Drug Calculations Review

Transcription

1 Drug Calculations Review By Richard Lakeman 2008 Contents Revision - Fractions and decimals Units of measurement and metric conversions Tablet dosage calculations Fluid dosage calculations Making up solutions IV Flow rates Dosages by weight

2 Drug Calculations for Health Professionals Download at Converting Fractions to Decimals 4 Is the same as 4 Step : Find a number you can multiply by the bottom of the fraction to make it 0, or 00, or 000, or any followed by 0s. Step 2: Multiply both top and bottom by that number. Step 3. Then write down just the top number, putting the decimal place in the correct spot (one space from the right for every zero in the bottom number) 4 goes into 00 (00 4) 25 times x x =

3 Converting Fractions to Decimals 3 4 Is the same as 3 4 Step : Find a number you can multiply by the bottom of the fraction to make it 0, or 00, or 000, or any followed by 0s. Step 2: Multiply both top and bottom by that number. Step 3. Then write down just the top number, putting the decimal place in the correct spot (one space from the right for every zero in the bottom number) 4 goes into 00 (00 4) 25 times 3 x = x 25 = 00 Converting Fractions to Decimals 2 5 Is the same as 2 5 Step : Find a number you can multiply by the bottom of the fraction to make it 0, or 00, or 000, or any followed by 0s. Step 2: Multiply both top and bottom by that number. Step 3. Then write down just the top number, putting the decimal place in the correct spot (one space from the right for every zero in the bottom number) 5 goes into 0 (0 5) 2 x 2 4 = = x times 3

4 Simplifying Fractions 4/8 => 2/4 => /2 Divide the top and bottom of a fraction by the same number that produces whole numbers until you can go no further. (try dividing by 2,3,5,7 etc) = 54 = 27 = Source: Simplifying Fractions Divide the top and bottom of a fraction by the same number that produces whole numbers until you can go no further. (try dividing by 2,3,5,7 etc) = 8 = Source: 4

5 Simplifying Fractions Divide the top and bottom of a fraction by the same number that produces whole numbers until you can go no further. (try dividing by 2,3,5,7 etc) = 5 6 Source: Simplifying Fractions Cancel out common factors on the top and bottom = As long as they have the same number of zeros they cancel out 5

6 Converting Fractions to Decimals Using Long Division. How many times can four go into 3? ¾ 0.75 number under the division symbol 4) times. Put a seven on the top line. 7 x 4 is 28. Subtract this from How many times can 4 go into 2? - 20 can 4 go into 20? Put a 0. on the top line and add a zero to the 4)30 3. How many times can four go into 30? 6. Add a zero to the two. How many times 7. 5 times. Add a five to the top line. 4 x 5 is 20. Subtract 20 from 20. The answer of the subtraction is zero so the decimal is exactly equivalent to the fraction. Multiplying fractions a b c x = a x c d b x d Multiply the numerator (the top) by the numerator Multiply the denominator (the bottom) by the denominator 2 x = x 2 2 x 2 = 4 7 x 3 = x =

7 Converting Fractions to Decimals Dividing by multiples of 0, 00 and so forth. 0 To divide by Move the decimal point places to the left i.e. Add up the zeroes and move that many places to the left = 0.3 = 0.39 = Multiplying by multiples of 0 0 To multiply by Move the decimal point places to the right i.e. For whole numbers just add zeros 3 x 0 = x 00 = x 000 =

8 Calculate your Body Mass Index BMI = foot = 2 inches metre = 00 centimetre stone = 4 pounds inch = 2.54 centimetres (weight in pounds * 703 ) height in inches² or weight in kilograms height in meters² Calculate your Body Mass Index BMI = of person that weighs 72 kg and is 5 foot 7 inches tall Convert to metric: 5 7 = (5 x 2) + 7 = = 67 inches 67 inches = 67 x 2.54 cm = 70.8 = 70 cm (rounded) 70 cm =.70 metres (decimal place moved two places to the left i.e. dividing by 00) Place numbers in formula: weight in kilograms = = = 24.9 height in meters².7 x

9 Metric Conversions Base Units: Length = metre (m) Area = square metre (m²) Volume = litre (l) Mass = gram (g) Time = second (s) Metric Conversions Prefix mega kilo hecto deka Unit deci centi milli micro Symbol M k h da d c m mc / µ Multiplication factor,000,

