Haematology Watch

Haematology Watch, Vol.4 , Issue 2.

LABORATORY HAEMATOLOGY

The Case for SI units

B. J. Bain, UK

Medicine has been plagued for decades by the use of multiple units for the measurement of the same biological variable. This can impair communication between physicians and can lead to misunderstandings that may endanger patient safety. For example, a point-of-care instrument in use in the UK was found to be measuring glucose in mmol/l but printed out the results as if they were in mg/dl.

The International System of Units (or SI units, from Système International d’Unités) was established in 1960 by the General Conference on Weights and Measures. It is the only internationally agreed system of units and symbols. Acceptance of SI units within medicine has been slow but in the interests of good communication and patient safety, should be pursued by all of us. The system is based on seven base units (which include the metre, the kilogram, the second and the mole) (Table 1) and derived units; the derived units have their own names and symbols, and are related to the base units by 103, 10-3 or multiples thereof (Table 2); abbreviations of base units take the lower case unless, like the Kelvin and the Ampere, they are derived from a proper name.

Table 1

Base Units and Symbols of the SI (Système International)

Unit (base quantity)	Symbol
Metre (length)	m
Kilogram (weight)	kg
Second (time)	s
Ampere (electric current)	A
Kelvin (thermodynamic temperature)	K
Mole (amount of substance)	m
Candela (luminous intensity)	cd

Table 2

SI (Système International) Prefixes and Abbreviations

Factor	Prefix	Abbreviation	Examples
10-18	atto	a	attogram (ag) or attolitre (al)
10-15	femto	f	femtogram (fg) or femtolitre (fl)
10-12	pico	p	picogram (pg) or picolitre (pl)
10-9	nano	n	nanogram (ng) or nanolitre (nl)
10-6	micro	μ	microgram (μg) or microlitre (μl)
10-3	milli	m	milligram (mg) or millilitre (ml)
10-2	centi	c	centigram (cg) or centilitre (cl)
10-1	deci	d	decigram (dg) or decilitre (dl)
10	deca	da	gram (g)
103	kilo	k	kilogram (kg), kilodalton (kD)
106	mega	M	megabyte
109	giga	G	gigabyte
1012	tera	T
1015	peta	P
1018	exa	E

The SI system also permits the use of certain non-SI units that can be defined precisely in relation to SI units. Those that are used in medicine are shown in Table 3.

Unit (quantity)	Symbol	Definition
Minute (time) Hour (time) Day (time)	min h d	60 s 3600 s 86 400 s
Volume	l or L	1 dm3

It will be noted that, exceptionally, two abbreviations are permitted for the litre, which can be expressed either as a lower case ‘l’ or an upper case ‘L’. The reason is that in some circumstances, and using some fonts, a lower case ‘l’ could be confused with the number ‘1’.

Care must also be taken in use of commas and decimal points. Both a decimal comma and a decimal point are sanctioned by the CGPM (Conférence Générale des Poids e Mesures). However, in English language publications the decimal point is almost always used, and this is recommended for medical publications in English. Because of the use of a decimal comma, the comma should not be used to separate groups of figures. Instead groups of more than four figures should be separated by spaces, thus 3600 and 86 400 in Table 3.

Correct usage is for any figure to be separated from its units by a gap, thus 5 mg not 5mg.

It follows from what is explained above that a white cell count should be expressed in the form 5.4 x 109/l and not as 5400 per mm3. The haemoglobin concentration can be expressed in the format 82 g/l or 82 g/L. It is not recommended that the haemoglobin concentration be expressed in mmol/l as this has the potential to cause confusion and endanger patients and expressing it per litre conforms better to the SI system than expressing it in terms of decilitres.

Many US-published journals that do not yet use SI units give the SI equivalent. If they do not, conversion tables are available.

Barbara J. Bain

Professor in Diagnostic Haematology,

St Mary’s Hospital Campus,

Imperial College London.