EN ISO 19036:2019

SLOVENSKI STANDARD
01-januar-2020
Mikrobiologija v prehranski verigi - Ocena merilne negotovosti pri kvantitativnem
določanju (ISO 19036:2019)
Microbiology of the food chain - Estimation of measurement uncertainty for quantitative
determinations (ISO 19036:2019)
Mikrobiologie der Lebensmittelkette - Feststellung von Messunsicherheiten bei
quantitativen Bestimmungen (ISO 19036:2019)
Microbiologie de la chaîne alimentaire - Estimation de l'incertitude de mesure pour les
déterminations quantitatives (ISO 19036:2019)
Ta slovenski standard je istoveten z: EN ISO 19036:2019
ICS:
07.100.30 Mikrobiologija živil Food microbiology
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 19036
EUROPEAN STANDARD
NORME EUROPÉENNE
November 2019
EUROPÄISCHE NORM
ICS 07.100.30
English Version
Microbiology of the food chain - Estimation of
measurement uncertainty for quantitative determinations
(ISO 19036:2019)
Microbiologie de la chaîne alimentaire - Estimation de Mikrobiologie der Lebensmittelkette - Feststellung von
l'incertitude de mesure pour les déterminations Messunsicherheiten bei quantitativen Bestimmungen
quantitatives (ISO 19036:2019) (ISO 19036:2019)
This European Standard was approved by CEN on 21 September 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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, 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 19036:2019 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 19036:2019) has been prepared by Technical Committee ISO/TC 34 "Food
products" in collaboration with Technical Committee CEN/TC 463 “Microbiology of the food chain” the
secretariat of which is held by AFNOR.
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 May 2020, and conflicting national standards shall be
withdrawn at the latest by May 2020.
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.
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, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 19036:2019 has been approved by CEN as EN ISO 19036:2019 without any modification.

INTERNATIONAL ISO
STANDARD 19036
First edition
2019-10
Microbiology of the food chain —
Estimation of measurement
uncertainty for quantitative
determinations
Microbiologie de la chaîne alimentaire — Estimation de l'incertitude
de mesure pour les déterminations quantitatives
Reference number
ISO 19036:2019(E)
©
ISO 2019
ISO 19036:2019(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
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Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
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Published in Switzerland
ii © ISO 2019 – All rights reserved

ISO 19036:2019(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols . 4
4 General considerations . 5
5 Technical uncertainty . 6
5.1 Identification of main sources of uncertainty . 6
5.1.1 General aspects . 6
5.1.2 Sampling uncertainty . 7
5.1.3 Bias . 7
5.1.4 Critical factors . 7
5.2 Estimation of technical uncertainty . 8
5.2.1 General aspects . 8
5.2.2 Reproducibility standard deviation derived from intralaboratory
experiments, s . 8
IR
5.2.3 Reproducibility standard deviation derived from interlaboratory studies .13
6 Matrix uncertainty .14
6.1 General aspects .14
6.2 Case of homogeneous laboratory (or test) sample .15
6.3 Multiple test portions from laboratory samples .15
6.4 Known characteristic of the matrix .16
7 Distributional uncertainties .17
7.1 General aspects .17
7.2 Colony-count technique — Poisson uncertainty .17
7.3 Colony-count technique — Confirmation uncertainty .17
7.4 Most probable number uncertainty .18
8 Combined and expanded uncertainty .19
8.1 Combined standard uncertainty .19
8.1.1 General considerations .19
8.1.2 Combined standard uncertainty based on separate technical, matrix, and
distributional standard uncertainties .19
8.1.3 Combined standard uncertainty based on reproducibility standard
deviation alone .20
8.2 Expanded uncertainty .20
8.3 Worked examples .20
8.3.1 Example 1 — Technical, matrix and Poisson components of uncertainty .20
8.3.2 Example 2 — Poisson component negligible .20
8.3.3 Example 3 — Poisson, matrix and confirmation components .21
8.3.4 Example 4 — Technical, matrix and most probable number components .21
9 Expression of measurement uncertainty in the test reports .22
9.1 General aspects .22
9.2 Results below the limit of quantification .23
9.2.1 General aspects .23
9.2.2 Example .23
Annex A (informative) Calculation of standard deviations with two or more than two
test portions (intralaboratory reproducibility standard deviation and matrix
uncertainty standard deviation).25
ISO 19036:2019(E)
Annex B (informative) Matrix effect and matrix uncertainty .30
Annex C (informative) Intrinsic variability (standard uncertainty) of most probable
number estimates .32
Annex D (informative) Correction of experimental standard deviations for unwanted
uncertainty components .34
Bibliography .37
iv © ISO 2019 – All rights reserved

ISO 19036:2019(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 19036:2006, which has been technically revised. It also
