6 Measurement Systems and Their Equivalents

Knowledge and Accuracy Healthcare professionals who deal with administration of medications must be: Knowledgeable of weights and measures Accurate in calculations Careful in checking and verifying calculations

Units of Measure Pharmacology uses 3 systems of measurement: Metric: most common, most accurate, and safest Apothecary: older system Household: less accurate

Metric System Based on decimals Gram (g): unit of weight Liter (L): unit of volume Meter (m): unit of length Prefixes denote multiples or fractions of 10

Metric Prefixes (Latin) 0.1 or 1/10 deci- (d) 0.01 or 1/100 centi- (c) 0.001 or 1/1,000 milli- (m) 0.000001 or 1/1,000,000 micro- (mc) Value Prefix

Metric Prefixes (Greek) 1,000,000 mega- (M) 1,000 kilo- (k) 100 hecto- (h) 10 deca- (da) Value Prefix

Metric System 1 cm = 10 mm Length 1 L = 1,000 mL Volume 1 kg = 1,000 g 1 mg = 1,000 mcg = 0.001 g 1 g = 1,000 mg Weight

Système International (SI) Internationalized system of measurement based on gram for weight, liter for volume, and meter for length. 1 g = 1,000 mg = 1,000,000 mcg 1 L = 1,000 mL 1 m = 100 cm = 1,000 mm

Apothecary System Apothecary system measures volume in: Minims (basic unit), drams (fluidrams), ounces, (fluidounces), pints (volume), quarts, gallons Apothecary system measures weight in: Grains (basic unit)

Apothecary System Apothecary system is used for only a few drugs (acetaminophen, aspirin, and phenobarbital).

Apothecary Measurements gallon (gal) quart (qt) = pt ii pint (pt) = qti = 0 32 pound (lb) fluid ounce (℥) = pti = 16℥ ounce (oz or ℥) fluiddram (fl dr) dram (dr or ℨ) Minim (♏) grain (gr) Volume Weight

Household System Household system used in most American homes but is not accurate. Only household units used in measuring drugs are units of volume: Drop Tablespoon Cup Quart Teaspoon Ounce (fluid) Pint Gallon

Household Measurements drop (gtt) 15 gtt = 1 mL teaspoon (tsp) 1 tsp = 5 mL tablespoon (T) 1 T = 3 tsp ounce (oz) 1 oz = 2 T cup 1 cup = 8 oz pint (pt) 1 pt = 2 cups quart (qt) 1 qt = 4 cups = 2 pt gallon (gal) 1 gal = 4 qt

Milliequivalents Milliequivalents (mEq): used to indicate strength of certain drugs Defined as number of grams of equivalent weight of a drug contained in 1 mL of normal solution Some drugs measured in mEq include potassium chloride, sodium bicarbonate, sodium chloride

Units and International Units Units mainly measure potency of heparin, insulin, penicillin, and some vitamins. International Units (IU) are used to measure vitamins. Standardized by international agreement Units and mEq cannot be directly converted into the metric, apothecary, or household systems.

Conversion Within and Between Measurement Systems Drug doses are usually ordered in metric grams, milligrams, liters, and milliliters. You must know how to convert between systems and within systems.

Approximate Volume Equivalents 2 pt = 1 qt 32 oz 960 mL 2 cups = 1 pt 16 oz 480 mL 8 oz = 1 cup 8 oz 240 mL 2 T = 1 oz 1 oz 30 mL 1 T 3–4 dr 15 mL 1 tsp 1 dr 5 mL Household Apothecary Metric

Approximate Weight Equivalents 220 lb 1 kg 2.2 lb, gr xv, 15 gr 1 g (1,000 mg) gr vii ss (7-1/2 gr) 0.5 g gr i (1 gr) 60 mg gr 1/2 30 mg gr 1/4 15 mg gr 1/60 1 mg Apothecary Metric

Temperature Conversion Two common scales measure temperature: Celsius (C) and Fahrenheit (F). Fahrenheit is used mostly in the U.S.; Celsius is used elsewhere. Water freezes at 0°C, and at 32°F. Water boils at 100°C and at 212°F.

Converting from Fahrenheit to Celsius (°C x 1.8) + 32 = °F (°F – 32) × 1.8 = °C OR (°F – 32) × 5/9 = °C

Figure 6-1 Thermometer showing the temperatures at which water freezes (0°C and 32°F) and boils (100°C and 212°F).