SYSTEMS USED FOR DRUG ADMINISTRATION
Objectives
• List the basic units and subunits of weight, volume, and length of the metric system.
• Explain the rules for changing grams to milligrams and milliliters to liters.
• Give abbreviations for the frequently used metric units and subunits.
• List the basic units of measurement for volume in the household system.
• Convert units of measurement within the metric system and within the household system.
The three systems used for measuring drugs and solutions are the metric, apothecary, and household systems. The metric system, developed in 1799 in France, is the chosen system for measurements in the majority of European countries. The metric system, also referred to as the decimal system, is based on units of 10. Since the enactment of the Metric Conversion Act of 1975, the United States has been moving toward the use of this system. The intention of the act is to adopt the International Metric System worldwide. The metric system is known as the International System of Units, abbreviated as SI units. Eventually, it will be the only system used in drug dosing.
The apothecary system dates back to the Middle Ages and has been the system of weights and measurements used in England since the seventeenth century. It was brought to the United States from England. The system is also referred to as the fractional system because anything less than one is expressed in fractions. In the United States, the apothecary system is rapidly being phased out and is being replaced by the metric system. You may omit the apothecary system if you desire.
Standard household measurements are used primarily in home settings. With the trend toward home care, conversions to household measurements may gain importance.
METRIC SYSTEM
The metric system is a decimal system based on multiples of 10 and decimal fractions of 10. There are three basic units of measurement. These basic units are as follows:
Prefixes are used with the basic units to describe whether the units are larger or smaller than the basic unit. The prefixes indicate the size of the unit in multiples of 10. The prefixes for basic units are as follows:
| Prefix for Larger Unit | Prefix for Smaller Unit | ||
| Kilo | 1000 (one thousand) | Deci | 0.1 (one-tenth) |
| Hecto | 100 (one hundred) | Centi | 0.01 (one-hundredth) |
| Deka | 10 (ten) | Milli | 0.001 (one-thousandth) |
| Micro | 0.000001 (one-millionth) | ||
| Nano | 0.000000001 (one-billionth) | ||
Abbreviations of metric units that are frequently written in drug orders are listed in Table 1-1. Lowercase letters are usually used for abbreviations rather than capital letters.
TABLE 1-1
Metric Units and Abbreviations
| Names | Abbreviations | |
| Weight | Kilogram | kg, Kg |
| Gram | g, gm, G, Gm | |
| Milligram | mg, mgm | |
| Microgram | mcg | |
| Nanogram | ng | |
| Volume | Kiloliter | kl, kL |
| Liter | l, L | |
| Deciliter | dl, dL | |
| Milliliter | ml, mL | |
| Length | Kilometer | km, Km |
| Meter | m, M | |
| Centimeter | cm | |
| Millimeter | mm |
The metric units of weight, volume, and length are given in Table 1-2. Meanings of the prefixes are stated next to the units of weight. Note that the larger units are 1000, 100, and 10 times the basic units (in bold type) and the smaller units differ by factors of 0.1, 0.01, 0.001, 0.000001, and 0.000000001. The size of a basic unit can be changed by multiplying or dividing by 10. Micrograms and nanograms are the exceptions: one (1) milligram = 1000 micrograms, and one (1) microgram = 1000 nanograms. Micrograms and nanograms are changed by 1000 instead of by 10.
TABLE 1-2
Units of Measurement in the Metric System with Their Prefixes
| Weight per Gram | Meaning |
| *1 kilogram (kg) = 1000 grams | One thousand |
| 1 hectogram (hg) = 100 grams | One hundred |
| 1 dekagram (dag) = 10 grams | Ten |
| *1 gram (g) = 1 gram | One |
| 1 decigram (dg) = 0.1 gram (1/10) | One-tenth |
| 1 centigram (cg) = 0.01 gram (1/100) | One-hundredth |
| *1 milligram (mg) = 0.001 gram (1/1000) | One-thousandth |
| *1 microgram (mcg) = 0.000001 gram (1/1,000,000) | One-millionth |
| *1 nanogram (ng) = 0.000000001 gram (1/1,000,000,000) | One-billionth |
| Volume per Liter | Length per Meter |
| *1 kiloliter (kL) = 1000 liters | 1 kilometer (km) = 1000 meters |
| 1 hectoliter (hL) = 100 liters | 1 hectometer (hm) = 100 meters |
| 1 dekaliter (daL) = 10 liters | 1 dekameter (dam) = 10 meters |
| *1 liter (1, L) = 1 liter | 1 metric (m) = 1 meter |
| *1 deciliter (dL) =0.1 liter | 1 decimeter (dm) = 0.1 meter |
| 1 centiliter (cL) = 0.01 liter | 1 centimeter (cm) = 0.01 meter |
| *1 milliliter (mL) = 0.001 liter | 1 millimeter (mm) = 0.001 meter |
Conversion within the Metric System
Drug administration often requires conversion within the metric system to prepare the correct dosage. Two basic methods are given for changing larger to smaller units and smaller to larger units.
Method A
To change from a larger unit to a smaller unit, multiply by 10 for each unit decreased, or move the decimal point one space to the right for each unit changed.
When changing three units from larger to smaller, such as from gram to milligram (a change of three units), multiply by 10 three times (or by 1000), or move the decimal point three spaces to the right.
Change 1 gram (g) to milligrams (mg):
When changing two units, such as kilogram to dekagram (a change of two units from larger to smaller), multiply by 10 twice (or by 100), or move the decimal point two spaces to the right.
Change 2 kilograms (kg) to dekagrams (dag):
