Chapter 4- Measurement Systems, Units, & Equivalencies

Metric System of Measurements

- 1,000 units Weight: Kilogram Volume: Kiloliter Length: Kilometer - 100 units Weight: Hectogram Volume: Hectoliter Length: Hectometer - 10 units Weight: Dekagram Volume: Dekaliter Length: Dekameter Base Unit - 1/10th unit Weight: Decigram Volume: Deciliter Length: Decimeter - 1/100th unit Weight: Centigram Volume: Centiliter Length: Centimeter - 1/1,000th unit Weight: Milligram Volume: Milliliter Length: Millimeter - 1/1,000,000th unit Weight: Microgram Volume: Microliter Length: Micrometer

Household Measurements of Volume

- Drops: abbreviation (gtt) - Teaspoon: abbreviation (tsp). 1 teaspoon= 60 drops (depending on the size of the dropper & the viscosity of the medication) - Tablespoon: abbreviation (tbsp). 1 tablespoon= 3 teaspoons - Ounce: abbreviation (oz). 1 ounce= 2 tablespoons or 6 teaspoons - Cup: abbreviation (c). 1 cup= 8 ounces - Pint: abbreviation (pt). 1 pint= 2 cups or 16 ounces - Quart: abbreviation (qt). 1 quart= 2 pints, 4 cups, or 32 ounces - Gallon: abbreviation (gal). 1 gallon= 4 quarts, 8 pints, 16 cups, or 128 ounces

Household Measurements of Length

- Inch: abbreviation (in) - Foot: abbreviation (ft). 1 foot= 12 inches - Yard: abbreviation (yd). 1 yard= 36 inches or 3 feet

Electrolytes

Elements such as sodium (Na), potassium (K), magnesium (Mg), & calcium (Ca) that are necessary for normal body functions.

Metric Measurements of Volume in Pharmacy

L __________________ mL 1 L = 1,000 mL

Metric Equivalents Used in Pharmacy

- Liter: abbreviation (l). 1 Liter= 1,000 mL or 1,000 cc - Milliliter, Cubic Centimeter : abbreviation (mL) (cc). 1 milliliter= 0.001 Liter - Gram: abbreviation (g). 1 gram= 1,000 milligrams or 0.001 kilograms - Kilogram: abbreviation (kg). 1 kilogram= 1,000 grams - Milligram: abbreviation (mg). 1 milligram= 0.001 gram or 1,000 micrograms - Microgram: abbreviation (mcg) (μg). 1 microgram= 0.000001 gram or 0.001 milligram - Meter: abbreviation (m). 1 meter= 100 centimeters or 1,000 millimeters

Household Measurements of Weight

- Ounce: abbreviation (oz) - Pound: abbreviation (lb). 1 pound= 16 ounces - Ton: abbreviation (T). 1 ton= 2000 pounds

Technical Note: The Household System

- The household system uses fractions and Arabic numerals. - Teaspoon and tablespoon are common household measurements. - For patient safety, household utensils should not be used except when absolutely necessary. - Household utensils are only approximations. - For correct dosing, use the measuring device provided with the medication.

Technical Notes: The Metric System

- The metric system is a system of "place values" with the base being the unit from which all place values are measured. The gram, meter, & liter are at place 1, or the base site; kilo is at place 103 (the thousands place); milli is 10-3 (the one-thousandths place); & micro is 10-6 (the one-millionths place). - When using the metric system of measurement, the numeral is written before the abbreviation for the quantity, with a full space between the number & abbreviation (e.g., 10 mg, 2.5 mm, 1.5 L). - All fraction parts are written as a decimal number (e.g., 1.5 mL, 2.5 g, or 2.75 m). - Be sure that any number with a value less than zero has a "0" preceding the decimal point, a leading zero. - Any unnecessary, or trailing, zeros should be eliminated. - Avoid using a decimal number if a whole number can be used.

Technical Note: Apothecary System

- The quantity should be expressed in lowercase Roman numerals, frequently with a line placed over the entire numeral and a dot above the line with the numeral i, although this line is not used as often as it used to be. - An amount greater than 10, other than 20 and 30, may be written in Arabic numerals. - Numbers less than 1 are written as fractions, except 12, which is written ss. -Do not combine Roman and Arabic numerals in the same measurement. -The symbol or abbreviation is written before the quantity.

Cubic Centimeter

A cubic centimeter is another way to express a milliliter. One cubic centimeter is the amount of space that is required to hold one milliliter of liquid. The abbreviation "cc" is not recommended because of the danger of misreading it as "00" (double zeros), but it is occasionally still found on prescriptions & medication orders. The preferred abbreviation is mL. The abbreviation"μg"for microgram may be misinterpreted as mg, causing a 1,000-fold error. The preferred abbreviation is mcg.

