In the US, most laboratory test results are reported in what are termed conventional units; the rest of the world reports results in Système International d’Unités (SI) or international units (IU). The unit basis for SI is updated periodically by a panel.

Many SI units are the same as units used in the US system; however, SI units for concentrations are not. SI concentrations are reported as moles (mol) or decimal fractions of a mole (eg, millimole, micromole) per unit volume in liters (L). Conventional units are reported as mass (eg, grams, milligrams) or chemical equivalency (eg, milliequivalents) per unit volume, which may be in liters or decimal fractions of liters (eg, deciliters, milliliters). Results reported in amount per 100 mL (1 dL) are sometimes expressed as percent (eg, 10 mg/dL may be written as 10 mg%).

Moles, milligrams, and milliequivalents: A mole is an Avogadro’s number (6.023 × 10^{23}) of elementary entities (eg, atoms, ions, molecules); the mass of 1 mole of a substance is its atomic weight in grams (eg, 1 mole of sodium = 23 g, 1 mole of calcium = 40 g). Similarly, the mass of a given quantity of substance divided by its atomic weight gives the number of moles (eg, 20 g sodium = 20/23, or 0.87, mol).

An equivalent is a unit that integrates charge and moles; 1 equivalent represents one mole of charges and is calculated by multiplying the number of moles of charged particles in a substance times the valence of that substance. Thus, for ions with a +1 or −1 charge (eg, Na^{+}, K^{+}, Cl−), 1 mole is 1 equivalent (1 × 1 = 1); for ions with a +2 or −2 charge (eg, Ca^{2+}), ½ mole is 1 equivalent (½ × 2 = 1), and so forth for other valence values. A milliequivalent (mEq) is 1/1000 of an equivalent.

The following can be used to convert between mEq, mg, and mmol:

mEq = mg/formula wt × valence = mmol × valence

mg = mEq × formula wt /valence = mmol × formula wt

mmol = mg/formula wt = mEq/valence

(Note: Formula wt = atomic or molecular wt.)

Alternatively, conversion tables are available in print and on the Internet.

Table

Metric System

Unit

Equivalent Subunit

Mass

1 kilogram (kg)

1000 grams (10^{3} g)

1 gram (g)

1000 milligrams (10^{3} mg)

1 milligram (mg)

1000 micrograms (10^{-3} g)

1 microgram (μg)

1000 nanograms (10−^{6} g)

1 nanogram (ng)

1000 picograms (pg; 10−^{9} g)

Volume

1 liter (L)

1000 milliliters (mL)

1 liter (L)

1000 cubic centimeters (cc)

Table

Metric–Nonmetric Equivalents

Metric Unit

Equivalent Nonmetric Unit*

Liquid

30 milliliters (mL)

1 fluid ounce (oz)

250 mL

8+ fluid oz

500 mL

1+ pint

1000 mL (1 liter)

1+ quart

Weight

65 mg

1 grain (gr)

28.35 g

1 oz

1 kg

2.2 pounds (lb)

Linear

1 millimeter (mm)

0.04 inch (in)

1 centimeter (cm)

0.4 in

2.54 cm

1 in

1 meter (m)

39.37 in

Household

4 mL

1 teaspoon (tsp)

5 mL

1 teaspoon, medical

8 mL

1 dessert spoon

15 mL

1 tablespoon (tbsp—½ fluid oz)

240 mL

1 cup (8 fluid oz)

*Approximate.

Table

Atomic Weight of Some Elements Important in Medicine

Element

Symbol

Atomic Weight*

Hydrogen

H

1

Carbon

C

12

Nitrogen

N

14

Oxygen

O

16

Sodium

Na

23

Magnesium

Mg

24

Phosphorus

P

31

Chlorine

Cl

35.5

Potassium

K

39

Calcium

Ca

40

*Approximate.

Table

Centigrade–Fahrenheit Equivalents*

Application

°C

°F

Freezing for water at sea level

0

32

Clinical range

36.0

96.8

36.5

97.7

37.0

98.6

37.5

99.5

38.0

100.4

38.5

101.3

39.0

102.2

39.5

103.1

40.0

104.0

40.5

104.9

41.0

105.8

41.5

106.7

42.0

107.6

Pasteurization (holding),† 30 min at

62.8

145.0

Pasteurization (flash),† 15 sec at

71.7

161.0

Boiling for water at sea level

100.0

212.0

*Conversion:

To convert °F to °C, subtract 32, then multiply by 5/9 or 0.555.

To convert °C to °F, multiply by 9/5 or 1.8, then add 32.

†According to the FDA Code of Federal Regulations, 1991.