Dosing and calculation Flashcards
what are the common calculation errors seen in practise
- 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
describe how paediatrics calculate dosage
- 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
it is good practise for a …
second practioniner to check your dose calculation independently in order to minimise the risk of error
What is a sense check
- 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
the dosing regimen for some drugs is…
dependent on the weight of the patient
- therefore every patient should have a recent weight documented in their medical record
What is the actual body weight
- this is the weight you get when you stand a patient on a set of scales
Actual body weight….
works well provided the patient is neither too large or too small
How is lean body weight calculated
calculated by subtracting body fat weight from actual body weight
What are most drug trials conducted on
- average weight patient so limited data available for patients at the extremes of weight
drugs that mainly distribute into water…..
water are likely to need dosing on lean or ideal body weight
drugs that mainly distribute into fat…
are more likely to be dosed on actual body weight
What formulates are used in acute paediatric emergencies before weight can be used
- 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
what does body surface area (BSA) correlate more with
- it is thought to correlate more closely with variations in age, body size and body composition
What drugs are dosed according to body surface area
- toxic drugs
- systemic anticancer therapies
- some antibacterials/antivrials
Name an example of a drug dosed according to body surface area
- intravenous aciclovir for the treatment of herpes simplex for children
What method do you use to measure the BSA in children
- The Boyd method - estimates BSA from the weight only and is used in children
How are solid doses expressed as
- they are expressed as weight per unit
how are liquid doses expressed
- expressed as weight per ml or weight per 5ml or some other variation
How are injectable preparations expressed
- percentage
- amount per ml
- total quantity of drug per total volume
- ratio
How many
- Grams in a Kg
- how many mg in a g
- how many micrograms in an mg
- how many nanograms in a microgram
- 1Kg = 1000g
- 1g = 1000mg (milligrams)
- 1mg = 1000 micrograms
- 1 microgram = 1000 nanograms
what is % w/w
- 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
What is % w/v
- 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
what is % v/v
- 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
how much does
- 1 stone in pounds
- 1 pound in grams
- 1 stone in kilograms
- 1 stone contains 14 pounds
- 1 pound = 450g
- 1 stone = 6.35kg
How much is
- 1 foot in inches
- 1 inch in millimeters
- 1 foot in millimetres
- 1 foot contains 12 inches
- 1 inch is equal to 25.4 mm
- 1 foot is equal to 304.8 mm
in obese patients what should you use to calculate the dose needed for gentamicin
ideal body weight
what do paediatric doses vary on
- age
- weight
- body surface area
what is a drug dose equivalents
- 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
What do you have to think about when using an equivalent drug dose
- bioequivalence
- pharmacological profile
- renal function
- liver impairment
How do you withdraw benzodiazepines
- change this to an equivalent dose of diazepam and then carefully reduce the dose of this to 0
what are the equivalent doses to 5mg of diazepam
- chlordiazepoxide
- lorazepam
- nitrazepam
- temazepam
- chlordiazepoxide = 12.5mg
- lorazepam = 500 micrograms
- nitrazepam = 5mg
- temazepam = 10mg
why are corticosteroids difficult to do the dose on
- more complex as they have both glucocorticoid and mineralocorticoid activity
what are the equivalents to 5mg of prednisolone
- betamethasone
- dexamethasone
- hydrocortisone
- methylprednisolone
- betamethasone= 750 micrograms
- dexamethasone = 750 micrograms
- hydrocortisone = 20mg
- methylprednisolone = 4mg
prednisolone 1mg is equivalent to…
hydrocortisone 4mg
when switching oral morphine to fentanyl patches how do you do it
- 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
what is the conversion to oral and subcutaneous morphine
- oral morphine sulfate and subcutaneous morphine sulphate are not equivalent
- the subcutaneous dose of morphine is half the oral dose
what should you communicate with the health care professional administering the infusion
- 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
many IV drugs need…
further dilution before they are administered - usually because they are too concentrated to be given through a peripheral vein
what is used in the treatment of paracetamol overdose
- acetylcysteine
How do you calculate dosage in acetylcysteine
- using the patients actual body weight
- maximum body weight you should use is 110kg even if the patient is heavier
what do you dilute the acetylcystiene in
- it is preferable diluted in glucose 5%
what are the three parts of administration of acetylcystiene
- 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%
describe how the dose of acetylcysteine varies at each step
- first = 150mg/kg
- second = 50mg/kg
- third = 100mg/kg
- total dose of 300mg/kg over 21 hours
how do you calculate the volume of a parenteral drug required to be administered or added to an infusion
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
how do you calculate percentage change
- 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