Unit

A general term covering any quantity chosen as a standard; for a measurement to make sense, it must include a number and a unit; examples of units: mg, mL, teaspoon. The unit measurement is actually assigned according to a medication's activity in that system. Units are specific to each individual medication and do not relate to one another. Example: Units of insulin cannot be compared with units of heparin.

International Unit/Unit

A specific unit of measurement used for biologicals; a measurement of a medication's action as opposed to its weight (as with the units mcg, mg, g); specific to each particular medication.

Milliequivalents (mEq)

A type of unit used to express the concentration of electrolytes that are needed for normal body functions. Example: K-Dur 20 tablets contain 20 mEq of potassium per tablet.

Leading Zero

A zero, called a leading zero, should always be placed before a decimal when the number is less than 1 to prevent confusion and a possible error in interpretation. Example: .25 mg might be misread as 25 mg if the measurement was not written as 0.25 mg. The potential error would be 100 times too much medication being administered to a patient.

Trailing Zeros

Also no zeros, called trailing zeros, should be added at the end of a fractional portion after a decimal point. Example: To prevent errors, 2.50 mg should be written 2.5 mg, and 1.500 L should be written 1.5 L. Zeros found at the end of a number to the right of the decimal point may be removed without changing the value of the number.

Standards

An exact quantity agreed on for use in comparing measurements.

Technical Note: Household Measurements

Household measurements are expressed in Arabic numerals with fractions for expressing parts of a whole, such as 1/2, 2/3, or 3/4. The abbreviation for each measurement follows the number, such as 5 tsp or 2 1/2 pt.

Length

In pharmacy, length is used to measure medications that require application to the body that must be measured in inches, centimeters, or millimeters. In these instances, the means of application is usually pre-marked on a dispensing paper for ease in ensuring that the correct amount of medication is administered, such as with nitroglycerin ointment. Another pharmaceutical use of length is in finding body surface area (BSA) where height & weight are used for dosage calculations.

International System of Units (Metric System)

Internationally accepted system of measurement of mass, length, & time.

Mass (Weight)

Mass is the measurement of the amount of matter in an object & is commonly referred to as weight. Metric weight is the measurement used most often in pharmacy to express a dosage unit. Most medications are ordered & supplied by the weight of a drug in solid or liquid amounts. Most solid medications are supplied in micrograms, milligrams, or grams. A few older medications are still supplied in grains from the apothecary system. Household measurements of weight are not used in medication strengths.

Specific Gravity

Most measurements need a unit to have meaning. The one exception to this is specific gravity which is the ratio of the density of a substance to the density of water when dealing with liquids in pharmacy. In this calculation, the units cancel, leaving specific gravity without a unit. If the density of a substance is x g/mL, once it is divided by the density of water or 1 g/mL, the units cancel and the specific gravity of the substance is x. Specific gravity is used occasionally in pharmacy to calculate the weight or volume of a solution.

Apothecary System

One of the oldest measurement systems used to calculate drug orders using measurements such as grains & minims.

Technical Note: Basic Measurements

Pharmacy technicians should commit the following basic household equivalencies to memory. Volume - 3 tsp = 1Tbsp - 2 Tbsp = 1 oz - 8 oz = 1 c - 2 c = 1 pt - 2 pt = 1 qt - 4 qt = 1 gal Weight - 16 oz = 1 lb

Technical Note: Basic Metric Equivalencies

Pharmacy technicians should commit the following basic metric equivalencies to memory & can use methods presented in Chapter 5 to calculate any of the others needed. Weight - 1,000 mcg= 1 mg - 1,000 mg= 1 g - 1,000 g= 1 kg Volume - 1,000 mL= 1 L

Biologicals

Substances made from natural sources such as human, animal, or microorganism that are used as drug treatments or to prevent or diagnose diseases; tested for potency in a biologic system. Units, as a measurement, are assigned to various biologicals.

U.S. Customary System (Household System)

System of measurement based on common kitchen measuring devices. It uses measurements such as inches, teaspoons, & pounds.

International System of Units (Metric System)

The International System of Units, or the metric system, which is used in most of the remaining world, uses meters, liters, & grams.

Technical Note: Unit Measurement

The Joint Commission's Official "Do Not Use" List includes the use of the abbreviations u or U, which should be written "unit," & IU, which should be written "International unit.

