Quantities and Measurement Units

Transcription

1 RCPA-AACB Chemical Pathology Course 2018 Quantities and Measurement Units Graham White Chemical Pathology SA Pathology at Flinders Medical Centre

2 Quantities Quantity: property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference e.g. 4.1 mmol/l means x definition of 1 mole The mole is the SI unit for amount of substance in chemistry defined as: The mass of substance containing the same number of fundamental units as there are atoms in exactly g of 12C. Fundamental units may be atoms, molecules, or formula units, depending on the substance concerned.

3 Use of mole allows quantities to be added, subtracted, multiplied e.g. Calculation of anion gap, osmolality, solubility product for Ca/Phos etc. Adelaide at 20 C and at 40 C has not doubled in temperature Centigrade scale has no true zero

4 Technical Difficulty with using Mole for Many Analytes Analytes must be of known purity and a single structure e.g. Na, Ca, Mg, HbA1c etc. Not TSH, hcg, TropT etc. Which structure is of clinical importance? Require International Units (IU) often arbitrary units of measurement no relation to number of analyte entities

5 Measurements Always a comparison of response by unknown sample with that by reference Reference represents the appropriate measurement unit Quality of results depends on how well a reference represents the measurement unit Clinical biochemists need to know pedigree of calibrators used How are calibrator values assigned? Are calibrators commutable? Are calibrators reliable for comparing patient results with: Reference intervals? With results from other MPs for same measurand?

6 History of Measurement Units (~3000 BC) TRADE

7 Measurement Units Based on Body Parts

8 Foot; stride; arm length (elbow to extended fingers) cubit Smaller units: Finger breadth; 4 digits = 1 palm; 28 digits = 1 cubit Used for local trade unreliable between settlements Standardisation Pharaoh introduced a solid rod Royal Cubit Babylonians ~1700 BC used cubit (530 mm); Egyptian cubit (524 mm) Not reliable as trade spread further

9 1670: Metre invented as 1/10,000,000 of the distance from equator to North Pole on meridian line 1792: Traveller s diary. In France the infinite perplexity of the measures exceeds all comprehension. They differ not only in every province, but in every district and almost every town. ~ 800 names for ~250,000 differently sized measurement units 1795: France adopts metric system 1820: Metric system only legal system in Low Countries 1860s:Made legal in Britain; USA, but not compulsory 1875: Treaty of the Metre Conference (Paris) signed by 17 nations to improve trade and international scientific communication International System of Units (SI) Measurement units should be founded on constants of nature

10 1911: USA 8 different state-based volumes termed gallon Accurate and reliable measurements needed for: manufacturing industry, science, medical diagnostics and therapy, global climate predictions World-wide agreement needed on units of measurement and practical provision of accurate measurement standards for all such purposes Also need to demonstrate equivalence between national measurement standards Assured under the Convention of the Mètre

11 Metrology: Science and Practice of Measurement Objectives are: Stable Long term trends can be used for decision-making Comparable Results from different laboratories can be brought together Coherent Results from different methods can be brought together

12 The objectives of metrology achieved through the framework for metrologically traceable measurements. Traceability the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to measurement uncertainty Note: Traceability is the property of the result of a measurement, not of an instrument or calibration report or laboratory From the International Vocabulary of Basic and General Terms in Metrology; VIM, 3rd edition, JCGM 200:2008

13 SI units bring global coherence to scientific, technological, industrial and commercial measurements 1990s: Need for standard approach to: describing estimating the variability of measurements (MU)

14 Theoretical definition Realization Governed by General Conference of Poids et Mesures

15 BIPM responsible for SI UNITS

16 Planned re-definitions of the SI base units: kilogram, ampere, kelvin and mole will be in terms of constants of nature; based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (kb), and the Avogadro constant (NA), respectively.

17 7 SI Base Measurement Units SI unit of time is the second (s): Defined as the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. Unit of Length: Metre Was two marks on a platinum-iridium rod Now a constant of nature Definition: Length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second (BIPM 1998, p. 94). The international prototype of the kilogram arbitrary artefact made of platinum-iridium kept at the BIPM Realizations of theoretical definitions responsibility of National Metrology Institutes (also provide calibration services)

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20 Comparisons

21 Mole: SI Unit for Amount of Substance in Chemistry secondary RMP matrix-insensitive

22 Essential Features of a High Order Reference Material Simple analytes of unique structure e.g. Mg+; HbA1C Known purity by primary methods independent of MP e.g. Gravimetry; MS; NMR Also needs a High Order Reference Measurement Procedure (RMP) Must comply with specific ISO standards that address features of RMPs

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26 B-E NOT SI-TRACEABLE Termed Conventional References e.g. Hb e.g. Enzyme activity WHO FSH miu/ampule Immunoassays: e.g. tumour markers Must be commutable ASSIGNMENT OF END-USER CALIBRATOR VALUES

27 ISO 17511: 2003 specifies how to assure the metrological traceability of values assigned to calibrators and control materials intended to establish or verify trueness of measurement. EU made compliance with this Standard a Directive on IVD manufacturers. 10 years on No improvement in metrological traceability of patients results EU has now made compliance a Regulation much tougher

28 B-E NOT SI-TRACEABLE: Describes majority of reference materials used by pathology labs Majority of measurement units are arbitrary e.g. FSH etc. uses IU/L Acceptable for comparing patient results: using same MP over time with RI determined by same MP Not acceptable for comparing with results by different labs/methods or MPs

29 Important to check full traceability chain (calibration hierarchy) of IVD kit calibrators

30 Bibliography Update on current concepts and meanings in laboratory medicine - Standardization, traceability and harmonization N Greenberg Clinica Chimica Acta, 15 May 2014; 432: Mini-Review - Traceability in Clinical Enzymology I Infusino, R Bonora, M Panteghini Clinical Biochemist Reviews, November 2007; 28(4): Metrological traceability in clinical biochemistry GH White Annals of Clinical Biochemistry 2011; 48: International vocabulary of metrology Basic and general concepts and associated terms (VIM) JCGM 200:2012: (VIM) Located under Guides in Metrology The International System of Units (SI): BIPM - ISO 17511: Extensive revisions coming

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