Pharmaceutical Measurement Techniques

Pharmaceutical Measurement

• Pharmaceutical care involves accurate weighing, measuring volume, and combining components.
• Incorrect calculations impact dosage and can harm patients.
• Accurate component calculation and measurement are crucial steps in compounding.
• Three measurement systems are used: Metric, Apothecary, and Avoirdupois.

Metric System

• A decimal system based on a factor of 10.
• Common in pharmacy calculations for quick and easy conversion.
• Conversions from smaller to larger units involve moving the decimal to the left.
• Conversions from larger to smaller units involve moving the decimal to the right.
• Basic units: gram (g) for mass, meter (m) for length, and liter (L) for volume.
• 1 cubic centimeter (cc) of water is approximately equal to 1 milliliter (mL) and weighs 1 gram (g).

Apothecary System

• Uses weight and volume as divisions of measurement.
• Includes measurements of ounces, gallons, pints, and quarts.
• Historically used by apothecaries, pharmacists, and alchemists.
• Still used today by pharmacists as their primary measurement system.

Avoirdupois System

• A system of weight measurement only.
• The basic unit is the grain (gr), same as in the apothecary system.

Conversion Equivalents

• Length: 1 meter (m) = 39.37 inches (in), 1 inch (in) = 2.54 centimeters (cm).
• Volume: 1 milliliter (mL) = 16.23 minims (m), 1 fluid dram (fl dr) = 60 minims (m).
• Volume: Additional conversions for fluid ounces, pints, quarts, and gallons are provided.
• Mass: 1 gram (g) = 15.432 grains (gr), 1 kilogram (kg) = 2.20 pounds (lb), 1 ounce (oz) = 28.35 grams (g), 1 pound (lb) = 454 grams (g).
• Approximate equivalents: 1 fluid dram = 1 teaspoonful = 5 mL, 4 fluid ounces = 120 mL, 8 fluid ounces = 1 cup = 240 mL, 1 grain = 65 mg, 1 kg = 2.2 pounds (lb).
• Household equivalents Common household equivalents are given.

Measurement of Volume

• Common instruments: micropipets, burettes, graduated cylinders, and flasks.
• Instrument choice depends on needed precision.
• Cylindrical and conical graduates are common in pharmacy.
• Pipettes and calibrated syringes are used for smaller volumes.
• Erlenmeyer flasks, beakers, and prescription bottles are NOT volumetric glassware.

Guidelines for Selecting Liquid Measurement Devices

• Select the smallest device that accommodates the desired volume to reduce measurement error.
• Use graduated pipettes, syringes, or calibrated droppers for volumes less than 1 mL.
• Oily and viscous liquids might require specialized handling.
• Avoid using non-volumetric containers for accurate measurements unless calibrated.
• Use calibrated droppers, oral syringes, or similar devices for very small or exact dosages.

Measurement of Weight

• Class A prescription balances are required for compounding.
• These balances have a sensitivity requirement (SR) of 6 milligrams or less.
• Balances can handle a load of 10 grams in each pan.
• Weighing less than 120 milligrams can lead to errors greater than 5%.
• Most commercially available Class A balances have a maximum capacity of 120 grams.
• Sensitivity requirement is defined as the load causing a 1-unit change on the index plate.

Aliquot Method

• Used to obtain precision beyond instrument capacity.
• An aliquot is a fraction of a larger amount.
• Steps for weighing by the aliquot method are described.
• The method involves weighing a multiple of the desired quantity and diluting it with an inert substance.
• The resulting aliquot contains the desired quantity.

Minimum Measurable Quantity

• The minimum measurable quantity is determined using the sensitivity requirement and acceptable error to determine the smallest quantity that can be reliably measured.

Least Weighable Quantity Method

• An alternative approach to the aliquot method for obtaining small quantities of a drug substance.
• Weigh a quantity of the drug substance above the least weighable amount, dilute it with an inert substance, and weigh the calculated portion.
• This will contain the desired quantity of drug.

Percentage of Error

• Percentage error is crucial in pharmacy to ensure that measurements are as accurate as possible.
• Percentage error is calculated by multiplying the error by 100 and dividing it by the desired quantity.
• Use this formula to ensure errors fall within permissible limits for a given set of tools.

This quiz covers essential concepts in pharmaceutical measurement, including the importance of accuracy in weighing and measuring components. It explores the three primary measurement systems: Metric, Apothecary, and Avoirdupois, and their relevance in pharmacy calculations. Test your knowledge on these critical topics to ensure patient safety.