10 Metric Conversions Prefix mega kilo hecto deka Unit deci centi milli micro Symbol M k h da d c m mc / µ x factor,000, Converting between base units e.g. grams, and milligrams or micrograms is most commonly required in drug calculations To convert from a unit to the base unit multiply by the factor e.g. 500 mg = 500 * 0.00g (i.e. move the decimal place 3 place to the left) = 0.5g To convert from the base unit to another unit divide by the factor e.g. 0.5 g = 0.5 / 0.00mg (move the decimal place three places to the right) = 500 mg Metric Conversions Prefix mega kilo hecto Symbol M k h x factor,000, How many grams is 2 kg? 2 kg = 2000 kg (decimal place moved three places to the right) How many litres is 00mls? deka Unit deci centi milli micro da d c m mc / µ mls = 0. l (decimal place is moved three places to the left) How many micrograms is 0.005g? g = 5000 mcg (decimal place is moved 6 places to the right) How many millilitres is 0.5mls? 0.5 l = 500 mls (decimal place is moved 3 places to the right) 0

11 Metric Conversions Prefix mega kilo hecto deka Unit deci centi milli micro Symbol M k h da d c m mc / µ x factor,000, micrograms of digoxin is prescribed daily. On hand you have 0.25 milligram tablets? How many tablets would you give? First you must ensure that both units are the same. To convert milligrams to micrograms simply move the decimal place 3 places to the right ( microgram is /000 of a milligram). 0.25mg = 250 mcg Micrograms are clearly smaller than milligrams so moving the decimal place to the the right is logical. Or alternatively to convert from micrograms to milligrams move the decimal place three places to the left 25 mcg = 0.25mg Tablet Dosage Calculations Required dose (what you want) Stock dose (what you have got) = number of tablets Both need to be in the same units 25 micrograms of digoxin is prescribed daily. On hand you have 0.25 milligram tablets? How many tablets would you give? 25 micrograms 250 micrograms = 2 tablet

12 Tablet Dosage Calculations Required dose (what you want) Stock dose (what you have got) = number of tablets 7.5 milligrams of diazepam is prescribed tds. On hand you have 5 milligram tablets. How many tablets will you give? 7.5 milligrams milligrams =.5 tablets Tablet Dosage Calculations Required dose (what you want) Stock dose (what you have got) = number of tablets mg of glyceryl trinitrate is prescribed sublingual stat. On hand you have 500 microgram tablets. How many tablets will you give? 000 mcg mcg = 2 tablets 2

13 Tablet Dosage Calculations Required dose (what you want) Stock dose (what you have got) = number of tablets 7.5 mgs of glibenclamide is prescribed before breakfast. On hand you have 2.5 mg tablets. How many tablets will you give? 7.5 mg mg = 3 tablets Fluid Dosage Calculations Required dose (what you want) Stock dose (what you have got) X stock volume = Volume to administer 40 milligram of lamivudine is prescribed. On hand you have 0 milligram in millilitre. How many millilitres will you give? 40 milligram millilitre X milligram = 4 X = 4 millilitres 3

14 Fluid Dosage Calculations Required dose (what you want) Stock dose (what you have got) X stock volume = Volume to administer 6 milligram of haloperidol is prescribed. On hand you have 5 milligram in millilitre. How many millilitres will you administer? 6 milligram millilitre X milligram =.2 X =.2 millilitres Fluid Dosage Calculations Required dose (what you want) Stock dose (what you have got) X stock volume = Volume to administer 0 milligram of droperidol is prescribed. On hand you have 5 milligram in 2 millilitres? How many mls will you administer 0 milligram 2 millilitre X milligram = 2 x 2 = 4 mls 4

15 Making up solutions Strength required (what you want) Stock strength (what you have got) X total volume = required Sometimes you may be required to make up a solution to a desired strength. In which case you use this slight variation of the formula for fluid dosages. You have a 00% (:) disinfectant concentrate and need to make litre of 0% disinfectant (:0) solution. How much concentrate do you need to use? 0% X 000ml = 00 mls (+ 900mls water) 00% Amount of stock required Making up solutions Strength required (what you want) Stock strength (what you have got) X total volume = required Amount of stock required Calculate the volume of stock 8% saline and distilled water required to make up one litre of normal saline (0.9%) 0.9% X 000ml = 0.05 x 000 = 50 mls stock 8 % Distilled water required = = 950mls water 5

16 IV Calculations drops / min Volume (ml) x drop factor (drops / ml) drops / min to Time (minutes) = administer Delivery sets deliver a specific number of drops per ml. Look on the tubing and check the box. Common drop rates are: 0 drops/ml (blood set), 5 drops / ml (regular set) 60 drops / ml (microdrop). IV Calculations drops / min Volume (ml) x drop factor (drops / ml) Time (minutes) drops / min to = administer Convert time to minutes: 8 hours = (8 X 60) = 480 minutes. litre of normal saline is to be infused over 8 hours. The drop factor of the giving set is 5 drops/ml. What is the IV flow rate? Place the values in the formula:- 000 X = = 3.25 drops / minute Round the number to the nearest whole number = 3 drops / minute 6