incorporates the amendment ISO/TS 19036:2006/Amd.1:2009. The main changes compared with the
previous edition are as follows:
— provision has been made for the estimation of technical uncertainty, and also for other relevant
sources of uncertainty involved in quantitative microbiological tests, relating to:
— the matrix uncertainty (i.e. the uncertainty due to dispersion of microbes within the actual test
matrix);
— the Poisson uncertainty that relates to colony count techniques;
— the confirmation uncertainty associated with tests to confirm the identity of specific organisms
following a count for presumptive organisms;
— the uncertainty associated with most probable number (MPN) estimates;
— the experimental design for the estimation of intralaboratory reproducibility standard deviation
described in this document in connection with the technical uncertainty is now the same as the
design described in ISO 16140-3 for the verification of quantitative methods;
— worked examples have been added to illustrate ways in which uncertainty estimates should be
generated and reported;
— annexes have been added to provide details of some of the important, or alternative, procedures and
issues associated with uncertainty estimation.
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 19036:2019(E)
Introduction
The term “measurement uncertainty” (MU) is used to denote the lack of accuracy (trueness and
precision) that can be associated with the results of an analysis. In the context of quantitative
microbiology, it provides an indication of the degree of confidence that can be placed on laboratory
estimates of microbial numbers in foods or other materials.
ISO/IEC Guide 98-3 (also known as the “GUM”) is a widely adopted reference document. The principal
approach of ISO/IEC Guide 98-3 is to construct a mathematical or computer measurement model that
quantitatively describes the relationship between the quantity being measured (the measurand) and
every quantity on which it depends (input quantities). That measurement model is then used to deduce
the uncertainty in the measurand from the uncertainties in the input quantities.
ISO/IEC Guide 98-3 recognizes that it might not be feasible to establish a comprehensive mathematical
relationship between the measurand and individual input quantities and that in such cases the effect of
several input quantities can be evaluated as a group. ISO/IEC 17025 also recognizes that the nature of
the test method can preclude rigorous calculation of measurement uncertainty.
In the case of the microbiological analysis of samples from the food chain, it is not feasible to build
a comprehensive quantitative measurement model, since it is not possible to quantify accurately the
contribution of each input quantity, where:
— the analyte is a living organism, whose physiological state can be largely variable;
— the analytical target includes different strains, different species or different genera;
— many input quantities are difficult, if not impossible, to quantify (e.g. physiological state);
— for many input quantities (e.g. temperature, water activity), their effect on the measurand cannot be
described quantitatively with adequate precision.
For the reasons given above, this document mostly uses a top-down or global approach to MU, in which
the contribution of most input quantities is estimated as a standard deviation of reproducibility of the
final result of the measurement process, calculated from experimental results with replication of the
same analyses, as part of the measurement process. These quantities reflect operational variability and
result in technical uncertainty. In food chain quantitative microbiology, assigned values or reference
quantity values are usually not available so bias (which quantitatively expresses the lack of trueness)
cannot be reliably estimated and is not included in the uncertainty estimated by this document.
While reproducibility provides a general estimate of uncertainty associated with the measurement
method, it might not reflect characteristics associated with matrix uncertainty, resulting from the
distribution of microorganisms in the food matrix.
Also, microbiological measurements often depend on counting or detecting quite small numbers of
organisms that are more or less randomly distributed leading to intrinsic variability between replicates
and a corresponding distributional uncertainty. For colony-count techniques, the Poisson uncertainty
is determined, to which may be added, in certain cases, an uncertainty linked to confirmation tests
used to identify isolated organisms. An additional uncertainty component is also required for most
probable number (MPN) determinations. Relevant distributional uncertainty components, estimated
from statistical theory, are calculated from individual experimental data.
These three different kinds of uncertainty (technical, matrix and distributional uncertainties) are
combined using the principles of ISO/IEC Guide 98-3. This approach is similar to that followed by
ISO 29201 in the field of water microbiology.
