Dosing and calculation

Question 1

what are the common calculation errors seen in practise

Answer 1

• miscalculating the concentration of a medicine in a solution
• moving the decimal point in the wrong direction when converting dose units for example milligrams to grams
• over or under estimating the time interval between doses
• miscalculating the number of tablets to be administered
• miscalculating the rate of administration

Question 2

describe how paediatrics calculate dosage

Answer 2

• dose needs to be calculated based on age and weight
• dosing recommendations can be misinterpreted if the information is not read in full
• liquid preparations vary in strength
• there can be confusion between ml and mg for the prescriber or for the person administrating the dose
• if medicines are not licensed in a liquid form, the use of non standard unlicensed preparations can cause confusion
• a small dosing mistake with minimal effect in an adult can have a significant effect in a child

Question 3

it is good practise for a …

Answer 3

second practioniner to check your dose calculation independently in order to minimise the risk of error

Question 4

What is a sense check

Answer 4

• what is the approximate answer you are expecting from the calculation
• ask yourself if the dose seems reasonable to prescribe for the drug in question
• check that the calculated dose does not exceed the adult dose or maximum daily dose especially when dosing for neonates and children

Question 5

the dosing regimen for some drugs is…

Answer 5

dependent on the weight of the patient

- therefore every patient should have a recent weight documented in their medical record

Question 6

What is the actual body weight

Answer 6

• this is the weight you get when you stand a patient on a set of scales

Question 7

Actual body weight….

Answer 7

works well provided the patient is neither too large or too small

Question 8

How is lean body weight calculated

Answer 8

calculated by subtracting body fat weight from actual body weight

Question 9

What are most drug trials conducted on

Answer 9

• average weight patient so limited data available for patients at the extremes of weight

Question 10

drugs that mainly distribute into water…..

Answer 10

water are likely to need dosing on lean or ideal body weight

Question 11

drugs that mainly distribute into fat…

Answer 11

are more likely to be dosed on actual body weight

Question 12

What formulates are used in acute paediatric emergencies before weight can be used

Answer 12

• 0-12 months = (0.5 x age in months) + 4kg
• 1-5 years = (2 x age in years) + 8Kg
• 6-12 years = (3 x age in years) +7kg

Question 13

what does body surface area (BSA) correlate more with

Answer 13

• it is thought to correlate more closely with variations in age, body size and body composition

Question 14

What drugs are dosed according to body surface area

Answer 14

• toxic drugs
• systemic anticancer therapies
• some antibacterials/antivrials

Question 15

Name an example of a drug dosed according to body surface area

Answer 15

• intravenous aciclovir for the treatment of herpes simplex for children

Question 16

What method do you use to measure the BSA in children

Answer 16

• The Boyd method - estimates BSA from the weight only and is used in children

Question 17

How are solid doses expressed as

Answer 17

• they are expressed as weight per unit

Question 18

how are liquid doses expressed

Answer 18

• expressed as weight per ml or weight per 5ml or some other variation

Question 19

How are injectable preparations expressed

Answer 19

• percentage
• amount per ml
• total quantity of drug per total volume
• ratio

Question 20

How many

• Grams in a Kg
• how many mg in a g
• how many micrograms in an mg
• how many nanograms in a microgram

Answer 20

• 1Kg = 1000g
• 1g = 1000mg (milligrams)
• 1mg = 1000 micrograms
• 1 microgram = 1000 nanograms

Question 21

what is % w/w

Answer 21

• this represents percentage weight per weight
• this is used when a defined weight of a medicine or chemical is added to a weight of a diluent
• in a 1% w/w preparation there is 1g of drug in 100g of final product

for example

• hydrocortisone acetate BP cream 0.5% w/w
• this means that there is 0.5g of hydrocortisone in 100g of the cream

Question 22

What is % w/v

Answer 22

• this represents percentage weight per volume
• this is used when a defined weight of a medicine (or chemical) is added to a volume of diluent
• in a 1% w/v solution there is 1g of drug in 100ml of the final product

for example

• sodium chloride 0.9% w/v for infusion
• 0.9g of sodium in 100ml of the infusion

Question 23

what is % v/v

Answer 23

• this represents percentage volume per volume
• tends to be used for liquid chemicals
• in a 1% v/v solution there is 1ml of liquid drug/chemical in 100ml of the final product
  for example
• chloraPrep containing isopropyl alcohol 70% v/v
• contains 70ml of isopropyl alcohol in 100ml of solution

