Pharmaceutical Measurement Systems

Pharmaceutical Measurement

• Pharmaceutical care involves accurately weighing, measuring volumes, and combining components.
• Incorrect calculations can harm patients.
• Accurate calculation and measurement of ingredients are crucial for compounding.
• Pharmacists use three measurement systems: Metric, Apothecary, and Avoirdupois.

The Metric System

• A decimal system based on factors of 10.
• Commonly used for pharmacy calculations.
• Easy conversions between systems.
• Conversion from smaller to larger units involves moving the decimal to the left.
• Conversion from larger to smaller units involves moving the decimal to the right.

The Apothecary System

• Uses weight and volume (ounces, gallons, pints, quarts).
• Traditionally used in dispensing and prescribing medications.
• Originated with apothecaries, pharmacists, and alchemists.
• Still used by some pharmacists today.

The Avoirdupois System

• A system of weight measurement only.
• The grain is common to both the Apothecary and Avoirdupois systems.

Metric System - Basic Units

• Mass (g or gram)
• Length (m or meter)
• Volume (L or liter)
• 1 cubic centimeter (cc) of water is roughly equal to 1 milliliter (mL) and weighs 1 gram (g).

Apothecary System - 1. Volume (fluids)

• 60 minims (m) = 1 fluidrachm or fluidram (f 3) or (3)
• 8 fluidrachms (480 minims) = 1 fluid ounce (f 3 or 3)
• 16 fluid ounces = 1 pint (pt or 0)
• 2 pints (32 fluid ounces) = 1 quart (qt)
• 4 quarts (8 pints) = 1 gallon (gal or C)

Apothecary System - 2. Mass (weight)

• 20 grains (gr) = 1 scruple (3)
• 3 scruples (60 grains) = 1 drachm or dram (3)
• 8 drachms (480 grains) = 1 ounce (3)
• 12 ounces (5760 grains) = 1 pound (lb)

Avoirdupois System - 1. Volume

• 1 fluidrachm = 60 minims (m)
• 1 fluid ounce = 8 fl. dr. = 480 min
• 1 pint = 16 fl. oz. = 7680 min
• 1 quart = 2 pt. = 32 fl. oz.
• 1 gallon = 4 qt. = 128 fl. oz.

Avoirdupois System - 2. Mass (weight)

• 437.5 grains (gr) = 1 ounce (oz)
• 16 ounces (7000 grains) = 1 pound (lb)

Conversion (Exact Equivalents)

• Equivalents are used for conversions in formulas and compounding.
• Examples include conversions between meters and inches, milliliters and minims, and grams and grains.

Conversion (Approximate Equivalents)

• Examples include fluid dram = 1 teaspoonful = 5 mL, 4 fluid ounces = 120 mL, 8 fluid ounces = 1 cup = 240 mL, 1 grain = 65 mg, and 1 kg = 2.2 pounds (lb).

Interconversions (Common Household)

• Common household equivalents for imperial and metric units.

Measurement of Volume

• Common instruments include micropipets, burettes, large calibrated vessels, cylindrical and conical graduates, calibrated syringes, and pipettes.
• Choice of instrument depends on precision needed.
• Erlenmeyer flasks, beakers, and prescription bottles are not volumetric glassware.

Guidelines for Selecting Liquid Measurement Devices

• Choose the smallest device that accommodates the desired volume.
• Use a graduated pipette, syringe, or dropper for volumes less than 1 mL.
• Consider using a disposable syringe for oily or viscous liquids.
• Avoid using nonvolumetric containers.
• Utilize calibrated droppers, oral syringes, or similar devices for small or precise doses.

Measurement of Weight

• Class A prescription balances are used for compounding.
• Sensitivity requirement (SR) is 6 milligrams or less without load, and 10 grams with load in each pan.
• For 5% error, weigh amounts of 120 mg or more.
• Most commercial balances hold a maximum of 120 grams.

Sensitivity Requirement

• Defined as the load that causes a change of one division on the balance's index plate.
• Determined by procedures like leveling, finding the rest point, and identifying the smallest weight that shifts the index plate by one division.

Aliquot Method of Weighing and Measuring

• Achieves precision beyond instrument's capacity.
• Involves calculating and measuring in aliquot parts (fractions, portions, or parts).
• Useful for very small quantities.

Weighing by the Aliquot Method

• Calculate the smallest quantity for desired precision.
• Select a multiple of the desired quantity, weigh this multiple.
• Dilute the multiple with an inert substance.
• Weigh the aliquot portion of the dilution that contains the desired quantity.

Minimum Measurable Quantity

• Calculated based on the formula: 100% * Sensitivity Requirement (mg) / Acceptable Error (%) = Smallest Quantity (mg)
• Important for determining the minimal weight appropriate for the balance given an acceptable error value.

Step 1 of the Aliquot Method

• Select a multiple of the drug required that can be weighed with the desired precision (e.g., 25 times the amount required).

Step 2 of the Aliquot Method

• Dilute the multiple with an inert substance.

Step 3 of the Aliquot Method

• Weigh the aliquot portion of the dilution that contains the desired drug quantity.

Measuring Volume by the Aliquot Method

• Identical to the weighing method in principle.
• Replaces inert diluent with solvent.
• Calculate a volume to measure appropriately.

Percentage of Error

• Pharmacy measurements must be as accurate as possible.
• Pharmacists must understand limitations of measurement instruments and calculate percentage error.
• Percentage error is calculated by: (Error × 100%) / Quantity Desired

This quiz covers essential pharmaceutical measurement systems, including the Metric, Apothecary, and Avoirdupois systems. You'll learn about the importance of accurate weighing and measuring in pharmaceutical care and how different systems are applied in pharmacy calculations. Test your knowledge and ensure safe compounding practices.