Drug Calculations

Aims of this session review the importance of drug calculations review formulas used for drug calculations work through an examples booklet

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

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

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

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

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

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

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

Units of Weight 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: 500mg = 0.5g 250mg = 0.25g 10mg = 0.01g 1mg = 0.001g

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

GRAMS TO KILOGRAMS KILOGRAMS TO GRAMS 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 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)

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

Strength Required X Volume of stock solution = Volume 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

Workbook

Drug Dosages – Tablets/Mixtures Formula Strength Required X Volume of stock solution = Amount Stock Strength Required

Strength Required X Volume of stock solution = Amount 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 15 800 x 15 = 40 drops/minute 5 hrs 60 5 60 Formula Volume (ml) x Drops = Rates (drops/minute) Time (hours) 60

Workbook

Any Questions