistical methods for use in proficiency testing by interlaboratory comparison
ISO/IEC 17043:2010, Conformity assessment — General requirements for proficiency testing
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3534-1, ISO 3534-2, ISO 5725-1,
ISO 13528, ISO/IEC 17043 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
ISO 22117:2019(E)
NOTE 1 Some terms used in the text have different meanings in microbiology and statistics, e.g. homogeneity,
heterogeneity, test, sample, distribution. The context clarifies whether the terms refer to microbiological test
samples or data sets used for statistical analysis.
NOTE 2 Some providers of proficiency testing use the term “external quality assessment” (EQA) to indicate
schemes with broader application to all areas of operation of a laboratory and a particular educational remit. The
requirements of this document cover those EQA activities that meet the definition of proficiency testing.
3.1
target organism
microorganism that is the designated analyte for a proficiency testing sample
3.2
background flora
microorganisms included in a proficiency testing sample that are naturally present or can be introduced
to compete with or mimic the target microorganism
3.3
matrix
all the components of the sample
[SOURCE: ISO 16140-1:2016, 2.38, modified — In the term, “(product)” has been removed.]
3.4
reference strain
microorganism obtained directly from an official culture collection or reference laboratory and defined
to at least the genus and species level, catalogued and described according to its characteristics and
preferably originating from food, food production areas, primary production stages, animals or water,
as applicable
[SOURCE: ISO 11133:2014, 3.4.2, modified — In the term, “an official culture collection or reference
laboratory” has replaced “a reference culture collection, i.e. a culture collection, which is a member of
the World Federation of Culture Collections (WFCC) or the European Culture Collections’ Organisation
(ECCO)”, and “food production areas, primary production stages, animals" has replaced “animal feed,
the food or feed production environment”.]
3.5
recovery percentage
proportion of the assigned value of the target organism (3.1) recovered by the participant
Note 1 to entry: The recovery percentage is calculated by multiplying by 100 the number of recovered colony-
forming units (cfu) per volume or per mass and dividing by the assigned value.
Note 2 to entry: The recovery percentage can be significantly below 100 % when competitive microflora and
matrix effects are present in a proficiency testing sample.
4 Scheme design and purpose
4.1 General
General requirements for designing PT schemes are given in ISO/IEC 17043. This clause discusses
areas requiring special consideration for microbiological PT schemes in the context of these general
principles.
4.2 Scheme objectives
The primary objective of any PT scheme is to provide information to enable laboratories to have
confidence in the reliability of their results.
2 © ISO 2019 – All rights reserved

ISO 22117:2019(E)
The detailed requirements for a documented plan of a PT scheme are covered in ISO/IEC 17043:2010,
4.4.1.3, and the plan should also include reference to any relevant legislation. An example of a plan for a
typical microbiology food examination scheme is given in Annex A.
The studies required to establish a new PT scheme are extensive and shall be clearly defined in
the scheme objectives. These should include, as a minimum, the requirements listed in Clause 5.
Requirements for checking individual rounds of testing, including homogeneity and stability testing,
should also be established in the scheme design and be appropriate for the scheme objectives.
4.3 Laboratory requirements for schemes
General requirements for appropriate laboratory facilities to handle all aspects of PT schemes are given
in ISO/IEC 17043:2010, 4.3.1, and safety requirements are covered in ISO/IEC 17043:2010, 4.6.2.4.
For microbiology schemes, providers shall have a documented policy to bring hazards to the attention
of participants and ensure that relevant safety advice is given (see Clause 7). For example, food
microbiology laboratories shall have facilities for dealing with microorganisms of biosafety levels 1
and 2, as appropriate (see ISO 7218).
4.4 Choice of test matrices
General requirements to document test matrices in the scheme plan are given in ISO/IEC 17043:2010,
4.4.1.3, and choice of the matrices to reflect routine sample types in ISO/IEC 17043:2010, 4.4.2.3.
The reasons for the choice of matrix type should be stated (e.g. to provide levels of sample stability and
homogeneity that are fit for the intended purpose of the scheme).
The description of the test items shall specify the sample matrix (natural or simulated); whether
artificially or naturally contaminated; the source and country of origin to comply with international
transport regulations; and any method of preservation used (e.g. freeze-dried, air-dried).
