General rules on measurement units - Dynamic NPU Manual

General rules on measurement unit

Purpose

A quantitative estimation of a clinical laboratory analyte must be expressed together with an appropriate unit of measurement. Without units, the result is at best useless, at worst dangerously misleading and a great risk to patient care. Many potential forms of measurement unit are available for a given estimation, however the result will be of most benefit to the patient and clinician if the unit is scientifically relevant and commonly employed by local laboratories.

Recommendation

The International Union of Pure and Applied Chemistry (IUPAC) publishes recommendations for terminology and nomenclature in clinical laboratory medicine in the “Silver Book” (ref. 1).  The International System of Units (“SI units” – Systeme Internationale) offers measurement units based on physical quantities and are normally preferable for laboratory estimations in most parts of the world.

Patient safety

Estimations expressed with no unit create risks for patient management. The recipient of the estimation may assume an incorrect unit, with consequent misinterpretation of the result against a reference or therapeutic interval. Therapeutic drug estimations are an example of how dangerous such misinterpretations may be.

A digoxin result of 1.0 nmol/L communicated without units might be assumed to represent a value of 1.0 in mass concentration units (e.g. µg/L).  In fact a digoxin value of 1.0 nmol/L is equivalent to a value of 0.8 µg/L. A 20% difference is small enough to increase the risk of error of interpretation not being detected but large enough to risk patient health (e.g. if clinician elected to increase the patient’s drug dosage based on the misinterpreted “low” result).

All laboratory estimations, whether communicated verbally, printed report or electronically, must therefore include an appropriate unit of measurement records with the result.

Reference

  1. Ferard G, Dybkaer R, Fuentes-Arderiu X. Compendium of Terminology and Nomenclature of Properties in Clinical Laboratory Sciences : Recommendations 2016. 1 ed: Royal Society of Chemistry; 2016. 182 p. doi:10.1039/9781782622451.

Synonym to measurement unit

Unit, unit of measurement, reference

Definition of measurement unit

Real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number (reference 1)

Scope

Measurement unit is a generic concept of a quantity of any dimension, including non-SI unit and SI unit

Description

The specific kind-of-quantity specifies the type of the corresponding measurement unit, e.g. mmol/L is the corresponding measurement to the kind-of-quantity, substance concentration.

Note: There can be a many-to-many relation between the kind-of-quantity and the type of meaurement unit. Hence, a kind-of-quantity should be stated in the description of the measurement.

Examples

The corresponding measurement unit to mass concentration and density can be µg/L or mg/L

The corresponding measurement unit to mass fraction or volume fraction can be per mille or one out of thousand (symbol: ‰)

Reference

  1. International vocabulary of metrology, https://www.bipm.org/en/publications/guides/vim.html
  2. Ferard G, Dybkaer R, Fuentes-Arderiu X. Compendium of Terminology and Nomenclature of Properties in Clinical Laboratory Sciences : Recommendations 2016. 1 ed: Royal Society of Chemistry; 2016. 182 p. doi:10.1039/9781782622451.
  3. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.

Rule

Reason

Example

One expression per unit

To ensure unambiguity in reporting values, only one expression for a unit of a given magnitude should be used. Various expressions may cause errors in communication between health personnel and organisations with potential patient mistreatments as consequences Millimole per liter

Use multiples and submultiples

To present numerical values in the interval of 0.1–999 and to make values with very large or very small numerical values readable, the units can be combined with SI prefixes, expressed as either SI prefix symbols or SI prefix factors (numerical values)  

One SI prefix per unit

To ensure unambiguity in reporting values, only one expression for a unit of a given magnitude should be used. Various expressions may cause errors in communication between health personnel and organisations with potential patient mistreatments as consequences  

One or SI prefix factors are units for dimensionless kind-of-quantities 

 

 Mass fraction

Substance fraction

 

   

Reference

  1. International vocabulary of metrology, https://www.bipm.org/en/publications/guides/vim.html
  2. Ferard G, Dybkaer R, Fuentes-Arderiu X. Compendium of Terminology and Nomenclature of Properties in Clinical Laboratory Sciences : Recommendations 2016. 1 ed: Royal Society of Chemistry; 2016. 182 p. doi:10.1039/9781782622451.
  3. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.
 

 

Synonym to measurement unit

Unit, unit of measurement, reference

Definition of measurement unit

Real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number (reference 1)

Scope

Measurement unit is a generic concept of a quantity of any dimension, including non-SI unit and SI unit

Description

The specific kind-of-quantity specifies the type of the corresponding measurement unit, e.g. mmol/L is the corresponding measurement to the kind-of-quantity, substance concentration.

Note: There is a many-to-one relation between the kind-of-quantity and the type of meaurement unit. Hence, a kind-of-quantity should be stated in the description of the measurement.

Examples

The corresponding measurement unit to mass concentration and density can be µg/L.

The corresponding measurement unit to mass fraction or volume fraction can be ‰

Reference

  1. International vocabulary of metrology, https://www.bipm.org/en/publications/guides/vim.html
  2. Ferard G, Dybkaer R, Fuentes-Arderiu X. Compendium of Terminology and Nomenclature of Properties in Clinical Laboratory Sciences : Recommendations 2016. 1 ed: Royal Society of Chemistry; 2016. 182 p. doi:10.1039/9781782622451.
  3. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.