LPE&T Calculations I
You have 60 minutes to answer the following 20 calculation questions
Q1
What weight of potassium permanganate is required to prepare a
100ml solution such that 10ml diluted to 1 litre will give a 1 in 4000
solution?
A
2.5mg
B
25g
C
1.25g
D
2.5g
E
250mg
10ml – 1L (1g in 4000ml = 250mg/L)
10ml contains 250mg
Therefore x 10 for 100ml = 250 * 10 = 2500mg = 2.5g
Q2
A patient is prescribed prednisolone as a descending dose course. The
patient is to take 80mg OD reducing by 10mg every 2 days until a dose
of 10mg is reached. She is then to take 10mg for 7 days and 5mg for 7
days and then stop. What is the total dose of prednisolone taken?
A
725mg
B
755mg
C
775mg
D
805mg
E
825mg
80mg – 2 days
70mg- 2 days
60mg – 2 days
50mg- 2 days
40mg- 2 days
30mg- 2 days
20mg- 2 days
10mg- 7 days
5mg- 7 days
=
=
=
=
=
=
=
=
=
160
140
120
100
80
60
40
70
35
Total = 805mg
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Q3
Pripsen sachets contain 4gms piperazine citrate and 15.3mg
sennosides per sachet. It is recommended that they are prepared by
dissolving the granules in a glass of water or milk. Assuming that there
is no displacement, what is the approximate concentration of
sennosides if a standard glass of water is 150mls?
A
0.01%
B
0.015%
C
0.030%
D
0.060%
E
0.15 %
15.3mg/ 150ml
15.3/3 = 5.1
5.1 * 2 = 10.2 in 100ml
Approx 10mg in 100ml =
10/1000 = 0.01%
Q4
A patient is to be given infliximab for Crohns disease. The patient
weighs 54Kg and is to be given a dose of 2.5mg/kg/dose. Infliximab
comes as 100mg in 10ml and should be prepared in an intravenous
solution with a final volume of 250ml which is then to be administered
over 3 hours (The recommendation is a minimum of 2 hours). At what
rate is the infusion actually to be administered in micrograms/minutes?
A
1000mcg /minute
B
750mcg /minute
C
500mcg/minute
D
250mcg/minute
E
125mcg /minute
54 x 2.5 = 135mg dose
Infliximab comes as 10mg/ml
135mg in 250ml =
135mg/3 = 45mg/hour 45000mcg/60 = 4500/6 = 750mcg/minute
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Q5
A patient is admitted to hospital following a drug overdose. On
admission a plasma concentration of 72 micrograms/ml is measured.
Given that the half-life of the drug is 18 hours, what would you expect
the approximate plasma concentration to be after 45 hours? (You may
assume that the distribution is complete and the elimination follows
first order kinetics)
A
13.5 micrograms/ml
B
22.5 micrograms/ml
C
25.5 micrograms/ml
D
36.5 micrograms/ml
E
42.5 micrograms/ml
72mcg/ml – 36mcg/ml – 18mcg/ml after 36 hours
Then how much after 9 hours = 18mcg/ml – 9mcg/ml
Which is 13.5mcg/ml
Q6
A patient is being given pilocarpine eye drops 0.5% for their glaucoma.
How much drug is contained in 1 drop if each 1ml contains 20 drops?
A
1mg
B
500 micrograms
C
250 micrograms
D
125 micrograms
E
62.5 micrograms
0.5g/100ml 500mg/100ml 5mg/ml
5mg/20 = 5000mcg/20 = 250mcg
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Q7
A six year old child year old child weighing 21kg is to be treated with
diclofenac sodium to relive the pain of juvenile arthritis. Which one of
the following would you recommend as a treatment option?
A
100mg diclofenac sodium suppository bd
B
25mg diclofenac sodium tablet tds
C
50mg diclofenac dispersible tablet bd
D
75mg diclofenac sodium m/r capsules bd
E
2 x 50mg diclofenac sodium dispersible tablet bd
Answer: c) 50mg bd
Patient is 21kg.
c-BNF 2013-14, pg 503. Dose for 6 months- 18yrs = 1.5- 2.5 mg/kg bd
1.5 x 21 = 31.5mg
2.5 x 21 = 52.5mg
Dose range is 31.5- 52.5mg bd = 63mg – 105mg
Option a) dose is too high
Option b) within range, but tablets are probably not the most suitable
preparation for a 6 year old child
Option c) dose within range and dispersible tablets are a suitable preparation
Option c) not licensed for a 6 year old
Option d) dose over range
Q8
What volume of stock solution of chlorhexidine 5% when diluted to
250ml produces a chlorhexidene 1.5% solution.
