Pharmaceutical Calculations: The International System of Units Danielle DelVillano, Pharm.D.

Objectives Demonstrate an understanding of the International System of Units Convert measures within the International System of Units State equivalent measures and complete conversions between the International System of Units and other systems of measure used in pharmacy Apply the International System of Units correctly in calculations

Introduction International System of Units (SI) Formerly called metric system Base units Meter Kilogram Liter http://kaffee.50webs.com/Science/activities/Activity.SI.Unit.Conversions.2.htm

Guidelines for Correct Use Two symbols for microgram mcg μg Decimals are used, not fractions 5.25 mg NOT 5 ¼ mg Leading zero should be used before all decimals 0.5 mg NOT .5 mg Trailing zeros should not be placed after whole numbers 5 mg NOT 5.0 mg

Common Conversions Length Volume Weight 1 m = 100 cm 100 cm = 1000 mm 1 L = 1000 mL Weight 1 g = 1000 mg 1 g = 0.001 kg 1 mg = 1000 mcg

Useful Equivalents Ansel, H. C. (2009) Phamaceutical Calculations (13th Ed.). Philadelphia:Lippincott Williams & Wilkins, and Wolters Kluwer Publishers

Practice Problem 1: Reducing SI Units By decimal shift Reduce 9876 milligrams to grams To convert from smaller to larger number shift the decimal RIGHT 9876 mg = 9.876 g Reduce 2.525 liters to microliters To convert from larger to smaller number shift the decimal LEFT 2.525 L = 2525 mL = 2,525,000 μL

Practice Problem 2: Reducing SI Units Reduce 85 micrometers to centimeters By ratio and proportion: (KNOW 1 cm = 10000 μm) 1 cm = x cm 10000 mcg 85 mcg (85 mcg)(1 cm) = x cm (10000 mcg) x = 0.0085 cm

Practice Problem 2: Reducing SI Units Reduce 85 micrometers to centimeters By dimensional analysis: (KNOW 1 cm = 10000 μm) 85 μm X 1 cm__ = 0.0085 cm 10000 μm

Practice Problem 3: Addition Reduce to common denomination then add or subtract Add 1 kg, 250 mg, and 7.5 g 1 kg = 1000 g 250 mg = 0.25 g 1000 g 0.25 g + 7.5 g 1007.75 g

Practice Problem 4: Subtraction Reduce to common denomination then add or subtract Subtract 2.5 mg from 4.85 g 2.5 mg = 0.0025 mg 4.85 g - 0.0025 g 4.8475 g

Practice Problem 5: Multiplication Multiply 820 mL by 12.5 and express the result in liters 820 mL x 12.5 _ 10250 mL 10250 mL = 10.25 L

Practice Problem 6: Division Divide 0.465 g by 15 and express the result in milligrams 0.465 g ÷ 15__ 0.031 g 0.031 g = 31 mg

Questions

Reference Ansel, H. C. (2009) Phamaceutical Calculations (13th Ed.). Philadelphia:Lippincott Williams & Wilkins, and Wolters Kluwer Publishers

Additional Problems

Chapter 2 Problem 2 Add 7.25 L and 875 cL. Reduce the result to mL.

Chapter 2 Problem 8 A liquid contains 0.25 mg of a substance per milliliter. How many grams of the substance will 3.5 L contain? 0.875 g

Chapter 2 Problem 13 Aspirin tablets generally contain 325 mg of aspirin. How many such tablets may be prepared from the 5 kg of aspirin? 15,384 tablets

Chapter 2 Problem 15 Norgestrel and ethinyl estradiol tablets are available containing 0.5 mg of norgestrel and 50 mcg of ethinyl estradiol. How many grams of each ingredient would be used in making 10,000 tablets? 5 g norgestrel 0.5 g ethinyl estradiol

Chapter 2 Problem 20 A patient is instructed to take three 50 mcg tablets of pergolide mesylate (PERMAX) daily. How many mg of the drug would the patient receive weekly? 1.05 mg pergolide mesylate

Chapter 2 Problem 31 A prefilled syringe contains 20 mg of a drug in 2 mL of solution. How many micrograms of drug would be administered by an injection of 0.5 mL of the solution? 5000 mcg

Chapter 2 Problem 38 If an albuterol inhaler contains 18 mg of albuterol, how many inhalation doses can be delivered if each inhalation dose contains 90 mcg? 200 doses

Chapter 2 Problem 49 The product of biotechnology, filgrastim (NEUPOGEN), is available in vials containing 0.3 mg of drug in each milliliter. Which choice is equivalent in concentration? (a) 0.03 mg/0.1 dL (b) 300 mcg/0.01 dL (c) 3 mcg/0.01 cL (d) 300 mcg/10 cL (b) 300 mcg/0.01 dL