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 the following:
Mass: Grams measure mass, the amount of matter in an object. Mass is similar to weight, but weight is affected by gravity.
Volume: Liters measure volume, the amount of space an object occupies.
Length: 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).
+ 35 grams = 58 grams.
Osmolarity is a measure of the number of particles in a liter of liquid, and osmolality is a measure of the number of particles in a kilogram (kg) of liquid. Because 1 liter of water weighs 1 kg, osmolarity and osmolality are the same for substances dissolved in water. An osmole is the amount of a substance that dissolves in liquid to form 1 mole. For example, because table salt dissolves into sodium and chloride in water, one mole of table salt dissolved in 1 liter of water results in 1 mole of sodium and 1 mole of chloride. Thus, its osmolarity is 2 osmoles per liter, and its osmolality is 2 osmoles per kg.
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).
The units used for medical tests vary depending on the substance being measured. The units that are traditionally used in the United States are called conventional units. Conventional units usually express concentration as weight per volume, and the volume can vary. The International System of Units (SI units) always expresses concentration as moles per liter.
