Transcript Drug Calculations

Drug Calculations
Aims of this session
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review the importance of drug calculations
review formulas used for drug calculations
work through an examples booklet
Nurses need an understanding of
basic calculations:
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Fractions
Decimals
Percentages
SI Units; (e.g. millilitre-ml; milligram-mg)
Conversion of metric units
Dilution of solutions
Fractions
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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 =
3
12
3
or 32 =
7
4
4
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
8
= 13 or
8
41
= 9
2
2
Decimals
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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
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To the right of the decimal point are tenths,
hundredths, thousandths etc
Percentages
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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
Units of Weight
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1 milligram (mg)
= 1000 micrograms
(mcg)
1000 milligrams (mg) = 1 gram (g)
1000 gram (g)
= 1 Kilogram (Kg)
Milligrams are sometimes
expressed in terms of grams:
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500mg
250mg
10mg
1mg
=
=
=
=
0.5g
0.25g
0.01g
0.001g
Examples of conversions from
milligrams to micrograms:
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0.1mg = 100 micrograms
0.2mg = 200 micrograms
0.25mg = 250 micrograms
0.4mg = 400 micrograms
0.6mg = 600 micrograms
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
Percentage Calculations
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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
Drug Dosages - Injections
Formula
Strength Required X Volume of stock solution = Volume
Stock Strength
Required
Example
Cortisone 40mg is required ampoule contains
50mg/2ml. What Volume is required?
40mg x 2ml
50mg
1
40 x 2 = 8 or 1.6ml
50
1
5
Formula
Strength Required X Volume of stock solution = Volume
Stock Strength
Required
Workbook
Drug Dosages – Tablets/Mixtures
Formula
Strength Required X Volume of stock solution = Amount
Stock Strength
Required
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
Workbook
Drug Dosages –
Intravenous Infusion Rates
Formula
Volume (ml) x Drops = Rates (drops/minute)
Time (hours)
60
Example
800ml fluid to be given over 5 hours. Giving set
delivers 15 drops/ml. What rate is required?
800 X 15
5 hrs
60
800 x 15
5
60
= 40 drops/minute
Formula
Volume (ml) x Drops = Rates (drops/minute)
Time (hours)
60
Workbook
Any Questions