WESTGARD QC
ISO Glossary
This glossary covers the official definitions of terms according to ISO standards and guidelines.
GLOSSARY OF ISO METROLOGICAL AND RELATED TERMS AND DEFINITIONS RELEVANT TO CLINICAL LABORATORY SCIENCES
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Introduction
In April 1999 was held in Stockholm the conference "Strategies to set global quality specifications in laboratory medicine" which agreed conclusions are in the document Consensus statement. At this Conference one of the matters discussed and agreed was the need for agreement on concepts, definitions and terms related to quality specifications. Some parts of the content of different vocabularies published by the International Organization of Standardization (ISO) may serve to satisfy this need internationally.
The present glossary is a selection of terms and definitions concerning basic concepts of metrology, chemometrics and qualitology published by ISO which are relevant to clinical laboratory sciences.
In some instances there are some little terminological discrepancies between ISO normative documents prepared by different ISO technical committees. Thus, in order to avoid misunderstandings, some statistical definitions have been lightly modified to harmonize the glossary under a metrological point of view. The symbol '»' added to the bibliographic reference to indicate that this definition has been lightly modified.
In order to make some definitions more understandable to professionals of the clinical laboratory sciences, some notes and examples are not those the original ones.
The layout of this glossary follows the standard ISO 10241:1992 International terminology stadards. Preparation and layout.
Glossary
accuracy (of measurement): closeness of the agreement between the result of a measurement and a true value of the measurand (1)
NOTES
1. Accuracy is a qualitative concept. Its quantitative counterpart is error of measurement.
2. IFCC has used this term with the present meaning of trueness.
basic state: specific state of a system for use as a base for the evaluation of actual states of the system (2)
NOTE
In clinical laboratory sciences the basic state is represented by the zero value of the state variable.
calibration: set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material and the corresponding values realized by standards (1)
characteristic: property that helps to distinguish between items of a given population (3)
combined standard uncertainty (of a measurement): standard uncertainty 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 sum of terms, the terms being variances or covariances of these other particular quantities weighted according to how the measurement result varies with changes in these quantities (2)
conventional true value (of a quantity): value attributed to a particular quantity and accepted, sometimes by convention, as having an uncertainty appropriate for a given purpose (1)
NOTE
Conventional true value is sometimes called 'assigned value' or 'target value',
drift: slow change of a metrological characteristic of a measuring instrument (1)
error (of measurement): result of a measurement minus a true value of the measurand (1)
NOTES
1. When it is necessary to distinguish "error" from "relative error", the former is sometimes called 'absolute error of measurement'.
2. In many instances the error of measurement is called 'total error'.
expanded uncertainty (of a measurement): 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 (4)
influence quantity: quantity that is not the measurand but that affects the result of the measurement (1)
NOTE
In clinical laboratory sciences the component of an influence quantity is usually called 'analytical interferent'
EXAMPLE
bilirubin concentration in the measurement of the concentration of creatininium in plasma.
material measure: device intended to reproduce or supply, in a permanent manner during its use, one or more known values of a given quantity (1)
measurand: particular quantity subject to measurement (1)
measurement procedure: set of operations, described specifically, used in the performance of particular measurements according to a given method (1)
NOTES
1. In a quality system a measurement procedure is recorded as a working instructions document, and should be described in sufficient detail to enable an operator to carry out a measurement without additional information.
2. Metrological characteristics such a repeatability, systematic error or minimum detectable value can be assessed in measurement procedures, not in methods of measurement.
EXAMPLE
IFCC reference procedure for the measurement of the catalytic concentration of alanine aminotransferase in serum.
measurement: set of operations having the object of determining a value of a quantity (1)
measuring instrument: device intended to be used to make measurements, alone or in conjunction with supplementary device(s) (1)
measuring system: complete set of measuring instruments and other equipment assembled to carry out specified measurements (1)
NOTE
Many anlalyzers used in clinical laboratory sciences are measuring systems.
method of measurement: logical sequence of operations, describes generically, used in the performance of measurements (1)
EXAMPLE
Glucose oxydase/molecular absorption spectrometry method for the measurement of glucose concentration in serum.
metrology: science of measurement (1)
minimum detectable value (of the net state variable): true value of the net state variable in the actual state that will lead, with probability (1-b), to the conclusion that the system is not in the basic state (2)
NOTES
1. In clinical laboratory sciences the net state variable is conceptually equivalent to the state variable itself.
2. In clinical laboratory sciences and in analytical chemistry this concept is usually termed 'limit of detection'.
net state variable: difference between the state variable and its value in the basic state (2)
NOTE
In clinical laboratory sciences the net state variable is conceptually equivalent to the state variable itself.
nonconformity: nonfulfilment of a specified requirement (5)
precision: closeness of agreement between independent results of measurement obtained under stipulated conditions (»6)
NOTES
1. Precision is a qualitative concept. Its quantitative counterpart is imprecision, which is computed as a standard deviation or a coefficient of variation of the measurement results.
