17 Measurement Systems

State Standard 22) Calculate correct doses required when given a simulated prescription for a pediatric dose, adult dose, and geriatric dose based on weight (if applicable), length of administration, times per day of administration, and presence of other diseases/disorders.

Objectives Students will be able to… Identify measurements in the metric system Correctly convert measurements while completing dosage calculations.

The Metric System International and scientific standard system of measurement, based on the Meter Measures length (1 meter = ~ 3 feet, 4 inches) Liter Measures volume (1 liter = ~ 4 cups)

The Metric System International and scientific standard system of measurement, based on the Gram Measures weight (1 gram = ~ 1/29 ounce)

The Metric System Prefixes are added to indicate multiples of the base unit.

The Metric System Most commonly used in pharmacy kilo- = 1000 (one thousand of the base unit) centi- = 0.01 (one-hundredth of the base unit) milli- = 0.001 (one-thousandth of the base unit) micro- = 0.000001 (one-millionth of the base unit)

Table 17-1 Metric Units of Measurement.

Table 17-2 Metric System Prefixes with Standard Measures.

The Metric System Guidelines for metric notations Always place the number before the abbreviation. For example, write: 4 mg, not mg 4 Place a zero to the left of the decimal when the decimal is less than 1. For example, write: Synthroid 0.2 mg, not Synthroid .2 mg

The Metric System Guidelines for metric notations Never place a zero to the right of the decimal place when you have a whole number. For example, write: A patient weighs 20 kg, not 20.0 kg Always use decimals to reflect fractions when using the metric system. For example, write: 6.5 mL, not 6 1/2 mL

The Metric System Guidelines for metric notations Avoid unnecessary (trailing) zeros. For example, write: 3.2 g, not 3.20000 g When converting from small units to larger units, make sure your number decreases proportionately. For example, write: 2,000 mg = 2 g

The Metric System Guidelines for metric notations When converting from large units to smaller units, make sure your number increases proportionately. For example, write: 20 kg = 20,000 g When multiplying metric values by multiples of 10, move the decimal point one place to the right for each zero in the multiplier.

The Metric System Guidelines for metric notations When dividing metric values by multiples of 10, move the decimal point one place to the left for each zero in the divisor. When in doubt, do not guess about the correct meaning. Always check when clarification is needed. A one-decimal-place error in dosing can be fatal to a patient.

International Units Drugs like insulin, heparin, and pencillin are measured in IUs (International Units). IU measures a drug in terms of its action, not its physical weight.

International Units Commas are not used in IU unit values unless there are at least 5 digits (25,000 but 2500). Do not use the abbreviation IU as it could be misread "IV", either the Roman numeral or the abbreviation for "intravenous."

Milliequivalent (mEq) Unit of measurement based on the number of grams of a drug in 1 mL of a normal solution. Because different solutions have different densities and weights, the value of the milliequivalent will be different from substance to substance.

The Apothecary System Some medications still use this old English system of measurements (have not yet been replaced by the metric system). Primary unit of weight is the grain (gr).

The Apothecary System Volume/liquid measurements are: Minim Fluid dram Fluid ounce

Table 17-3 Apothecary Weights.

Table 17-4 Apothecary Fluid Measures.

The Avoirdupois Systems Only used for measuring weight (being replaced by the metric system) Based on 1 pound = 16 ounces Most commonly used in the U.S. and still used in Canada and U.K.

Table 17-5 Avoirdupois Weights.

The Household System The system of measurement commonly used in American households, usually related to food and beverages Pharmacy technicians must be familiar with this system to assist patient with measuring at home.

Table 17-7 Household Measures and Metric Equivalents.

Converting Measurements Metric system used almost exclusively in pharmacies but other systems used in certain cases Because pharmacies stock only a fraction of the drugs and dosage forms available, it is sometimes necessary to convert an order to match the stock on hand.

Converting Measurements Converting the household system Memorize conversion factors and use them. Conversions should be performed to and from the metric system.

Converting Measurements Converting the household system Some minor differences Apothecary system: 1 lb = 12 oz Household system: 1 lb = 16 oz Set up proportions as fractions and multiply the fractions to get the correct answer.

Table 17-9 Household to Metric Conversions.

Converting Measurements Converting the apothecary system Ancient system based on grains of wheat is more approximate than exact. The majority of apothecary-unit applications are with older drugs, such as codeine, pheno-barbital, and aspirin.

Table 17-9 Apothecary to Metric Conversion.

Temperature Conversions Celsius (centigrade) international unit of measurement for temperature Fahrenheit American unit of measurement for temperature Simple formula used with basic algebra to solve for either °C or °F: 9C = 5F − 160

Temperature Conversions To convert from Celsius to Fahrenheit: Multiply the Celsius temperature by 1.8 and add 32 to the result, or expressed as a formula: °C = 80 − 32/1.8 To convert from Fahrenheit to Celsius: Subtract 32 from the Fahrenheit temperature and divide the result by 1.8, or expressed as formula °C = °F − 32/1.8

Summary Solid knowledge of the systems of measurement and their units and abbreviations is the foundation for all pharmacy calculations. Every practice setting is individual and unique; use these charts and formulas until they are second nature to you.

Summary Miscalculation of a conversion could have drastic and irrevocable effects on a patient's health.

Activity Complete drug cards for the following… Lorazepam (Ativan) Exetimibe (Zetia) Estrogen (Premarin) Complete the Metric Conversions worksheet on the class website.