4.5 Information on test methods used by the PT provider
The general requirements for methods to be used by the PT provider are given in ISO/IEC 17043:2010,
4.4.1.3.
If the scheme is targeted at one or more tests specified in or required by legislation, the routine quality
control tests on the scheme samples (e.g. homogeneity and stability) shall be undertaken in accordance
with the methods stipulated in that legislation and this shall be stated (ISO/IEC 17043:2010, 4.5.1).
Participants shall be encouraged to use their routine methods but, where they are undertaking tests
in accordance with legislation, some degree of guidance shall be given, e.g. reference to ISO methods,
legislative texts, or peer-reviewed publications (ISO/IEC 17043:2010, 4.5.1).
4.6 Statistical design
General requirements for statistical design are given in ISO/IEC 17043:2010, 4.4.4.
An outline of the statistical design for PT schemes for microbiology shall indicate that the statistical
tests to be used are influenced by the level of homogeneity of the test material which, in turn, is
influenced by the random variation in distribution of the microorganisms.
Except for low numbers, a log-normal distribution is usually expected in quantitative testing data and
suitable statistical analysis methods shall be used for such data [ISO/IEC 17043:2010, B.3.1.4 d)]. Where
low numbers are required in quantitative test items (e.g. water or beverage examination), a Poisson
distribution is more applicable, as the variation in numbers of organisms between different units of
material becomes relatively large and can mask variations in performance.
Sample homogeneity shall normally be sufficient, such that it does not significantly influence the
observed variation between laboratories.
ISO 22117:2019(E)
Semi-quantitative enumeration tests and qualitative detection tests require different statistical
methods to analyse data and these are discussed further in 8.3 and 8.4.
The scheme plan shall clarify distinctions between performance testing for methods for detection and
those for enumeration (or quantification for viruses) of target microorganisms.
5 Technical requirements and guidance for sample design and content
5.1 Sources, characterization and traceability of organisms
The characteristics of the target organisms shall be established before use to assess performance
reliably, especially in schemes where participants may use different methodologies.
Target viruses shall give expected results when tested by reference methods. Target parasites can be
identified by microscopy or molecular methods depending on their size and/or other characteristics.
Both typical and atypical strains of target bacteria should be considered and included in the scheme
programme to challenge laboratory performance.
Recognized reference strains from international collections or reference laboratories should be used
where they are most suitable for the scheme purpose; however, laboratory isolates or so-called “wild”
strains isolated from the matrices used by PT schemes are useful to reflect routine situations more
closely. Where these are used, they should be sufficiently characterized according to the appropriate
International Standard reference methods, to ensure that any atypical reactions are apparent to the
organizers before use.
NOTE Strains, particularly wild isolates, can adapt to culture media and environment unless the number of
passages is kept to a minimum.
Spore suspensions can be used to inoculate samples intended to enumerate moulds as these help to
improve stability and homogeneity. A method to prepare these is given in Annex B.
In all cases, the organisms used in PT scheme samples should be traceable to the relevant reference
source or to valid characterization data held by the organizers.
Under certain circumstances, it is not possible to use reference cultures or materials from
internationally recognized collections or cultured laboratory strains, for example, PT schemes for non-
cultivable organisms such as human noroviruses or parasites. Naturally contaminated samples may
be used, if available, or clinical material can be used to contaminate (“spike”) a test matrix artificially,
either through immersion, spraying or, in the case of bivalve shellfish, through bioaccumulation.
The method of artificial contamination should be as close to the “natural” route of contamination as
possible. Extreme care should be used when manipulating human clinical material, faecal or vomitus
samples and these should be screened for additional pathogens before use.
For distributions to be used with serological or molecular methods, it may not be necessary to
distribute live, or even whole, microorganisms. Use of inactivated microorganisms, target antigens or
nucleic acid sequences will often be safer and these may be more stable. Stability of such materials
should be determined by the scheme provider and, in all cases, the targets should give expected results
in reference methods.
5.2 Target organisms level
The target organisms shall be provided at levels suitable to show that examination methods are fit-for-
purpose and to reflect levels likely to be found in the sample matrices being tested (ISO/IEC 17043:2010,
4.4.2.3). Where pathogenic microorganisms are the target, the levels should also take account of and
4 © ISO 2019 – All rights reserved

ISO 22117:2019(E)
reflect the levels likely to cause hazard to human health and, if appropriate, any limits specified in
microbiological criteria.
