Dosing calculations Flashcards
Deci-
What is the symbol and factor?
d
10(-1)
Centi-
What is the symbol and factor?
c
10(-2)
Milli-
What is the symbol and factor?
m
10(-3)
Micro-
What is the symbol and factor?
μ
10(-6)
Nano-
What is the symbol and factor?
n
10(-9)
What are the basic principles?
Check the BNF or SPC (summary of product characteristics) if required Check age or weight related dosing Check maximum doses Check co-mobidities / interactions e.g. renal impairment Apply logic to quantity / rate
What are the two main methods of calculating Body Surface Area (BSA)?
Dubois formula
Boyd method
Mr N is prescribed 25mg spironolactone in
liquid form (5mg/ml). What volume should he
take?
Concentration = Dose / Volume
Therefore volume required = Dose required / concentration = 5ml
A 35kg 8-yr old boy is prescribed amoxicillin at
30mg/kg intravenously. The drug is available as a
500mg/mL preparation.
How much volume is required?
Dose required = 30mg/kg x 35kg (= 1050mg)
Volume required = dose required / concentration
= 1050mg / 500mg/mL
= 2.1mL
Syringe drivers - what is the typical rate?
0.1 to 20 ml/hr
How do you calculate rate?
Rate required = Volume required / time
But usually need to count volume first i.e. volume = dose / concentration
Following a bolus, Mr N (80kg) is prescribed 10mg/kg
amiodarone over 24 hours. Intravenous amiodarone is made to a dilution to 500mg/500mL.
Give the rate (mL/min) required.
Dose required = 10mg/kg x 80kg = 800mg Volume required = Dose required /conc = 800mg / 500mg/500mL = 800mg / 1mg/mL = 800mL Rate required = Volume required /time = 800mL / 24hrs = 800mL / 24x60mins = 0.56mL/min
How do you calculate % (weight/volume)?
% (w/v) = grams of solute / 100ml of total solvent
What is the % (w/v) of glucose 5g/100ml?
5% Glucose
What is 1:1000 adrenaline in mg/ml?
1:1000 adrenaline
=1g/1000ml
=1mg/ml
During a femoral block procedure, 20ml of 0.5% w/v bupivacaine has been used. What is the quantity (in mg) of bupivacaine that has been used?
0.5% = 0.5g/100ml
Therefore 20ml = 20/100 x 0.5g = 0.1g = 100mg
How do you calculate % (v/v)?
% (v/v) = volume of solute / total volume
How many ml of ethanol is in 100ml of 12% ABV wine?
12ml of ethanol in 100ml
A 30 year-old man presents to A+E having consumed 200ml of methylated spirits (5% methanol). How much methanol (in ml) has he consumed?
5% x 200ml = 0.05 x 200ml
= 10ml methanol
Miss N, a 10-year-old girl has been diagnosed with otitis
media, and is prescribed amoxicillin. She weighs 45kg.
Please prescribe the first dose (in mg).
Otitis media, 500mg every 8 hours; CHILD 40mg/kg daily in 3 divided doses (max. 1.5g daily).
Daily dose = 45kg x 40mg/kg/day
= 1800mg daily
= BUT maximum dose 1500mg
= 500mg as first dose
Mr F, usually takes ferrous fumarate 210mg three times daily. He has recently has swallowing difficulties. It is planned that he only has liquids.
What is his equivalent syrup dose (in ml)?
Tablets, 68mg iron
Syrup, 45mg iron/5ml
68mg elemental iron three times a day
= 68 / (45mg/5ml) three times a day
= 7.6ml (rounded up) three times a day
Mr OP has been taking 30mg MST® tabs BD and 10ml
Oramorph® (10mg/5ml morphine) oral solution PRN
(5ml, twice a day) for malignancy-associated bone pain.
He now has difficulty swallowing and the pain team
suggested switching to a patch.
MST 30mg BD = 60mg per day Oramorph® oral solution = 10mg/5ml morphine So, 5ml BD = 20mg per day So, total Daily Dose = 80mg morphine Consider Transtec® ‘35’patch
A 4yr old child (weight 12kg) is to be given vincristine
injection. The recommended dosage is 2mg/m2.
What dose should be prescribed?
2mg/m2 / 0.56m2 = 3.57mg
Mr N, a 75kg man requires inotropic support with severe sepsis. He is prescribed noradrenaline at 0.1 micrograms/kg/min. Noradrenaline is available at 40
micrograms/mL.
Please give the required rate (mL/hr).
Dose required = 75kg x 0.1 micrograms/kg/min
= 7.5 micrograms/min
= 7.5 x 60 micrograms/hr
= 450 micrograms/hr
Rate required = 450 micrograms/hr / 40 micrograms/mL
= 11.25 mL/hr
An 48-year-old woman weighing 62 kg requires a
subcutaneous treatment dose of tinzaparin 175 u/kg daily. Her estimated glomerular filtration rate is 36 mL/min.
Subcutaneous tinzaparin is available in 10000 u/mL pre-filled syringes.
What amount of tinzaparin should the patient be given?
62 x 175 = 10850 units
An 48-year-old woman weighing 62 kg requires a
subcutaneous treatment dose of tinzaparin 175 u/kg daily. Her estimated glomerular filtration rate is 36 mL/min.
Subcutaneous tinzaparin is available in 10000 u/mL pre-filled syringes.
What volume of tinzaparin should the patient be given?
The dose of tinzaparin required is 62 x 175 = 10850 u.
The concentration of the tinzaparin available is 10000 u/mL.
Therefore volume required = 10850/10000 mL = 1.085 mL.
Volumes need to be rounded to most appropriate values. Thus the best answer is 1.1 mL
An 78-year-old man with atrial fibrillation developed an
embolic stroke that caused him to have an unsafe swallow. A naso-gastric tube was placed for nutrition and medications.
His daily oral digoxin dose of 0.125 mg in tablet form needs to converted to an equivalent liquid dose. Digoxin elixir is available in 50 micrograms/mL.
What volume of liquid should the patient be given?
He takes 0.125 mg digoxin = 125 micrograms
Elixir is 50 micrograms/mL
Therefore he requires 125 micrograms / 50micrograms/mL = 2.5 mL
An 28-year-old man weighing 90 kg presented to the ED with 4 tonic-clonic seizues in quick succession. He was treated as status epilepticus. He was on no regular medications. He required intravenous phenytoin loading at 20mg/kg at a maximum rate of 50mg/min.
What is the quickest time this infusion can be given over?
He requires 20mg/kg x 90kg = 1800 mg
Max arte is 50mg/min
Therefore max rate = 1800 mg/ 50mg/min = 36min
An 68-year-old woman weighing 72 kg required initation of dobutamine at 2.5 microgram /kg /min following cardiac surgery. 20mg dobutamine was made up to 100 mL crystalloid ready for infusion.
What infusion rate does she require per hour?
72 kg x 2.5 microgram/kg/imn = 180 microgram/min
180 x 60 = 10800 microgram/h = 10.8 mg / h
Infusion is 20mg/100mL
Therefore in 1 hour, she requires 10.8 mg/h / 20mg/100mL = 54 mL/h