2. Imprecision depends critically on the specified conditions.
3. Standard deviation expressing imprecision may depend on the value of the measurand; the phenomenon is called heteroscedasticity.
primary standard: standard that is designated or widely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of the same quantity (1)
principle of measurement: scientific basis of a measurement (1)
EXAMPLES
1. Molecular absorption spectrometry.
2. Chemiluminescence.
process in control: process in which each of the quality measures is in a state of statistical control (3)
process quality control: that part of the quality control that is concerned with maintaining process variability within the required limits (3)
quality: totality of features and characteristic of a product, process or service that bear on its ability to satisfy stated or implied needs (5)
quality assurance: all those planned and systematic actions necessary to provide adequate confidence that a product, process or service will satisfy given requirements for quality (5)
quality control: operational techniques and activities that are used to fulfil given requirements for quality (5)
quantity: attribute of a phenomenon, body or substance that may be distinguished qualitatively and determined quantitatively (1)
NOTES
1. The term 'quantity' may refer to a quantity in a general sense or to a particular quantity.
2. IUPAC and IFCC recommend the term 'kind-of-quantity' instead of 'quantity in a general sense' for the field of clinical laboratory sciences.
EXAMPLES
1. Kind-of-quantities: substance concentration, catalytic concentration, number fraction.
2. Particular quantities: substance concentration of glucose in serum of a given patient at a given time, mass rate of protein excretion of a given patient at a given day.
random error: result of a measurement minus the mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions (1)
NOTES
1. Random error is equal to error of measurement minus systematic error.
2. In practice, random error may be estimated from twenty or more repeated measurements of a measurand under specified conditions.
reference material: material or substance one or more of whose property values are sufficiently homogeneous and well established to be used for the calibration of an apparatus, the assessment of a measurement method, or for assigning values to materials (1)
NOTE
Some reference materials have properties which, because they cannot be correlated with an established chemical structure or for other reasons, cannot be measured by exactly defined physical and chemical procedures. Such materials include certain biological reference materials to which an arbitrary unit termed 'international unit' has been assigned by the World Health Organization.
EXAMPLES
1. Human serum SRM 909b from the National Institute of Standards and Technology.
2. Human prolactin IS 84/500 from the World Health Organization.
relative error: error of measurement divided by a true value of the measurand (1)
repeatability (of results of measurements): closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement (1)
NOTES
1. Repeatability is a qualitative concept. Its quantitative counterpart is standard deviation of repeatability or coefficient of variation of repeatability of the measurement results.
2. Repeatability may depend on the value of the measurand.
repeatability conditions: conditions where independent results of measurements are obtained with the same measurement procedure in the same laboratory by the same operator using the same equipment within short intervals of time (»6)
repeatability: precision under repeatability conditions (6)
reproducibility (of results of measurements): closeness of the agreement between the results of measurements of the same measurand carried out under changed conditions of measurement (1)
NOTES
1. The changed conditions may include: principle of measurement, method of measurement, observer, measuring instrument, reference standard, location, conditions of use, time.
2. The set of specified condition is termed 'reproducibility conditions'.
3. Reproducibility is a qualitative concept. Its quantitative counterpart is standard deviation of repeatability or coefficient of variation of repeatability of the measurement results.
4. Reproducibility may depend on the value of the measurand.
reproducibility conditions: conditions where results of measurements are obtained on the same measurand in different laboratories with different conditions (»6)
NOTE
The different conditions should be specified.
reproducibility: precision under reproducibility conditions (6)
result of a measurement: value attributed to a measurand, obtained by measurement (1)
NOTE
A complete statement of the result of a measurement includes information about the uncertainty of measurement.
secondary standard: standard whose value is assigned by comparison with a primary standard of the same quantity (1)
sensitivity: change in the response of a measuring instrument divided by the corresponding change in the stimulus (1)
NOTES
1. The sensitivity may depend on the value of the stimulus.
2. Sensitivity is not synonym of minimum detectable value (limit of detection).
3. Nosographic sensitivity (also called 'diagnostic sensitivity') should be clearly differentiated from (metrological) sensitivity.
specification: document that prescribes the requirements with the product, process or service has to conform (5)
standard (of measurement): material measure, measuring instrument, reference material or measuring system intended to define, realize, conserve or reproduce a unit of measurement or one or more values of a quantity to serve as a reference (1)
standard uncertainty (of a measurement): uncertainty of the result of a measurement expressed as a standard deviation (4)
state of statistical control: state in which the variations among the observed sampling results can be attributed to a system of chance causes which does not appear to change with time (3)
NOTE
In clinical laboratory sciences "sampling results" are the results obtained with control materials.
state variable: quantity describing the state of a system (2)
NOTE
State variable is conceptually the same that particular quantity.
