Drug dosage and calculation

• Metric system


  Decimal system that is the most logically organized
• Converting and computing metric units


  Simple multiplication and division
• Metric system

  • Each basic unit of measurement is organized into units of 10
  • Multiplying or dividing by 10 forms secondary units
  • In multiplication the decimal point moves to the right; in division the decimal moves to the left
• Metric system conversions

  • 10mg x 10= 100mg
  • 10 mg/10= 1mg
• Basic units of measurement in metric system

  • Meter (length)
  • Liter (Volume)
  • Gram (Weight)
• For medication calculations, use only the volumes and weights unit
• Metric system units

  • Gram = g or GM
  • Liter = l or L
  • Milligram = mg
  • Milliliter = mL
• Metric system prefixes

  • Deci-(1/10 or 0.1)
  • Centi- (1/100 or 0.01)
  • Milli- (1/1000 or 0.001)
• Many actual and potential medication errors happen with the use of fractions and decimal points
• Leading zero

  Always placed in front of a decimal (e.g., use 0.25, not .25) to make the decimal point more visible
• Solution

  A given mass of solid substance dissolved in a known volume of fluid or a given volume of liquid dissolved in a known volume of another fluid
• Metric system

  Decimal system that is the most logically organized
• Converting and computing metric units


  Simple multiplication and division
• Metric system


  • Each basic unit of measurement is organized into units of 10
  • Multiplying or dividing by 10 forms secondary units
  • In multiplication the decimal point moves to the right; in division the decimal moves to the left
• Metric system conversions


  • 10mg x 10= 100mg
  • 10 mg/10= 1mg
• Basic units of measurement in metric system


  • Meter (length)
  • Liter (Volume)
  • Gram (Weight)
• For medication calculations, use only the volumes and weights unit
• Metric system units


  • Gram = g or GM
  • Liter = l or L
  • Milligram = mg
  • Milliliter = mL
• Metric system prefixes


  • Deci-(1/10 or 0.1)
  • Centi- (1/100 or 0.01)
  • Milli- (1/1000 or 0.001)
• Many actual and potential medication errors happen with the use of fractions and decimal points
• Leading zero


  Always placed in front of a decimal (e.g., use 0.25, not .25) to make the decimal point more visible
• Solution


  A given mass of solid substance dissolved in a known volume of fluid or a given volume of liquid dissolved in a known volume of another fluid
• Concentration of a solution


  • Units of mass per units of volume (e.g., g/L, mg/mL)
  • Can also be expressed as a percentage

  
• Percentage solution


  • A 10% solution is 10 g of solid dissolved in 100 mL of solution
• Proportion


  Expresses concentrations
• Proportion


  • A ¹⁄₁₀₀₀ solution represents a solution containing 1 g of solid in 1000 mL of liquid or 1 mL of liquid mixed with 1000 mL of another liquid
• Converting measurements within one system


  1. Divide or multiply in the metric system

  2. To change milligrams to grams, divide by 1000, moving the decimal 3 points to the left

  3. To convert liters to milliliters, multiply by 1000 or move the decimal 3points to the right
• Metric system conversions


  • 100mg= 1g
  • 350 mg= 0.35g
  • 1L= 1000mL
  • 0.25 L= 250 mL
• To convert from one measurement system to another, always use equivalent measurements
• Dosage calculation methods


  • Ratio and Proportion Method
  • Formula Method
  • Dimensional analysis
• Ratio


  • Indicates the relationship between two numbers separated by a colon (:)
  • The colon (:) in the ratio indicates the need to use division
  • Think of a ratio as a fraction; the number to the left is the numerator, and the number to the right is the denominator
• Ratio


  • 1:2 is the same as 1/2
• Proportion


  An equation that has two ratios of equal value
• When multiplying the extremes, the answer is the same as when multiplying the means
• Solving a calculation using the ratio and proportion method

  1. Estimate the answer in your mind

  2. Set up the proportion, labeling all the terms

  3. Put the terms of the ratio in the same sequence (e.g., mg : mL = mg : mL)

  4. Cross multiply the means and the extremes and divide both sides by the number before the x to obtain the dosage

  5. Always label the answer; if the answer is not close to the estimate, recheck the calculation
• Ratio and proportion method example

  • The health care provider orders 500 mg of amoxicillin to be administered in a gastric tube every 8 hours. The bottle of Amoxicillin is labeled 400mg/ 5mL.
• Formula method


  Estimate the answer

  2. Set up the formula

  3. Calculate the answer

  4. Compare the estimate in Step1 with the answer in Step3
• Formula method formula


  Dose ordered = (Dose on hand / Amount on hand) x Amount to administer
• Formula method example


  • A liquid medication comes in the strength of 125 mg per 5 mL. In this case 125 mg is the dose on hand and 5mL is the amount on hand.
• Dimensional analysis


  Identify the unit of measure that you need to administer

  2. Estimate the answer

  3. Place the name or appropriate abbreviation for x on the left side of the equation

  4. Place available information from the problem in a fraction format on the right side of the equation

  5. Look at the medication order and add other factors into the problem

  6. Cancel out like units of measurement on the right side of the equation

  7. Reduce to the lowest terms if possible and solve the problem or solve for x. Label your answer.

  8. Compare the estimate from Step2 with the answer in Step7