Rules for use of SI-prefixes - Dynamic NPU Manual

Rules for use of SI-prefixes in laboratory medicine

Definition SI prefixes are a set of multiples and submulitples within the SI system
Purpose of the use of SI-prefixes

To prevent communication errors due to several expressions of the same unit

To ensure that the numerical values in laboratory results are human readable and understandable

Principles
  • One expression of a unit
  • Numerical values of laboratory measurements should be presented in the range of 0.1-999 (Ref 1)
  • Units for quantities of the same sort of system, sort of component(s), and kind-of-quantity should differ at least by a factor of one thousand to avoid confusions of unit and prefix symbols
  • A unit can be a SI factor (a number as sub- or multiples)
Rules
  • Unit of a given magnitude should only have one expression
  • One SI prefix per unit
  • SI prefix belongs in the numerator only
  • One thousand between SI prefixes (deci (d), centi (c), deca (Da), and hecto (H) are not allowed)
Reference
  1. Health informatics. Expression of results of measurements in health sciences. EN 12435:2006, 6.13
  2. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.
  3. https://www.bipm.org/en/measurement-units/
Factor (SI factor) Term (SI prefix name) Symbol (SI prefix)
10–30 quecto q
10–27 ronto r
10–24 yocto y
10–21 zepto z
10–18 atto a
10–15 femto f
10–12 pico p
10–9 nano n
10–6 micro µ
10–3 milli m
103 kilo K
106 mega M
109 giga G
1012 tera T
1015 peta P
1018 exa E
1021 zetta Z
1024 yotta Y
1027 ronna R
1030 quetta Q

Note: SI prefixes (deci (d), centi (c), deca (Da), and hecto (H) are not allowed or displayed due to a less distance than 1000 between the next SI prefixes.

Background

A measurement unit can be expressed with several combinations of SI prefix and SI factors that may placed in the denominator and numerator.

To avoid confusion and misunderstanding with various different expressions of the same unit, a preferred unit symbol should be used.

Examples

Examples of various unit symbols for the same measurement unit

Note: The last unit symbol, gram per decilitre, is not recommended to use because the distance between g/dL and g/L is only a factor 10.

Reference
  1. Health informatics. Expression of results of measurements in health sciences. EN 12435:2006, 6.13
  2. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.
  3. https://www.bipm.org/en/measurement-units/
Principle Kinds-of-quantity of dimension one (e.g. mass fraction, substance ratio, number concentration) may have “one” or SI factors as corresponding measurement units.
Background

There are several combinations of unit symbols for SI factors.

To avoid confusion and misunderstanding with several unit symbols for the same measurement unit, the measurement unit equation undergoes a linear algebraic reduction.

The linear algebraic reduction will change the unit equation into a simpler form.

Note 1: When SI factors are used as measurement units, it is in particular important to display the kind-of-quantity along the measurement unit, to understand the measurement unit

Note 2: The same SI factor may be used as measurement unit for different kinds-of-quantity

Examples

Example 1

Linear algebraic reduction of a measurement unit equation into a SI factor that is the preferred unit symbol.

Example 2

The same SI factor can be used as corresponding measurement unit for other kinds-of-quantity.

 

Reference
  1. Health informatics. Expression of results of measurements in health sciences. EN 12435:2006, 6.13
  2. Hansen YBL. Recommendations on Measurement Units – Why and How. EJIFCC. 2019;30(3):250-75.
  3. https://www.bipm.org/en/measurement-units/