U2.5.4^(LO 5.5, 5.6, 5.12). Pharmacy Units of Measurement^135^141^,,^8439^8725%
(LO 5.5, 5.6, 5.12)
Pharmacy Units of Measurement
Metric System of Measurement

The metric system is the official system for weights and measures, as stated in the United States Pharmacopeia and National Formulary. The dosage of a solid or semi-solid drug is usually measured in milligrams and liquid medications are often measured in milliliters. There are times when a pharmacy technician will be required to calculate the correct dosage for patients based upon their weight; the dosage will be based upon milligrams of drug per kilogram of patient weight. The basic units of measurement in the metric system include the gram (g; for weight or mass), liter (L; for volume), and meter (m; for distance). The metric system uses prefixes based on multiples of 10 (refer to Table 5-4). As a pharmacy technician, it is extremely important to know how to convert from a larger unit to a smaller unit and vice versa. When going from a larger unit to a smaller unit, you will multiply in multiples of 10 and going from a smaller unit to a larger unit, you will divide in multiples of 10.

 
Table 5-4
Metric Prefixes
  Kilo- Hecto- Deka- Base Deci- Centi- Milli- Micro-
Meaning × 1000 × 100 × 10   /10 /100 /1000 /1000000
Abbreviation k h da   d c m mc
Weight kg hg dag g dg cg mg mcg
Volume kL hL daL L dL cL mL mcL
Length km hm dam m dm cm mm mcm
* Note: Bold faced terms are more commonly used in pharmacy practice.
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EXAMPLE
Convert 15 g to milligrams

First, a gram (g) is larger than an milligram (mg). There are 1000 mg in 1 g. You may use a proportion in solving this problem:

Cross multiply (1 g × X mg) and (1000 mg × 15 g):

Divide both sides of the equation by 1 g to get the unknown number of mg:

Apothecary System of Measurement

The apothecary system of measurement is the traditional system of measuring and weighing ingredients in the practice of pharmacy. Although the system is rarely used today, a pharmacist or pharmacy technician may see a prescription or medication order where the prescriber has used it. The practice of pharmacy uses both the apothecary and metric system to measure out ingredients (Table 5-5). In both the apothecary and avoirdupois system, only the grain is the same.

 
Table 5-5
Apothecary Conversions
Apothecary Volume:
60 minims (m) = 1 fluid dram (3)
8 fluid drams (480 minims) = 1 fluid ounce (fl oz)
16 fluid ounces = 1 pint (pt)
2 pints (32 fluid ounces) = 1 quart (qt)
4 quarts (8 pints) = 1 gallon (gal)
Apothecary Weight:
20 grains (gr) = 1 scruple (Э)
3 scruples (60 grains) = 1 dram (3)
8 drachm (480 grains) = 1 ounce (oz)
12 ounces (5760 grains) = 1 pound (lb)
Avoirdupois System of Measurement

The avoirdupois system is the system used by commerce in the United States and Britain. It is based upon a pound being equal to 16 ounces. This system along with the metric system is how products are purchased and sold in the United States (Table 5-6).

Table 5-6
Avoirdupois Conversions
Avoirdupois Weight:
437.5 grains (gr) = 1 ounce (oz)
16 ounces (7000 gr) = 1 pound (lb)
Household System of Measurement

The household or U.S. customary system is used for the patient to understand the quantity of medication to be taken at a particular time. Unlike the other systems we have studied, the household system is an approximate system of measurement (Table 5-7). An individual uses the utensils readily accessible in the home to administer medicine.

Table 5-7
Household Conversions
Household Volumes:
5 milliliters (mL) = 1 teaspoon (tsp)
3 teaspoons (tsp) = 1 tablespoon (tbsp)
2 tablespoons (tbsp) = 1 fluid ounce (fl oz)
8 fluid ounces (fl oz) = 1 cup
2 cups = 1 pint (pt)
2 pints (pt) = 1 quart (qt)
4 quarts (qt) = 1 gallon (gal)
Household Weights:
16 ounces (oz) = 1 pound (lb)
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In various situations, the pharmacist or pharmacy technician may find that a conversion is needed to perform a pharmacy calculation. The USP has issued a table of exact equivalents for both weights and measures in the metric, apothecary, and avoirdupois systems. As a pharmacy technician, you will be required to memorize apothecary, avoirdupois, and household conversions. You can always solve these conversions by using proportions. Refer to Table 5-8 for these conversions.

Table 5-8
USP Conversions for Weights and Measures
Conversion Equivalents of Volume:
1 milliliter (mL) = 16.23 minims (m)
1 minim (m) = 0.06 mL
1 fluid drachm (fl 3) (dram) = 3.69 mL
1 fluid ounce (fl oz) = 29.57 mL
1 pint (pt) = 473 mL
1 gallon (U.S.) = 3785 mL
Conversion Equivalents of Weight:
1 gram (g) = 15.432 grains (gr)
1 kilogram (kg) = 2.20 pounds (lb)
1 grain (gr) = 0.065 gram (g) or 65 mg (can also be 60 mg)
1 ounce (oz − avoirdupois) = 28.35 grams (g)
1 ounce (oz) = 31.1 grams (g)
1 pound (avoirdupois) = 454 grams (g)
1 pound (apothecary) = 373.2 grams (g)
Other Equivalents:
1 oz (avoirdupois) = 437.5 grains (gr)
1 ounce (oz) = 480 grains (gr)
1 gallon (U.S.) = 128 fluid ounces (fl oz)
1 fluid ounce (fl oz) = 455 grains (gr)

