Categories
Introduction to Nursing Pharmacology Nursing Pharmacology

Dosage Calculation

Dosage Calculations

Dosing Considerations

  • Sex
  • Weight
  • Age
  • Physical condition
  • Other drugs that the patient is taking

Accurate Dosing

  • Dependent on:
    • The prescriber who orders the drug
    • The pharmacist who dispenses the drug
    • The nurse who administers the drug

Measuring Systems

  • Metric system
  • Apothecary system
  • Household system
  • Avoirdupois system

Metric System

  • Solids: gram (g)
    • 1 milligram (mg) = 0.001 g
    • 1 microgram (mcg) = 0.000001 g
    • 1 kilogram (kg) = 1000 g
  • Liquids: liter (L)
    • 1 milliliter (mL) = 0.001 L
    • 1 mL = 1 cubic centimeter = 1 cc

Apothecary System

  • Solids: grain (gr)
    • 60 gr = 1 dram (dr)
    • 8 dr = 1 ounce (oz)
  • Liquids: minim (min)
    • 60 minim = 1 fluidram (f dr)
    • 8 f dr = 1 fluidounce (f oz)

Household System

  • Solids: pound (lb)
    • 1 lb = 16 ounces (oz)
  • Liquids: pint (pt)
    • 2 pt = 1 quart (qt)
    • 4 qt = 1 gallon (gal)
    • 16 oz = 1 pt = 2 cups (c)
    • 32 tablespoons (tbsp) = 1 pt
    • 3 teaspoons (tsp) = 1 tbsp
    • 60 drops (gtt) = 1 tsp

Conversion Table

Commonly Accepted Conversion Between Systems of Measurement
Commonly Accepted Conversion Between Systems of Measurement

Conversion Between Systems

amount of drug available amount of drug prescribed
one tablet or capsule #           of tablets or capsules to give

amount of drug availableamount of drug prescribed
volume available                       volume to administer

Solving for X

amount of drug available =   amount of drug prescribed
one tablet or capsule           # of tablets or capsules to give

10 mg = 40 mg
1                x

Examples

1 oz =   6 oz
30 mL        X

1 oz x X = 6 oz x 30 mL
1(oz)X = 180 (oz) (mL)

X = 180 (oz) (mL)
1 oz
X = 180 mL

Calculating IV Drip Rate

drops/min = mL of solution prescribed per hr x drops delivered per mL
60 min/hr

Difference Between Children and Adults

  • Children absorb, distribute, metabolize, and excrete drugs differently than adults
  • Children’s organs are not as developed as adults’ organs

Pediatric Dosage Calculation

  • Fried’s rule
  • Young’s rule
  • Clark’s rule
  • Surface area calculation

Fried’s Rule

This rule assumes that an adult dose would be appropriate for a child who is 12.5 years (150 months) of age

child’s dose (age <1 year) =   infant’s age (in months)
150 months x average adult dose

Young’s Rule

child’s dose (age 1 to 12 years)  =

child’s age in years
child’s age in years + 12 x average adult dose

Clark’s Rule

Uses the child’s weight to calculate the appropriate dose and assumes the adult dose is based on a 150-lb person

child’s dose  =   weight of child in pounds
150 pounds x average adult dose

Surface Area Calculation

  • Determine the child’s surface area with the use of a nomogram (the height and weight of the child are taken into consideration in this chart)

child’s dose = surface area in square meters
1.73 x average adult dose

Nomogram

Nomogram
Nomogram