17 IV Calculations drops / min Volume (ml) x drop factor (drops / ml) Time (minutes) drops / min to = administer unit of blood (450mls) is to be given over 2 hours. The drop factor is 0 drops/ml. What is the IV flow rate? Convert time to minutes: 2 hours = (2 X 60) = 20 minutes. Place the values in the formula:- 450 X = = = 37.5 drops / minute Round the number to the nearest whole number = 38 drops / minute IV Calculations drops / min Volume (ml) x drop factor (drops / ml) Time (minutes) drops / min to = administer Convert time to minutes: 4 hours = (4 X 60) = 240 minutes. 500 mls of 5% dextrose & 0.9% Sodium Chloride is to be given over 4 hours. The drop factor is 5 drops/ml. What is the IV flow rate? Place the values in the formula:- 500 X = = = 3.25 drops / minute Round the number to the nearest whole number = 3 drops / minute 7

18 IV Calculations time to complete Volume (ml) x drop factor (drops / ml) Rate (drops / minute) 840 millilitres of 0% Dextrose in water remains and is being infused at 70 drops / minute. The drop factor is 0. How long will the infusion take to complete at this rate? = Time (min) Place the values in the formula:- 840 X = = = 20 minutes = 2 hours IV Calculations time to complete Volume (ml) x drop factor (drops / ml) Rate (drops / minute) 360 millilitres of 0.45% Sodium Chloride remains and is being infused at 60 drops / minute. The drop factor is 5. How long will the infusion take to complete at this rate? = Time (min) Place the values in the formula:- 360 X = = = = 90 minutes = /2 hours 8

19 IV Calculations Time to complete Volume (ml) x drop factor (drops / ml) Rate (drops / minute) = Time (min) 60 millilitres of 0% Dextrose in water remains and is being infused at 50 drops / minute. The drop factor is 5.. How long will the infusion take to complete at this rate? Place the values in the formula:- 60 X = = = 48 minutes IV Calculations Rates / hr Dose = Rate x Concentration or Rate = Dose Concentration You need to administer 000 U/hr of Heparin. The label states there are 25,000U/500ml of fluid. What is the rate (ml/hr) for the IV pump? Concentration= 25000U = = 50 U/ml 500ml Rate = = 20 ml / hr 50 9

20 IV Calculations Rates / hr Dose = Rate x Concentration or Rate = Dose Concentration You have an order to administer theophylline at 50 mg / hr. There is g of theophylline in 500 mls. What is the rate (ml/hr) for the IV pump? Concentration= 000 mg = 2mg/ml 500ml 50 Rate = = 25 ml / hr 2 IV Calculations Rates / hr Dose = Rate x Concentration or Rate = Dose Concentration You have an order to administer a lidocaine infusion at 2mg / min. On hand you have lidocaine 2g/500ml. At what rate (ml/hr) will you infuse the pump? Concentration= 2000 mg = 4mg/ml 500ml 2 mg/min Rate = = 0.5 ml/min ml/min x 60 min = 30 ml/hr 20

21 Weight dependent calculations Some prescriptions (particularly for children) may be expressed as a dosage / kg (often up to a maximum dosage). Oral, injectable or IV infusion dose forms may be prescribed. Regardless of the form work out the actual dose needed based on the person s weight (N.B. Weight needs to be determined by a reliable scale rather than estimate). Dose to give = Dose/kg x weight (kgs) A drug is prescribed at 5mg/kg. The person weighs 30 kg. How much is the dose to give? 5mg/kg x 30kg = 90mg Weight dependent calculations Dose to give = Dose/kg x weight (kgs) Amoxicillin / clavulanate K is prescribed at 25 mg per kg / 24 hrs in 3 divided doses. The patient's weight is 33 kg. On hand you have 25 mg in 5 ml Suspension. How many millilitres will you administer in a single dose? First determine the dose required in 24 hours: = 25 mg X 33 kg = 825 mg Now determine the amount required in a single dose: = 825 / 3 = 275 mg Now use the result in the formula to calculate fluid dosages Required dose (what you want) Stock Volume X Stock dose (what you have got) 275 milligram 5 millilitre X milligram = 2.2 X 5 = mls 2

22 Weight dependent calculations Dose to give = Dose/kg x weight (kgs) The dose to give is 0mcg x 60kg = 600 mcg / min The doctor orders you to start dobutamine at 0mcg/kg / min. You have a premixed bag with g/500ml. What rate (ml/hr) will you infuse the drug for a patient weighing 60 kgs? The dose per hour would be 600 x 60 = mcg/hr mcg = 36 mg (N.B units need to be the same) The concentration is 000mg / 500 ml = 2 mg / ml Rate = Dose / Concentration = 36mg/hr / 2mg/ml = 8 ml / hr Drug Calculations for Health Professionals For more practice visit 22