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Calculating Drug Dosages: Formulas and Methods for Accurate Administration

This chapter teaches the formulas and methods for calculating drug dosages of tablets, capsules, and liquids. It also covers dosage calculations for parenteral injections and flow rates for infusions.

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Calculating Drug Dosages: Formulas and Methods for Accurate Administration

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  1. Chapter 9 Calculating Drug Dosages

  2. Chapter 9 Lesson 9.1

  3. Learning Objectives • Use formulas to determine the dosages of tablets, capsules, or liquids • Use formulas to determine the total number of tablets or capsules or the amount of liquid to be ordered for a specified time • Use information about the apothecaries', metric, and household measurements systems to accurately calculate drug dosages

  4. Learning Objectives (cont.) • Calculate dosages for parenteral injections, including those for special preparations such as insulin • Calculate flow rates for infusions

  5. Calculating Medication Dosages Three Steps • Verify the drug available is the same measurement system as the drug dosage desired (convert if needed) 2. Reduce to lowest terms 3. Calculate dosage quantity to be administered

  6. Drug Calculation Methods • Fraction Method • 600 mg = 200 mg x tablets 1 tablet Solve for x • Ratios or Proportion Method • 600 mg : x tablets :: 200 mg : 1 tablet • Solve for x

  7. Drug Calculation Methods (cont.) • Desired over Available Method Desired units (conversion factor) x Quantity of drug form = Quantity to give Quantity available (x conversion factor)

  8. Forms of Oral Medications • Capsules • Cannot be broken or divided • If amount to be given is more than 0.5, round to next whole number • Tablets • Only divide if scored • Coated tablets are not to be broken • Liquids • May be measured in a medication cup, syringe, or calibrated dropper

  9. Scored and Unscored Tablets

  10. Parenteral Medications • Medication available in three forms: • Prefilled syringe labeled with specific dosage • For example: meperidine (Demerol) 100 mg in 1 mL • Single-dose ampule or multiple-dose vial labeled with a specific dosage per volume • For example: epinephrine (Adrenalin) 1:1000 in 0.1 mL • A vial with powder that requires a specific fluid be added to it to obtain a specific dosage (Reconstitution)

  11. Insulin • A critical medication that replaces the insulin not being produced by the patient’s pancreas • Insulin comes in a standardized measure called a “Unit” • Smallest amounts may be given; errors are critical

  12. U-100 Vial

  13. Insulin (cont.) • Strengths • U-100 (100 Units of insulin per 1 mL) • U-500 (500 Units of insulin per 1 mL) • Preparation 5 times stronger, rarely used • Syringe • Calibrated in Units also • Tuberculin syringe used in emergency • Minims used; 16 minims = 1 mL

  14. U-100 Syringe

  15. Tuberculin Syringe

  16. Intravenous Medications • Medications administered into the vein • IV push • IV hanging by gravity (flow rate formula) • IV pump (mL/min or hr)

  17. Flow Rate Formula • Gtts/min = Volume to be administered × gtt factor Time in minutes • Drop factor of tubing: Macrodrip = 10, 15, or 20 gtt/mL Microdrip = 60 gtt/mL

  18. Chapter 9 Lesson 9.2

  19. Learning Objectives • List the rule used to calculate medication dosages for children • Calculate flow rates for infusions for children

  20. Clark’s Rule • Formula Weight of the child ________________ x Adult dose = Child’s dose Weight of the adult

  21. Body Surface Area • Body surface area (BSA) = the total tissue area • A nomogram is used to easily calculate the BSA in square meters • BSA formula Surface area of the child (M2) × Usual adult dose Surface area of an adult (1.73 M2) = Child’s dose

  22. Dimensional Analysis • Steps • Numbers in the dosage calculation problem are placed on a grid along with their labels • The labels are cross-canceled to assure only one label is left (one for answer) • Numbers in calculation are placed along grid next to their labels

  23. Dimensional Analysis (cont.) • Numbers are cross-canceled • Numbers are multiplied across the top and bottom of the grid to yield a fraction • The fraction is divided, and the remaining label is applied to the answer

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