Overview :

SI Units is the abbreviation for le Système international d'Unités  which is the international standard for reporting laboratory data, providing a uniform method which allows comparison of information throughout the world. While endorsed in this country for over 20 years, most laboratory data in the United States is still reported in the "conventional" units used historically; these unfortunately can be arbitrary or confusing.

In addition to simplifying the exchange of information globally, certain practical issues make the SI unit system attractive. For example, the weight of a compound present is of less import than knowing the number of moles or ion equivalents present when calculating metabolic balance equations.

The SI recognizes 7 "base" units, and a number of units derived from these base units.

Units derived from these base units include area, volume, clearance, pressure, density, molality, concentration and energy. For enzyme activity the unit is the katal, which indicates the number of moles of substrate converted per second under defined conditions.

Each specimen has an abbreviation, which can be further specified by a prefix.

When very large or small units are involved, a symbol is used for the appropriate multiple of 10 involved, according to the following table:

Some style conventions (after Young, 1987):

(1) A symbol or abbreviation should be in lowercase, except for kelvin (K), ampere (A) and liter (L).

(2) Symbols are not followed by a period (.) except at the end of a sentence.

(3) Symbols are not followed by an "s" if multiple ones are present.

(4) When numbers are expressed, the associated abbreviation is preferred to the textual term, and this should be separated from the number by a blank space.

(5) Do not mix the textual term (kilogram, etc.) with abbreviations (s, mol, etc.).

(6) A product of units is indicated by a "•" (not "*" or "x").

(7) If more than one solidus ("/") occurs in the expression (example: mL per s per meter square), only one "/" is shown with symbols to the right of it expressed as a product (example: mL / (s • m^2)).

(8) Decimals are preferred to fractions.

(9) A "0" is placed before decimals less than 1.

(10) Spaces are preferred to commas for separating long numbers, with insertion every 3 columns from the right.

Some analytes require that the reference intervals be specified for:

(1) gender

(2) race

(3) age

(4) methodology

Symbols for Expressing non-SI Unit Time Intervals

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