Technical uncertainty is often the largest of these three kinds and is estimated from a reproducibility
standard deviation, which inevitably includes some contributions from the other two kinds. The
preferred estimate of technical uncertainty is based on intralaboratory reproducibility, in the same
way as ISO 16140-3. If consistent with laboratory protocols and client requirements, a general value of
uncertainty may be reported as based only on a reproducibility standard deviation.
vi © ISO 2019 – All rights reserved

INTERNATIONAL STANDARD ISO 19036:2019(E)
Microbiology of the food chain — Estimation of
measurement uncertainty for quantitative determinations
1 Scope
This document specifies requirements and gives guidance for the estimation and expression of
measurement uncertainty (MU) associated with quantitative results in microbiology of the food chain.
It is applicable to the quantitative analysis of:
— products intended for human consumption or the feeding of animals;
— environmental samples in the area of food production and food handling;
— samples at the stage of primary production.
The quantitative analysis is typically carried out by enumeration of microorganisms using a colony-
count technique. This document is also generally applicable to other quantitative analyses, including:
— most probable number (MPN) techniques;
— instrumental methods, such as impediometry, adenosine triphosphate (ATP) and flow cytometry;
— molecular methods, such as methods based on quantitative polymerase chain reaction (qPCR).
The uncertainty estimated by this document does not include systematic effects (bias).
2 Normative references
There are no normative references in this document.
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions 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/
3.1.1
sample
one or more items (or a proportion of material) selected in some manner from a population
(or from a large quantity of material) intended to provide information representative of the population,
and, possibly, to serve as a basis for a decision on the population or on the process which had produced it
[SOURCE: ISO/TS 17728:2015, 3.2.2, modified — Note 1 to entry has been deleted.]
3.1.2
laboratory sample
sample (3.1.1) prepared for sending to the laboratory and intended for inspection or testing
[SOURCE: ISO 6887-1:2017, 3.1]
ISO 19036:2019(E)
3.1.3
test sample
sample (3.1.1) prepared from the laboratory sample (3.1.2) according to the procedure specified in the
method of test and from which test portions (3.1.4) are taken
Note 1 to entry: Preparation of the laboratory sample before the test portion is taken is infrequently used in
microbiological examinations.
[SOURCE: ISO 6887-1:2017, 3.4]
3.1.4
test portion
measured (volume or mass) representative sample (3.1.1) taken from the laboratory sample (3.1.2) for
use in the preparation of the initial suspension
Note 1 to entry: Sometimes preparation of the laboratory sample is required before the test portion is taken, but
this is infrequently the case for microbiological examinations.
[SOURCE: ISO 6887-1:2017, 3.5]
3.1.5
measurand
particular quantity subject to measurement
[SOURCE: ISO/IEC Guide 98-3:2008, B.2.9 modified — The example and the Note 1 to entry have been
deleted.]
3.1.6
trueness
measurement trueness
closeness of agreement between the average of an infinite number of replicate measured quantity
values and a reference quantity value
Note 1 to entry: Trueness is not a quantity and thus cannot be expressed numerically, but measures for closeness
of agreement are given in ISO 5725 (all parts).
Note 2 to entry: Trueness is inversely related to systematic measurement error, but is not related to random
measurement error.
Note 3 to entry: “Measurement accuracy” should not be used for “trueness” and vice versa.
[SOURCE: ISO/IEC Guide 99:2007, 2.14, modified — The preferred term has been changed from
“measurement trueness” to “trueness”.]
3.1.7
bias
measurement bias
estimate of a systematic measurement error
[SOURCE: ISO/IEC Guide 99:2007, 2.18, modified — The preferred term has been changed from
“measurement bias” to “bias”.]
3.1.8
intralaboratory reproducibility
intermediate precision
closeness of agreement between test results obtained with the same method on the same or similar test
materials in the same laboratory with different operators using different equipment
[SOURCE: ISO 8199:2018, 3.6]
2 © ISO 2019 – All rights reserved

ISO 19036:2019(E)
3.1.9
measurement uncertainty
MU
parameter, associated with the result of a measurement, that characterizes the dispersion of the values
that could reasonably be attributed to the measurand (3.1.5)
Note 1 to entry: The parameter may be, for example, a standard deviation (or a given multiple of it), or the half-
width of an interval having a stated level of confidence.