Apothecary System of Measurement

The apothecary system is one of the oldest systems of measurement. First used by an apothecary (early pharmacist), this system is gradually being replaced with the metric system. In ancient times, a minim (drop) of water was considered to weigh the same as a grain of wheat. Therefore the basic unit of liquid in the apothecary system is a minim & the basic unit of weight is a grain (gr). Apothecary measurements of volume such as minims, fluid drams, & fluid ounces are still found on some pharmacy bottles & medication-dispensing cups even though the use of these measurements is discouraged. These apothecary symbols are found on the ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations. Although the metric system is preferred, the apothecary grain is still used with some older medications, such as nitroglycerin sublingual tablets (Nitrostat). In the apothecary system, lowercase Roman numerals are used for expression of numbers 10 and below, rather than Arabic numerals as found in the metric & household systems. The Roman numerals should be expressed with lines placed over them to tie them together, such as i, ii, iii, iv, v and x , & lowercase i's are often expressed with the dots above that line. Arabic numerals may be used for numbers higher than 10, except for 20 ( xx ) and 30 ( xxx ), for which Roman numerals are required. Over the years the use of the line over the Roman numeral has gradually diminished, but using it is correct. Fractions other than 1/2 , which is ss , are expressed using Arabic numerals. Roman numerals and Arabic numerals are never used together in one measurement, so when using a fraction other than ss, use Arabic numerals for the whole number as well. Therefore seven and three- fourths would be 7 3/4, not vii 3/4. Decimals are not used with the apothecary system. The unit abbreviation is written first with Roman numerals placed to the right of the unit of measure.

Apothecary System

The apothecary system, which uses grains & drams, is an older system used in pharmacy for many years, but less frequently today.

Technical Note:

The household pound (#) is 16 ounces; the apothecary pound is 12 ounces. The household pound (16 oz) will be used for conversions with body weight in pharmacy.

Metric System of Measurement

The metric system of measurement is the most widely used throughout the world & is the most commonly used system for measuring medications & dosages because of its accuracy. Most prescriptions are written in the metric system, & most liquid drugs are administered using this system. The U.S. Pharmacopeia considers the metric system the appropriate system for use on drug labels. Medication labels should be written in the appropriate metric units. Some medication labels include household measurements as well as metric measurements. Additionally, some labels still contain non-recommended abbreviations, such as μg (microgram) found on the labels for digoxin (Lanoxin). The metric system is based on units of 10. The basic measurements are the gram for weight, liter for volume, and meter for length. Prefixes for the base measurements are used to indicate the multiple or submultiple of the base that is being described.

Technical Note: Common Prefixes In Pharmacy

The prefixes most commonly used in pharmacy are micro, milli, & kilo.

Three Measurement Systems

Three measurement systems are presently used in the medical field to calculate length, volume, & weight, although length is not as commonly used in the pharmaceutical field as in other medical disciplines. Each system has a unique set of measurement units that have been chosen as standards. Numbers without units are meaningless. Consider telling someone that you will be gone for 3. Without the unit designating minutes, hours, days, months, or years, there is no meaning to the All three systems—household, metric, & apothecary—have units of measure for weight & volume that are used in pharmacy, but only household & metric systems have commonly used units for length. Knowledge of the units for weight & volume in the three systems & length in the household & metric systems is necessary to interpret medication orders & prescriptions.

Chapter Summary

Three measurement systems are used in the field of medicine in the United States. The household system is the most commonly used on a daily basis in homes in the United States. However, using the household system provides only approximate measurements because of the differences in utensils; it may lead to inaccurate dosing when used for administering medication. The use of actual measuring spoons as opposed to individual flatware can help decrease the variation. The metric system is used as the standard unit of measure throughout the rest of the world and is structured on multiples of 10. Most medications are designated in the metric system today. The final system, rarely used today but found with some medications, is the apothecary system, which is based on grains for weight and minims, fluid drams, and fluid ounces for volume. Although this system is not popular and its use is being discouraged, these measurements are still seen occasionally with some medications.

Volume

Volume is the amount of space something occupies & is used to measure amounts of liquids. A derived unit that is used to describe liquid medication strengths is density, which is weight divided by volume. Most liquid medications are described in terms of mg/mL. Occasionally some products, such as Milk of Magnesia, are still labeled with the amount of active ingredient per tablespoon (15 mL).

Viscosity

When looking the smallest measurement of volume, a drop, it is totally dependent on the size of the opening in the dropper & the viscosity (thickness) of the liquid; therefore the 60 drops per teaspoon often found in tables of household measurements is only an approximation. Drops should not be used for measuring drugs unless there is a specific dropper calibrated for the medication.

Metric Measurements of Mass in Pharmacy

kg __________________ g __________________ mg __________________ mcg (1 kg =1,000 g) (1 g =1,000 mg) (1 mg =1,000 mcg)