EXAMPLE
Substance concentration of a component in serum.
statistical quality control: that part of quality control in which statistical techniques are used (3)
systematic error: mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions minus a true value of the measurand (1)
NOTES
1. Systematic error is equal to error of measurement minus random error.
2. Systematic error may be constant or proportional to the value of the measurand.
3. In practice systematic error is estimated from twenty or more repeated measurements of a measurand under specified conditions.
4. In many instances the systematic error is called 'bias', but the International Vocabulary of Basic and General Terms in Metrology (1) only uses this term as a characteristic of a measuring instrument.
5. Systematic error is practically equivalent to the IFCC classical concept of "inaccuracy".
tolerance interval: variate values between and including tolerance limits (3)
tolerance limits: specified variate values giving upper and lower limits to permissible values (3)
traceability: property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties (1)
true value (of a quantity): value consistent with the definition of a given particular quantity (1)
NOTES
1. This is a value that would be obtained by a perfect measurement. True values are by nature indeterminate.
2. The indefinite article 'a', rather than the definite article 'the' is used in conjunction with 'true value' because there may be many values consistent with the definition of a given particular quantity.
trueness: closeness of agreement between the mean obtained from a large series of results of measurement and a true value or a conventional true value (»6)
NOTES
1. Trueness is a qualitative concept. Its quantitative counterpart is systematic error.
2. Trueness is practically equivalent to the IFCC classical concept of "accuracy".
uncertainty of measurement: parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand (1,4)
NOTE
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.
unit (of measurement): particular quantity, defined and adopted by convention, with which other quantities of the same kind are compared in order to express their magnitudes relative to that quantity (1)
value (of a quantity): magnitude of a particular quantity generally expressed as a unit of measurement multiplied by a number (1)
working standard: standard that is used routinely to calibrate or check material measures, measuring instruments or reference materials (1)
NOTE
In clinical laboratory sciences a working standard is usually called 'calibration material' or simply 'calibrator'.
Biographical details of Xavier Fuentes-Arderiu, Pharm.D., Ph.D.
Dr. Fuentes-Arderiu graduated in pharmacy at the University of Barcelona in 1973, and received a Ph.D. in 1978 at the same university. In 1975 he joined the Catalan Institute for Health, working in non-hospital clinical laboratories as general clinical pathologist during five years, then he moved to an university hospital, within the same Institute, where he is the quality manager of the clinical biochemistry service. In 1992 he became assistant professor of clinical biochemistry in the University of Barcelona.
Besides his usual task at hospital and university, he has served during ten years as associate, titular member and chairman of the Commission/Committee on Nomenclature, Properties and Units of IUPAC/IFCC. He is also the chairman of the IFCC Working Group on Spanish terminology and Nomenclature in Clinical Chemistry, and titular member of the IUPAC Interdivisional Committee on Nomenclature and Symbols.
He is actively involved in the Technical Committee 212 of the International Organization for Standardization (ISO), the Technical Committee 140 of the European Committee for Standardization (CEN), and the Technical Committee 129 of the Spanish Association for Standardization and Certification (AENOR), all three committees devoted to the standardization of clinical laboratories and in vitro diagnostic products. He is vice-president of the Catalan Association for Clinical Laboratory Sciences and member of the Technical Council and chairman of the Commission on Metrology of the Spanish Society of Clinical Biochemistry and Molecular Pathology.
He has produced more than two hundred publications, including research articles, scientific letters, chapters in books and books, on a diversity topics related to clinical laboratory sciences, such as qualitology, metrology, chemometrics, scientific terminology and biometrics applied to clinical laboratory data communication and interpretation.
References
- International Bureau of Weights and Measures, International Electrotechnical Commission, International Organization for Standardization, International Organization of Legal Metrology, International Federation of Clinical Chemistry, International Union of Pure and Applied Chemistry and International Union of Pure and Applied Physics. International Vocabulary of Basic and General Terms in Metrology. ISO: Geneva, 1993.
- International Organization for Standardization. Capability of detection - Part 1: Terms and definitions. ISO 11843-1. ISO: Geneva, 1997.
- International Organization for Standardization. Statistics - Vocabulary and Symbols. Part 2: Statistical quality control. ISO 3534-2. Geneva: ISO, 1993.
- International Organization for Standardization, International Electrotechenical Commission, International Organization of Legal Metrology, International Bureau of Weights and Measures. Guide to the expression of uncertainty in measurement. Geneva: ISO, 1993.
- International Organization for Standardization. Quality management and quality assurance - Vocabulary. ISO 8402. Geneva: ISO, 1994.
- International Organization for Standardization. Statistics - Vocabulary and Symbols. Part 1: Probability and general statistical terms. ISO 3534-1. Geneva: ISO, 1993.