Note 2 to entry: Measurement uncertainty comprises, in general, many components. Some of these components
may be evaluated from the statistical distribution of the results of a series of measurements and can be
characterized by experimental standard deviations. The other components, which also can be characterized
by standard deviations, are evaluated from assumed probability distributions based on experience or other
information.
Note 3 to entry: It is understood that the result of the measurement is the best estimate of the value of the
measurand and that all components of uncertainty, including those arising from systematic effects, such as
components associated with corrections and reference standards, contribute to the dispersion.
[SOURCE: ISO/IEC Guide 98-3:2008, 2.2.3, modified — The preferred term has been changed from
“uncertainty of measurement” to “measurement uncertainty”.]
3.1.10
standard uncertainty
u
uncertainty of the result of a measurement expressed as a standard deviation
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.1, modified — The symbol has been added.]
3.1.11
combined standard uncertainty
u (y)
c
standard uncertainty (3.1.10) of the result of a measurement when that result is obtained from the values
of a number of other quantities, equal to the positive square root of a sum of terms, the terms being the
variances or covariances of these other quantities weighted according to how the measurement result
varies with changes in these quantities
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.4, modified — The symbol has been added.]
3.1.12
expanded uncertainty
U
quantity defining an interval about the result of a measurement that may be expected to encompass a
large fraction of the distribution of values that could reasonably be attributed to the measurand (3.1.5)
Note 1 to entry: The fraction may be regarded as the coverage probability or level of confidence of the interval.
Note 2 to entry: To associate a specific level of confidence with the interval defined by the expanded uncertainty
requires explicit or implicit assumptions regarding the probability distribution characterized by the
measurement result and its combined standard uncertainty (3.1.11). The level of confidence that may be attributed
to this interval can be known only to the extent to which such assumptions may be justified.
Note 3 to entry: An expanded uncertainty U is calculated from a combined standard uncertainty u ( y) and a
c
coverage factor k (3.1.13) using:
U = k × u (y)
c
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.5, modified— The symbol has been added and Note 3 to entry
has been replaced.]
ISO 19036:2019(E)
3.1.13
coverage factor
k
number larger than one by which a combined standard uncertainty (3.1.11) is multiplied to obtain an
expanded uncertainty (3.1.12)
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.6, modified— The symbol has been added and the definition
has been reworded.]
3.1.14
technical uncertainty
uncertainty resulting from operational variability associated with the technical steps of the analytical
procedure
Note 1 to entry: Technical uncertainty includes the variability of the taking, mixing, and dilution of the test portion
(3.1.4) taken from the laboratory sample (3.1.2) to prepare the initial suspension and subsequent dilutions. It also
includes the effects of variability in incubation and media.
Note 2 to entry: Adapted from ISO 29201:2012, 3.4.2.
3.1.15
matrix uncertainty
uncertainty resulting from the extent to which the test portion (3.1.4) is not truly representative of the
laboratory sample (3.1.2)
3.1.16
distributional uncertainty
uncertainty resulting from intrinsic variability associated with the distribution of microorganisms in
the sample (3.1.1), the initial suspension and subsequent dilutions
Note 1 to entry: In microbiological suspensions, intrinsic variability is usually modelled by the Poisson
distribution. When partial confirmation is practised or the MPN principle is used, the resulting distribution may
differ from the Poisson distribution.
Note 2 to entry: Adapted from ISO 29201:2012, 3.4.3.
3.2 Symbols
For the purposes of this document, the following symbols apply.
ΣC for colony-count methods, total number of counted colonies used to calculate the
measurement results
n , n for colony-count methods with partial confirmation, number of presumptive colonies
p c
tested, and number of confirmed colonies, respectively
s reproducibility standard deviation
R
s intralaboratory reproducibility standard deviation
IR
s intralaboratory reproducibility standard deviation, corrected by subtraction of matrix and
IR : c o r r
distributional components
s repeatability standard deviation
r
s repeatability standard deviation, corrected by subtraction of distributional components
r : c o r r
S sum of squares of unwanted components
unwanted
u standard uncertainty
4 © ISO 2019 – All rights reserved

ISO 19036:2019(E)
u distributional standard uncertainty
distrib
u technical standard uncertainty
tech
u confirmation standard uncertainty
conf
u matrix standard uncertainty
matrix
u most probable number standard uncertainty
MPN
u standard uncertainty of the unwanted component
unwanted
u Poisson standard uncertainty
Poisson
u (y) combined standard uncertainty (of output estimate)
c
k coverage factor
U expanded uncertainty (of output estimate) = k × u (y)
c
4 General considerations
MU associated with any measurement value includes multiple components.