Question 24

how much does

• 1 stone in pounds
• 1 pound in grams
• 1 stone in kilograms

Answer 24

• 1 stone contains 14 pounds
• 1 pound = 450g
• 1 stone = 6.35kg

Question 25

How much is - 1 foot in inches - 1 inch in millimeters - 1 foot in millimetres

Answer 25

- 1 foot contains 12 inches - 1 inch is equal to 25.4 mm - 1 foot is equal to 304.8 mm

Question 26

in obese patients what should you use to calculate the dose needed for gentamicin

Answer 26

ideal body weight

Question 27

what do paediatric doses vary on

Answer 27

- age - weight - body surface area

Question 28

what is a drug dose equivalents

Answer 28

- this is when you need to change a patients treatment regimen to another medicine in the same class as the drug is not available or not available in the formulation that you need

Question 29

What do you have to think about when using an equivalent drug dose

Answer 29

- bioequivalence - pharmacological profile - renal function - liver impairment

Question 30

How do you withdraw benzodiazepines

Answer 30

- change this to an equivalent dose of diazepam and then carefully reduce the dose of this to 0

Question 31

what are the equivalent doses to 5mg of diazepam - chlordiazepoxide - lorazepam - nitrazepam - temazepam

Answer 31

- chlordiazepoxide = 12.5mg - lorazepam = 500 micrograms - nitrazepam = 5mg - temazepam = 10mg

Question 32

why are corticosteroids difficult to do the dose on

Answer 32

- more complex as they have both glucocorticoid and mineralocorticoid activity

Question 33

what are the equivalents to 5mg of prednisolone - betamethasone - dexamethasone - hydrocortisone - methylprednisolone

Answer 33

- betamethasone= 750 micrograms - dexamethasone = 750 micrograms - hydrocortisone = 20mg - methylprednisolone = 4mg

Question 34

prednisolone 1mg is equivalent to...

Answer 34

hydrocortisone 4mg

Question 35

when switching oral morphine to fentanyl patches how do you do it

Answer 35

- need to change the oral morphine to a patch that is either slightly less or slightly more potent that the morphine dose - it is normal practise to use the lowest dose in the conversion Ange and then adjust according to a response and tolerance

Question 36

what is the conversion to oral and subcutaneous morphine

Answer 36

- oral morphine sulfate and subcutaneous morphine sulphate are not equivalent - the subcutaneous dose of morphine is half the oral dose

Question 37

what should you communicate with the health care professional administering the infusion

Answer 37

- the name of the active drug - total quantity of the drug to be added to the infusion - the name the concentration of the diluent - the total volume of the syringe or infusion bag - how long the infusion is to be administered over - the infusion rat e - the infusion range - the intended dose - the route of administration

Question 38

many IV drugs need...

Answer 38

further dilution before they are administered - usually because they are too concentrated to be given through a peripheral vein

Question 39

what is used in the treatment of paracetamol overdose

Answer 39

- acetylcysteine

Question 40

How do you calculate dosage in acetylcysteine

Answer 40

- using the patients actual body weight | - maximum body weight you should use is 110kg even if the patient is heavier

Question 41

what do you dilute the acetylcystiene in

Answer 41

- it is preferable diluted in glucose 5%

Question 42

what are the three parts of administration of acetylcystiene

Answer 42

- the first is given over 1 hour in 200ml of glucose 5% - the second is given over the next 4 hours in 500ml of glucose 5% - the third and final is given over the next 16 hours in 1L of glucose 5%

Question 43

describe how the dose of acetylcysteine varies at each step

Answer 43

- first = 150mg/kg - second = 50mg/kg - third = 100mg/kg - total dose of 300mg/kg over 21 hours

Question 44

how do you calculate the volume of a parenteral drug required to be administered or added to an infusion

Answer 44

volume required = (dose prescribed x volume of solution)/ amount of drug in solution same as - Volume required = (what you want/what you've got) x what's in it

Question 45

how do you calculate percentage change

Answer 45

- first we convert the percentage to a decimal by dividing it by 100 (X/100) - Step 2 - X=25 (a 25% decrease in dose, 25/100 = 0.25 - Step 3 - then we multiply this by the number we need to affect 0.25 x 2300 mg = 575mg - step 4 = 2300-575 = 1725mg twice a day