NOTE The level causing hazard to human health is not always known with accuracy and depends on the
susceptibility of individuals. The main aim of examination for all pathogenic organisms (bacteria, viruses,
parasites) is to prevent illness, but also to detect pathogenic bacteria at a very low level, before they can grow to
a higher level.
For quantitative (enumeration) methods, the target level shall be appropriate for the levels routinely
found in, and any specifications applicable to, the sample matrices used. The target level should also
sometimes be near to the limit of quantification of routine methods to challenge the performance of the
participants across the applicable range of the method. However, samples should not be dispatched with
organism levels so low that, when using routine methods and dilutions, the expected mean number of
organisms in a sample is fewer than 10 colonies per plate or less than 1 MPN/g (< 100 MPN/g for bivalve
molluscs).
For traditional qualitative (detection) methods, target bacteria shall be at a sufficiently low level to
provide a valid challenge to the methods and contribute validation data for performance criteria or
verify levels of detection for individual participant laboratories.
5.3 Non-target organisms and interferences
The total microflora of PT samples, either naturally or artificially contaminated, is usually chosen
to assess the ability of participants to detect and/or enumerate target bacteria in the presence of
background flora. This background flora can include non-target strains typical of the sample matrix and
presumptive target organisms which, without appropriate confirmation tests, can lead to false-positive
results. However, basic schemes intended for specific purposes may provide samples containing only
the target bacteria.
Any strains added to matrices to simulate background flora shall meet the requirements of 5.1 for
characterization and traceability.
Determine any adverse effects of the background flora of artificially contaminated samples on the
target bacteria (e.g. inhibition or other interference) before such samples are used.
5.4 Matrix selection and effects
All matrices shall be evaluated before use to check for any effects on spiked target and background
flora, for example, whether a matrix reduces the recovery percentage of the spiked organisms. It
may be useful to include information for participants on food matrices known to affect recovery of
microorganisms adversely (e.g. those which bind and retain cells, such as fatty materials) or have
bactericidal or bacteriostatic properties.
Include suitable and validated (or verified) preparation procedures for the proficiency samples in the
information for participants.
Sample matrices used for microbiology PT schemes are often, but not necessarily, sterilized before
use. Alternatively, the absence of the target is checked by other means (e.g. use of special sources of
matrices).
Where natural, unsterilized sample matrices are distributed, the organizers shall determine the effect
of any background microflora on the target organisms before use. Also, absence of target organisms in
natural samples is usually required if these are to be artificially spiked. For example, in a PT scheme to
detect parasitic Anisakidae larvae in fish fillets, use only freshwater fish fillets as the larvae are found
only in sea fish.
ISO 22117:2019(E)
6 Sample verification by the provider
6.1 General
General requirements for sample verification are given in ISO/IEC 17043 and ISO 13528. This clause
expands the specific requirements and any particular issues for homogeneity and stability testing in
materials containing living microorganisms.
6.2 Sample homogeneity testing — General considerations
(See also ISO/IEC 17043:2010, 4.4.3 and B.5.)
Proficiency tests may involve the preparation of a bulk test material, which is then subdivided into
individual portions, as similar as possible to each other, for distribution to participants. Alternatively,
test portions may be individually inoculated for distribution.
Whatever preparation method is used, assess the test material for homogeneity, usually before but also
at the time of testing for less stable fresh materials.
Perform a homogeneity test, based on relevant statistical principles (ISO/IEC 17043:2010, 4.4.3.2 and
B.5), on each batch of samples. Such tests are given in ISO 13528 or, as an alternative, Annex C.
The number of samples to be tested from each batch should also be sufficient to obtain ongoing
information on the homogeneity of the batch; 10 samples (tested in duplicate if appropriate) is
suggested.
A test material that is less than sufficiently homogenous may still be used in a proficiency test round
(ISO/IEC 17043:2010, 4.4.3.1, Note 3) provided suitable statistical principles are used to take account of
the greater variance between samples (see ISO 13528). A statistical plan for such materials, including
replicate analysis of several samples (see ISO 5725-5), should be used to minimize the effects of lack of
homogeneity on the evaluation of participant performance.
For parasite detection, each test material is generally spiked with a known number of organisms
and homogeneity checked by microscopy counts of each sample, by at least two operators, since
homogeneous distribution of target parasites in bulk material is difficult to achieve.