As indicated in the Scope (see Clause 1), the uncertainty estimated by this document does not include
contributions from systematic effects (bias). In food chain quantitative microbiology, assigned values
or reference quantity values are usually not available so bias cannot be reliably estimated.
This document considers three distinct types of uncertainty component:
— technical uncertainty;
— matrix uncertainty;
— distributional uncertainty.
Technical uncertainty arises from operational variability and is estimated, using a global approach,
from a reproducibility standard deviation of the final result of the measurement process (see Clause 5).
This global approach means that the technical uncertainty estimate comes from final test results rather
than by calculation using estimates of uncertainty at every individual stage of the test.
Matrix uncertainty arises from imperfect mixing of the laboratory sample, resulting in poor
reproducibility of microbial levels between test portions, which can be large for solid matrices, and
especially for composite food products. Matrix uncertainty is estimated for each kind of matrix
(see Clause 6).
Even for homogeneous materials, the random distribution of microorganisms leads to distributional
uncertainty (see Clause 7), of which three potential kinds are considered in this document. The
relevance of each depends on the method used:
— for colony-count techniques:
— Poisson uncertainty (see 7.2);
— confirmation uncertainty (see 7.3);
— for MPN techniques: MPN uncertainty (see 7.4).
The uncertainty for each distributional uncertainty source is estimated mathematically.
ISO 19036:2019(E)
This document presents two options for estimating the combined uncertainty for a reported
measurement.
a) Technical, matrix and distributional uncertainty components for a reported value may be estimated
separately from each other (see Clauses 5, 6 and 7), after which the three components are combined
(see 8.1.2).
b) A general value of uncertainty may be reported as based only on a reproducibility standard
deviation, if consistent with laboratory protocols and client requirements (see 8.1.3). Technical
uncertainty is indeed often the largest of the three uncertainty components.
5 Technical uncertainty
5.1 Identification of main sources of uncertainty
5.1.1 General aspects
It can be helpful to consider the sources of technical uncertainty usually associated with the main
stages in a microbiological method. Typical sources for colony-count or MPN techniques are:
— taking a test portion from the laboratory (or test) sample;
— preparation of the initial suspension;
— serial dilution;
— inoculation;
— incubation;
— counting of colonies in a colony count technique, and/or detection of growth (as in a MPN technique);
— confirmation (if appropriate).
Figure 1 shows the main sources of uncertainty in food chain microbiology considered in this document.
6 © ISO 2019 – All rights reserved

ISO 19036:2019(E)
Key
the sequential procedures
the factors that affect uncertainty estimation
these factors are not covered by this document
Figure 1 — Main sources of uncertainty in food chain microbiology covered in this document
5.1.2 Sampling uncertainty
Sampling uncertainty, i.e. error associated with the drawing of the laboratory sample from a lot under
[18]
investigation, can contribute significantly to the overall error , but it is not part of the uncertainty
linked to the measurement itself and is not covered by this document.
Matrix uncertainty that arises from the inability of the test portion to perfectly represent a
heterogeneous laboratory sample or test sample is covered in Clause 6. The extent of such inability can
depend on the size of the test portion taken for examination (see ISO 6887-1).
5.1.3 Bias
As indicated in Clauses 1 and 4, MU estimated by this document does not include contributions from
systematic effects that is bias.
5.1.4 Critical factors
Examples of critical technical factors that can influence uncertainty and need to be controlled include:
the source and type of culture media and/or other reagents (such as the ones used for confirmation),
the dilution, inoculation and incubation procedures, the counting techniques (manual or automated),
and changes to the operator or group of operators, etc.
ISO 19036:2019(E)
5.2 Estimation of technical uncertainty
5.2.1 General aspects
Technical uncertainty is estimated from the standard deviation of reproducibility, s , on the final
R
result of the measurement process. As such, technical uncertainty is a characteristic of the method and
technical uncertainty estimated for one method cannot be applied to other methods.
Ongoing estimation of MU should be made to show that the estimate of uncertainty remains relevant
and that the test results are under control. Reassessment of MU estimate shall be made following
changes to any (critical) factor (see 5.1.1 and 5.1.4) that is likely to affect the results obtained with that
method in any significant way.
