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In medicine, precise measurements are necessary—for example, when various substances are measured in laboratory tests to evaluate health or make a diagnosis. Different units of measure may be used depending on the substance. Usually, the metric system, based on multiples of 10, is used to measure mass, volume, and length. Grams measure mass, the amount of matter in an object. Mass is similar to weight, but weight is affected by gravity. Liters measure volume, the amount of space an object occupies. Meters measure length.
Prefixes, indicating which multiple of 10 is meant, can be attached to the basic unit, such as meter (m), liter (L), or gram (g). Using prefixes helps make a number more readable. Commonly used prefixes include kilo (k), deci (d), centi (c), milli (m), and micro (µ).
Other units measure different properties of a substance. For example, a mole (mol) is the number of particles (molecules or ions) in a substance. Regardless of the substance, 1 mole always equals the same number of particles. However, the number of grams in 1 mole varies greatly from substance to substance. One mole equals the molecular (atomic) weight of a substance in grams. For example, the molecular weight of calcium is 40, and 1 mole of calcium equals 40 grams. Osmoles (Osm) and milliosmoles (mOsm) refer to the number of particles in a specific amount of liquid. Equivalents (Eq) and milliequivalents (mEq) measure a substance's ability to combine with another substance. A milliequivalent is roughly equivalent to a milliosmole.
Formulas are used to convert a measurement from one unit to another. The same amount can be expressed in terms of different units. For example, the concentration of calcium in the blood is normally about 10 milligrams in a deciliter (mg/dL), 2.5 millimoles in a liter (mmol/L), or 5 milliequivalents in a liter (mEq/L).
Last full review/revision February 2003
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