There are specific tools used to measure both volumes and weights in pharmacy practice. The proper selection of the equipment chosen is based upon the desired precision of the measurement. Volumes are measured using either a cylindrical or conical graduate; small volumes may be measured using calibrated syringes or a pipette. Cylindrical graduates are calibrated using metric units, such as milliliters; conical graduates are calibrated in both metric and apothecary units. It is extremely important that the proper size graduate is chosen; the graduate should not contain more than five times the volume to be measured. If the graduate measures more than five times the desired volume, the possibility of a measuring error will increase.

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Many different types of balances and scales may be used in a pharmacy. Each type of scale or balance must meet specific standards that are established by the USP for sensitivity, capacity, and accuracy. Class A balances are used to weigh ingredients for the filling of prescriptions or small-scale compounding. Class A balances must have a sensitivity requirement of 6 mg or less and should not weigh less than 120 mg and not be able to weigh more than 120 g. Electronic balances are capable of weighing very small quantities such as 0.1 mg and often are self-calibrating with a digital readout. Electronic balances have the capability of weighing 60 to 120 g.

In hospitals, the pharmacy is responsible for preparing IV bags for patients upon receipt of a medication order from a physician. The pharmacy technician may be asked to reconstitute a vial of medication or prepare a medicated IV bag for a patient. The pharmacy technician will use a syringe in these preparations. He or she should select a syringe that contains no more than five times the volume being measured to ensure accuracy. Refer to Figure 5-1 for syringes used to prepare IVs.

Figure 5-1
Parts of a standard syringe.
Conversion Factors

It is often necessary to convert a measurement from one unit to another. We have already discussed conversions within the metric system. Now let us examine how to use the equivalents given in the above tables to convert units between different systems. A conversion factor is a fraction made up of two values that are equal to one another, but which are expressed in different units. Table 5-8 lists equivalents that can be used when writing conversion factors. The key to writing a conversion factor is to put the proper part of the equivalent into the numerator and denominator of the fraction. The following rule can be used when writing conversion factors from equivalents: The part of the equivalent containing units that you want to convert from must go in the bottom of the conversion factor. The part containing units that you want to convert to must go in the top of the conversion factor.

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EXAMPLE

What conversion factor would you use to convert from pounds to kilograms?

  1. Find the equivalent containing pounds and kilograms.

  2. Rewrite the equivalent as a fraction, putting the units you are converting from on the bottom and the units you are converting to on the top.

EXAMPLE

What conversion factor would you use to convert from milliliters to fluid ounces?

  1. Find the equivalent containing milliliters and fluid ounces.

  2. Rewrite the equivalent as a fraction, putting the units you are converting from on the bottom and the units you are converting to on the top.

Using Conversion Factors

To use a conversion factor, multiply the number that is being converted by the numerator of the conversion factor, and divide the answer by the denominator of the conversion factor. You can then cancel the units that are the same in the numerator and denominator.

EXAMPLE
Convert 110 lb to kilograms
  1. Find the equivalent containing pounds and kilograms.

  2. Rewrite the equivalent as a fraction, putting the units you are converting from on the bottom and the units you are converting to on the top.

  3. Multiply the number being converted by the numerator of the conversion factor, while dividing by the denominator. Cancel any units found in both the numerator and denominator.

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EXAMPLE
Convert 12.5 mL to teaspoons
  1. Find the equivalent containing milliliters and teaspoons.

  2. Rewrite the equivalent as a fraction, putting the units you are converting from on the bottom and the units you are converting to on the top.

  3. Multiply the number being converted by the numerator of the conversion factor, while dividing by the denominator. Cancel any units found in both the numerator and denominator.

Tech Check
  1. How many milligrams are in 25 g? (LO 5.6)

  2. How many milligrams are in 7.5 gr? (LO 5.6)

  3. How many teaspoons are contained in 1 pt of solution? (LO 5.6)

  4. A patient weighs 150 lb. How many kilograms does the patient weigh? (LO 5.6)

  5. How many millilitres are 2½ pt of syrup? (LO 5.6)

  6. A patient is to receive 0.25 mg of medication. How many micrograms of medication is this? (LO 5.6)

  7. How many grams are in 4 oz (wt)? (LO 5.6)

  8. How many pounds does a 60 kg person weigh? (LO 5.6)

  9. How many fluid ounces are equal to 75 mL? (LO 5.6)

  10. How many gallons are equal to 4 L? (LO 5.6)

  11. How many grams are equal to 500 mg? (LO 5.6)

  12. A Nitrostat tablet contains 1/150 gr. How many milligrams are equal? (LO 5.6)

  13. How many milliliters are equal to 6 fl oz? (LO 5.6)

  14. A patient is to receive 5 gr of aspirin. How many milligrams are equal? (LO 5.6)

  15. You have been asked to prepare 1/2 lb of an ointment for a dermatologist. How many grams should you prepare? (LO 5.6)