Three different possibilities are presented in this document for estimation of the standard deviation
of reproducibility. They are based upon repeated measurements of nominally identical material. The
preference order is as follows:
— option 1: intralaboratory reproducibility, i.e. reproducibility estimated within a laboratory (see 5.2.2);
— option 2: reproducibility derived from results of a method validation interlaboratory study (see
5.2.3.1);
— option 3: reproducibility derived from results of an interlaboratory proficiency test (PT) (see
5.2.3.2).
5.2.2 Reproducibility standard deviation derived from intralaboratory experiments, s
IR
5.2.2.1 General aspects
Option 1, intralaboratory reproducibility, is the preferred option for deriving technical MU since it
enables a laboratory to attach the MU value to the results that it reports, in line with the definition of MU.
The experimental protocol described in this clause should take into account as many as possible of the
uncertainty sources identified in (see 5.1).
5.2.2.2 Experimental protocol
5.2.2.2.1 General aspects
The protocol for analysis of exactly two test portions for each laboratory sample is shown in Figure 2,
for which the corresponding calculations are provided in 5.2.2.3. For other cases (i.e. more than two
test portions for some or all laboratory samples), the protocol and calculations are given in Annex A.
For each test method, perform the experimental protocol of Figure 2 for at least ten laboratory samples
and repeat it to give at least two acceptable results for each laboratory sample. 5.2.2.3.1 provides
indications of acceptable values. Depending on the circumstances, this can require more than ten
laboratory samples and/or more than two test portions for each laboratory sample.
The data from different laboratory samples are collected over a period of time as part of a special
exercise or as part of a laboratory’s routine quality management procedure. In that case, it should be
ensured that the experimental design principles in this clause are followed.
8 © ISO 2019 – All rights reserved

ISO 19036:2019(E)
Figure 2 — Experimental protocol for estimation of intralaboratory reproducibility — Two
determinations on each laboratory sample
5.2.2.2.2 Choice of laboratory samples
The estimation of intralaboratory reproducibility is designed to exclude contributions from
heterogeneity within the laboratory sample, so it is not necessary to repeat this estimation for different
matrices, and this estimate may be based on a single matrix.
The calculation (see 5.2.2.3) uses log-transformed data to normalize the intralaboratory reproducibility
variance, so it is not necessary to repeat the experimental protocol for different contamination levels.
However, where possible, the laboratory samples should be chosen to cover the expected natural
variation in contamination levels.
5.2.2.2.3 Samples from interlaboratory proficiency tests
If a laboratory participates in interlaboratory PTs, the results of that laboratory’s analyses may be used
to contribute to the intralaboratory reproducibility estimate of uncertainty, provided that:
a) the PT samples are representative of routine samples analysed by the laboratory (matrix type, test
portion size);
ISO 19036:2019(E)
and
b) the laboratory carries out estimates on two, or more, test portions under different measurement
conditions, as indicated in 5.2.2.2.6.
However, if the intralaboratory reproducibility estimates from PT samples differ widely from in-house
estimates on real samples of a similar type, the differences shall be recognized and recorded since they
can reflect differences in the matrix and microbial inoculum used in the PT samples.
5.2.2.2.4 Preparation of laboratory sample
In order to minimize matrix uncertainty contributions, the laboratory sample or the test sample,
in cases where the laboratory sample is too big to homogenize, should be made as homogeneous as
possible. Laboratory samples that comprise the following should be mixed well prior to drawing test
portions:
— non-viscous liquids and powders (e.g. milk, coconut milk, dried milk);
— minced/finely chopped solids or suspensions/emulsions (e.g. minced meat, mechanically separated
meat, sausage meat, crushed meat, whipped cream, dairy ice cream, soya cream).
Prior to drawing test portions, other laboratory samples or test samples should be mixed using an
appropriate homogenization procedure. For possibilities suited to each type of sample material, see
ISO 6887 (all parts).
5.2.2.2.5 Test portions
Take at least two test portions from each laboratory (or test) sample to allow repeated measurements.
5.2.2.2.6 Initial suspension, artificial contamination (if needed) and conditions of analysis
If artificial contamination is required, perform it in the initial suspension. Detailed procedures for the
preparation of artificially inoculated food are described in ISO 16140-3.
Perform the analyses on each test portion as in routine testing (e.g. preparation of one series of decimal
dilutions, inoculation of one or two plates per dilution)
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