ISO/TS 27878:2023

TECHNICAL ISO/TS
SPECIFICATION 27878
First edition
2023-01
Reproducibility of the level of
detection (LOD) of binary methods in
collaborative and in-house validation
studies
Reproductibilité de la limite de détection (LD) des méthodes binaires
pour des études de validation internes et collaboratives
Reference number
© ISO 2023
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
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 2
5 General principles . 3
5.1 General considerations. 3
5.2 Considerations for the conventional approach . 3
5.3 Considerations for the factorial approach . 3
6 Conventional approach . 4
6.1 Experimental design . 4
6.2 Statistical model for methods for continuous measurands. 4
6.3 Statistical model for methods for discrete measurands . 7
6.4 Reliability of precision estimates . 10
7 Factorial approach .10
8 In-house validation .13
9 Software .13
Bibliography .15
iii
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 69, Applications of statistical methods,
Subcommittee SC 6, Measurement methods and results.
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.
iv
Introduction
An appropriate approach for the validation of binary methods will often differ considerably from that
of quantitative methods. Nevertheless, core concepts from the validation of quantitative methods can
be successfully carried over to binary methods. In particular, the precision of a method – a performance
characteristic usually associated with quantitative methods – can be determined for the level of
detection (LOD) of binary methods.
In analytical chemistry, one of the fundamental indicators of method performance is the reproducibility
[1]
of quantitative test results as described in ISO 5725 (all parts) . This aspect of method performance is
not usually taken into consideration in the validation of binary methods. However, in the last few years,
novel validation approaches have been proposed in which the reproducibility of a binary method can be
determined and meaningfully interpreted.
Why is it important to determine a method’s reproducibility? In order to answer this question, consider
an example from the field of microbiology. Take the case that, in the validation study, a method’s LOD
is determined as 3 CFU/ml (CFU = colony forming unit), but that the LOD is sometimes much higher
depending on the laboratory or on the test conditions. Failing to detect the occasional unreliability of
the method could lead to mistakes in routine laboratory determinations. On the other hand, if an LOD
of 300 CFU/ml is obtained in the validation study, the method will not be validated even though this
excessive LOD is not representative of its average performance. Accordingly, both the average LOD
value and the reproducibility parameter – describing the variability of the LOD across laboratories or
test conditions – capture important information about the performance of the method and should be
determined in the course of the validation process.
In order to accomplish this, a suitable approach should be identified for the conversion of the binary
results into quantitative ones. In this standard, two parametric models for the calculation of the LOD
will be used: one model for methods for discrete measurands, e.g. microbiological and Polymerase
Chain Reaction (PCR) methods, and one model for methods for continuous measurands, e.g. chemical
methods.
Two different study designs will be applied. In the conventional approach, test conditions vary randomly
from one laboratory to the other, whereas in the factorial approach, at least to some extent, test
conditions are controlled. The factorial approach makes it possible to assess different sources of errors
such as the variability arising in connection with different analysts, different instruments, different
lots of reagents, different elapsed assay times, different assay temperatures etc. Such an approach also
allows a reduction in workload and fewer participating laboratories.
v
TECHNICAL SPECIFICATION ISO/TS 27878:2023(E)
Reproducibility of the level of detection (LOD) of binary
methods in collaborative and in-house validation studies
1 Scope
This document provides statistical techniques for the determination of the reproducibility of the level
of detection for
a) binary (qualitative) test methods for continuous measurands, e.g. the content of a chemical
substance, and
b) binary (qualitative) test methods for discrete measurands, e.g. the number of RNA copies in a
sample.
The reproducibility precision is determined according to ISO 5725 (all parts).
Precision estimates are subject to random variability. Accordingly, it is important to determine
the uncertainty associated with each estimate, and to understand the relationship between this
uncertainty, the number of participants and the experimental design. This document thus provides not
only a description of statistical tools for the calculation of the LOD reproducibility precision, but also
for the standard error of the estimates.
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 5725-1, Accuracy (trueness and precision) of measurement methods and results — Part 1: General
principles and definitions
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3534-1 and ISO 5725-1 and
the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
factor
binary or quantitative parameter within the method that can be varied at two or more levels within the
limits of the specified method
EXAMPLE Technician.
3.2
factor level
value of the factors (3.1) within the experimental design
EXAMPLE Technician 1, Technician 2, etc.
3.3
level of detection
LOD
concentration from which on the POD (3.4) is not below a specified limit, e.g. 0,5 or 0,95 (LOD or
50%
LOD ).
95%
Note 1 to entry: This definition is mathematically equivalent to the definitions for “level of detection” in
[2] [3] [4]
ISO 16140-1 , ISO 16140-2 and ISO 16140-4 . It differs from the definition used for chemical and physical
methods for which a “limit of detection” is defined as the lowest quantity of an analyte that can be distinguished
from the absence of that analyte with a stated confidence level.
Note 2 to entry: In this document, the term concentration (or concentration level) is used as a generic term to
mean not only the actual concentration in the case of a measurand that can be quantified on a continuous scale,
but also the number of colony forming units or DNA copies per aliquot in the case of measurands which are
quantified on a discrete scale.
3.4
probability of detection
POD
probability of a positive analytical outcome of a qualitative test method at a given concentration for a
specific sample type
Note 1 to entry: This definition is based on the two slightly different definitions for “probability of detection” in
[6]
ISO/TS 16393 and ISO 16140-1, ISO 16140-2 and ISO 16140-4.
Note 2 to entry: The POD is a measure of the probability of a positive analytical result and thus a theoretical
value which can be approximated by a mathematical model.
3.5
rate of detection
ROD
proportion of positive analytical outcomes in a test series, when a qualitative method is performed
several times with a specific sample
Note 1 to entry: The ROD is not a theoretical value based on a mathematical model [like the POD (3.4)] but the
result of a series of repeated tests performed on a given sample.
4 Symbols
p number of participating laboratories
between-laboratory variance
σ
L
POD = P probability of detection
x concentration level (see Note 1 to entry 3.3) at which the POD is calculated
ROD rate of detection
LOD = L 50 % of the level of detection
50% 50
LOD = L 95 % of the level of detection
95% 95
L, H, B, C global model parameters for the four-param
...