Non-SI units - Dynamic NPU Manual
Definition

Measurement unit that is outside the SI system

Note:

Non-SI units have magnitudes (values) that do not belong to the 7 physical dimensions or basic kinds-of-quantity  (length, mass, time, temperature, amount-of-substance, electric current and luminous intensity). Some may be converted to SI units.

Scope

The use of non-SI units should be limited and only used when there are no available and relevant SI units.

Non-SI unit is a heterogenous group comprising of units, recognized at local, regional or international levels. Due to patient safety, it is recommendable to use a commonly used non-SI unit.

Description

The recognition level is determined by the arbiter that establishes the magnitude of a non-SI unit experimentally. The highest-level arbiter can be e.g. WHO or another similar organisation.

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.
Definition

Measurement unit that is outside the SI system

Note:

Non-SI units have magnitudes (values) that do not belong to the 7 physical dimensions or basic kinds-of-quantity  (length, mass, time, temperature, amount-of-substance, electric current and luminous intensity). Some may be converted to SI units.

Scope

The use of non-SI units should be limited and only used when there are no available and relevant SI units.

Non-SI unit is a heterogenous group comprising of units, recognized at local, regional or international levels. Due to patient safety, even though not correct, it is recommendable to use a commonly used non-SI unit.

 

Description

The recognition level is determined by the arbiter that determines the magnitude of a non-SI unit. The highest-level arbiter can be e.g. WHO or another similar organisation.

Examples include units such as hour, or day (versus SI unit of second) or WHO “international units” (IU).

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.
Symbol  IU
Definition A group of heterogenous measurement units with magnitudes determined by WHO or other equivalent organisation
Scope The use of non-SI units should be limited and only used when there are no available and relevant SI units
Description

The definition and magnitude of each “WHO International Unit” are defined by an international certified reference material (CRM), (e.g. a WHO International Standard) for a specific quantity.

For a quantity that has a “WHO International Unit”, the arbitrary kind-of-quantity and the specific CRM must be stated along the corresponding unit.

Example:

NPU19634 Plasma—Ampicillin antibody(IgE); arbitrary substance concentration(IRP 75/502;c203;proc.) = ? × 10³ IU/L

Note 1: “Arbitrary substance concentration” is an arbitrary kind-of-quantity

Note 2: “IRP 75/502” is the identifier of the WHO international standard that has established the magnitude of this specific “IU”. (see Traceability of reference materials of higher order – a short explanation)

Rules when used with SI prefix

The use of SI prefix factors (e.g. 10³) is allowed in descriptions of very small or very large values, because the international CRM has a well-defined magnitude.

SI prefixes (e.g. µ, m, k) are not recommended in combination with IU expressions due to confusion with the symbol for the ‘enzyme unit’, U. E.g. ‘kU/L’ can be mistaken for “kIU/L”, and “mU/L” for “mIU/L”.

Non-recommended symbol U (can be confused with the enzyme unit, U)
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.
Symbol  p.d.u.
Definition A group of heterogenous measurement units with magnitudes determined by proprietary organisations or persons
Scope
Description Procedure defined unit is a placeholder for the symbol (or term) of an non-SI unit that is not defined by an international organisation with the highest hierachal authority. The local laboratory can define and determine the non-SI unit in the placeholder (e.g. arbitrary unit, ELISA unit).

The actual magnitude of the unit depends on the analytical measurement procedure, and it is the responsibility of the laboratory to com-municate the required information for clinical evaluation of the laboratory reports.

Rules when used with SI prefix
Non-recommended 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.
Should anybody be able to establish a proprieraty international unit? It may seem convenient to establish a proprietary international unit (e.g. “company X unit”) when there is no available SI unit. However, it will, in time, create a surplus of proprietary international units that, by their overflooding number, cause confusion and potentially jeopardize patient safety. The number of units should be manegeable and limited.

This recommendation still applies, even though in some cases results may not be comparable using different assays of the same measurement. Information about the analytical procedures of the assays should be attached to the respective results.