Drug Calculations

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Drug Calculations
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Aims of this session

• review the importance of drug calculations
• review formulas used for drug calculations
• work through an examples booklet

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Nurses need an understanding of basic
calculations

• Fractions
• Decimals
• Percentages
• SI Units (e.g. millilitre-ml milligram-mg)
• Conversion of metric units
• Dilution of solutions

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Fractions

• Fractions are made up of numerators and
  denominators
• Numerator is the top number
• Denominator is the bottom number
• An improper fraction is one in which the
  numerator is larger than the denominator e.g. 5
• 
  3
• A proper fraction has a numerator smaller than
  the denominator e.g. 1
• 4

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• Improper fractions can be converted to mixed
  numbers, e.g.
• 5 1 2 or 32 4 4
• 3 3 7 7

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• Mixed numbers are partly whole numbers, partly
  fractions e.g. 15 or 41
• 8 2

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• Mixed numbers can be converted to improper
  fractions e.g.
• 15 13 or 41 9
• 8 8 2
  2

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Decimals

• Generally used to mean a number that includes a
  decimal point.
• e.g. 6.25 has 2 decimal places
• 0.625 has 3 decimal places
• 62.5 has 1 decimal place
• To the right of the decimal point are tenths,
  hundredths, thousandths etc

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Percentages

• This is the number of parts per hundred parts.
• e.g. 9 9 parts per 100 parts
• 3.5 3.5 parts per 100 parts

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Units of Weight

• 1 milligram (mg) 1000 micrograms
• 
  (mcg)
• 1000 milligrams (mg) 1 gram (g)
• 1000 gram (g) 1 Kilogram (Kg)

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Milligrams are sometimes expressed in terms of
grams

• 500mg 0.5g
• 250mg 0.25g
• 10mg 0.01g
• 1mg 0.001g

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Examples of conversions from milligrams to
micrograms

• 0.1mg 100 micrograms
• 0.2mg 200 micrograms
• 0.25mg 250 micrograms
• 0.4mg 400 micrograms
• 0.6mg 600 micrograms

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• GRAMS TO KILOGRAMS
• Divide by 1000
• Move decimal point 3 places to the LEFT
• E.G. 4000 g 4 kg
• KILOGRAMS TO GRAMS
• Multiply by 1000
• Move decimal point 3 places to the RIGHT
• E.G. 4kg 4000 g

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Percentage Calculations

• Measurement of concentration commonly used is
  percentage.
• Example a 1 volution is
• 1g in 100ml 1 w/v (solid in liquid)
• 1g in 100g 1 w/w (solid in solid e.g.
  
  
• 
  ointment)
• 1ml in 100ml 1 v/v (liquid in liquid)

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• 1 solution 1g in 100ml
• 1000mg in 10ml
• 10mg in 1ml
• 50mg in 5ml

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Drug Dosages - Injections

• Formula
• Strength Required X Volume of stock solution
  Volume
• Stock Strength Required

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Example

• Cortisone 40mg is required ampoule contains
  50mg/2ml. What Volume is required?
• 40mg x 2ml 40 x 2 8 or 1.6ml
• 50mg 1 50 1 5
• Formula
• Strength Required X Volume of stock solution
  Volume
• Stock Strength
  Required

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Workbook
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Drug Dosages Tablets/Mixtures

• Formula
• Strength Required X Volume of stock solution
  Amount
• Stock Strength
  Required

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Example

• 30mg tablets Phenobarbitone.
• How many are required for a dose of 45mg?
• 45mg x 1 1.5 tablets
• 30mg
• Formula
• Strength Required X Volume of stock solution
  Amount
• Stock Strength
  Required

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Workbook
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Drug Dosages Intravenous Infusion Rates

• Formula
• Volume (ml) x Drops Rates (drops/minute)
• Time (hours) 60

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Example

• 800ml fluid to be given over 5 hours. Giving set
  delivers 15 drops/ml. What rate is required?
• 800 X 15 800 x 15 40
  drops/minute
• 5 hrs 60 5 60
• Formula
• Volume (ml) x Drops Rates (drops/minute)
• Time (hours) 60

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Workbook
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Any Questions