For virus methods, the bacteriological terminology used in 6.3 and 6.4 may not be applicable, but
similar principles apply to ensure sufficient homogeneity in distributed PT samples.
6.3 Homogeneity testing for quantitative (enumeration) samples
General requirements and procedures for testing homogeneity of quantitative proficiency test
materials are given in ISO/IEC 17043:2010, 4.4.3 and B.5, and ISO 13528, but alternative procedures
may sometimes be required for microbiological materials.
Materials that show between-unit variation large enough to affect the assessment of laboratory
performance significantly should not be used in interlaboratory studies, unless special requirements
and methods of data analysis apply (e.g. low numbers of microorganisms in drinking water and other
samples).
The criterion for “sufficiently homogenous” is defined by the requirements of the interlaboratory
comparison. However, in general, a material is considered sufficiently homogenous if the between-unit
standard deviation (on the appropriately transformed scale) is ≤ 0,3σ , where σ is the target standard
pt pt
deviation used to assess the performance of laboratories (see ISO 13528).
Any alternative homogeneity test should meet the following criteria:
a) the probability of rejecting a sufficiently homogenous test material should be ≤ 5 %;
6 © ISO 2019 – All rights reserved

ISO 22117:2019(E)
b) the probability of rejecting a test material where between-unit variation is 1,5σ , in which σ is
pt pt
the acceptable between-laboratory variation (expressed as a target standard deviation), is ≥ 80 %.
An 80 % probability of rejecting a material where between-unit variation is 1,5σ is based on simulation
pt
studies of the duplicate analysis of 10 test units using a method with an analytical standard deviation
of 0,5σ (i.e. 0,125 log units) and a critical value for T (see Annex C) that meets criterion a) in the
pt 2
previous paragraph. It represents what is achievable with a reasonable amount of analytical effort.
Homogeneity tests are based upon estimations of between-unit and analytical (repeatability) variance
obtained under repeatability conditions. Suitable methods of testing for such variation are given in
Annex C.
The analytical (repeatability) variance should be estimated from replicate analyses of the initial
[17][18]
suspensions obtained from test portions . This analytical variance can also be calculated from
[13]
the number of counted colonies and the precision of analytical materials in use .
In microbiology, the between-unit variance shall be estimated under repeatability conditions (in one
run). If that is not possible, the between-unit variance will include the within-laboratory reproducibility
and can perhaps lead to false rejection of a satisfactory material.
When the number of counted colonies is sufficiently high (more than 35 to 40 colonies per plate), the
analytical standard deviation, σ , generally satisfies Formula (1):
an
σ
an
< 05, (1)
σ
pt
where σ is the target standard deviation.
pt
In such cases, the test for sufficient homogeneity proposed in Reference [14] should be used (see
Annex C). If the number of counted colonies is low (fewer than 35 to 40 colonies per plate), the T − T
1 2
test is recommended (see Annex C).
When replicated test units are provided to participating laboratories, the between-unit variability
obtained by participants should be examined by the provider to assess the homogeneity of the material.
Although this variability includes within-laboratory reproducibility, the higher number of replicates
increases the statistical power of the analysis and can be a good indicator for successive rounds.
Where the PT material contains low numbers of cfu (e.g. fewer than 20), the between unit (i.e. between
replicate samples) variation shall be measured to demonstrate that it does not exceed random (Poisson)
variation in order to provide a meaningful assessment. The index of dispersion test on n samples (where
n is a minimum of 10) should not exceed the χ value at 0,95 probability.
n−1
6.4 Homogeneity testing for qualitative methods
Similar principles apply to homogeneity testing for qualitative (detection) bacterial methods, but
special consideration is required where the level of target organisms is low (see 8.4).
The homogeneity of qualitative samples for bacteriological methods may be tested by enumerating the
spike. For levels of < 100 cfu/g use a most probable number (MPN) technique and for levels of 100 cfu/g
and above use the appropriate plate count enumeration.
If enumeration is possible, the homogeneity tests detailed in 6.3 may be used, depending on how the
samples are artificially contaminated and on how the spike is enumerated. The number of samples
tested should be proportional to the number of test materials produced.
These approaches may also be used with molecular methods (e.g. detection of viruses in foods). However,
because there ma
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