A
75 ml
B
125ml
C
150 ml
D
7.5ml
E
55ml
1.5% = 1.5g in 100ml
250ml contains 1.5 x 2.5 (250/100) = 3.75g
5g
in 100ml
3.75g in xml
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
3.75/5 x 100 = 375/5 = 75ml
Q9
Drug X, a class 1 anti-arrhythmic is renally excreted. It is administered
by intravenous infusion. The data sheet provides the following
recommendations:
Creatinine Clearance Dosage Frequency:
• >50ml/min 5mg/kg
12 hrly
• 25-50ml/min 2.5mg/kg 12 hrly
• 10-25ml/min 2.5mg/kg 24 hrly
• 0-10ml/L kg 1.25mg/kg 24 hrly
Creatinine Clearance =
(ml/minute)
1.02 x (140- age) x weight (Kg)
Serum creatinine (mmol/litre)
You have a patient who is a 60yr old non obese female. She weighs
50kg and her last plasma creatinine was measured as 300mmol/L.
Which of the following doses of Drug X would be appropriate?
A
275mg x 12hrs
B
125mg x 12 hrs
C
125mg x 24 hrs
D
75mg x 12 hrs
E
75mg x 24 hrs
Crcl = 1.02 * 80 * 50 = 1.02 * 40 = 40.8 = 13.6
300
3
3
2.5mg/kg 24 hourly = 2.5 * 50 = 125mg/ 24 hourly
Q10
Mr R has a body surface area of 1.6m2 and has been prescribed Drug X
at a dose of 30mg/m2 OD for 5 days. The doctor has told you to round
the daily dose to the nearest number of tablets. How many 10mg
tablets are required for the 5 day course?
A
20
B
25
C
30
D
40
E
48
30 x 1.6 = 48mg
48 x 5 = 240mg
240mg/10mg = 24 tablets
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Nearest is 25 tablets
Q11
A family of two adults and their 6 year old child (who weighs 20kg) ask
your advice for malaria chemoprophylaxis for a 3 week holiday to South
Asia. You advise a regime of chloroquine and proguanil for the family.
The child is able to swallow tablets. Which one of the following options
relating to the provision of Avlocor® tablets is correct?
A
In total you supply 48 tablets for the whole family
B
The child receives 62mg/kg of chloroquine (expressed as base) over the full
course of treatment
C
In total you supply 6g of chloroquine (expressed as base) for the whole
family
D
The child receives a total of 3g of chloroquine phosphate over the
full course of treatment
E
The total number of tablets to be supplied for the two adults is 20
Answers
a) Avloclor® = chloroquine phosphate 250mg tabs (=155mg
chloroquine as base). Regime is one tablet weekly starting 1 week
before travel, during period of travel and for 4 weeks after.
The family will require treatment for 8 weeks (1 week before, 3 weeks
away and for 4 weeks after)
b) c-BNF 2013-14, pg 337; child 4-8 yrs = dose of 1 tablet weekly
BNF 65, pg 426; adults = dose of 2 tablets weekly
Child = 8 weeks x 1 tablets = 8 tablets
Adult= 8 weeks x 2 tablets =16 tablets
Total tablets needed = 16 + 16 + 8 = 40 tablets, therefore a) is
incorrect
c) Child receives 8 tablets = 155mg base x 8 = 1240mg in total
1240mg / 20 = 62mg/kg, therefore b) is correct
d) 8 tablets = 250 x 8 = 2g = chloroquine phosphate, therefore d) is
incorrect
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Q12
A child weighing 5.8kg is prescribed a dose of 8mg/kg/day
ciprofloxacin to be given by IV infusion in 2 divided doses. Which of the
following is the correct volume of ciprofloxacin infusion 2mg/ml that
should be given for each dose?
A
10.3ml
B
11.2ml
C
11.6ml
D
12.6ml
E
23.2ml
5.8 x 8 = 46.4mg
46.4/ 2 = 23.2 per divided dose
2ml/ml = 23.2/2 = 11.6ml
Q13
Miss A is currently on a heparin infusion. It’s 2pm and you notice her
pump is bleeping showing an occlusion and she presses the ‘stop’
button. The last rate set on the pump was 2.5ml/hour and it had not
altered since it was set up at 8am. How much heparin has Miss A
received? Her prescription reads 25,000 units in 50ml sodium chloride
0.9%.
A
75 000 units
B
750 units
C
15 000 units
D
1500 units
E
7500 units
8am- 2pm = 6hrs
2.5ml/hr = 2.5 x 6 = 15ml
15 x 25000 = 15 x 500 = 7500 units
50
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Q14
A patient needs to receive 60mmols Potassium Chloride intravenously
as part of their TPN. The maximum dose the patient is to receive is
5mmols / hour and they are to receive it by intravenous infusion over a
maximum of 24 hours. 30mls of Potassium Chloride Concentrate
Sterile (2mmol/ml) is included as part of a 2.5 litre TPN Infusion bag
which is to be administered. Which of the following is the nearest
maximum drop rate per minute assuming 20 drops per ml.
A
700 drops per minute
B
420 drops per minute
C
210 drops per minute
D
70 drops per minute
E
35 drops per minute
30ml KCL = 60mmol
So they need to receive 30ml so the entire 2.5L bag
60/5 = 12 hours
2.5L/12 hours
2500ml/12 hours
50,000 = 12 hours
50,000ml = 720ml
5000 = 72ml
69.4ml
Approx 70 drops per min
Q15
The recommended dose of drug A is 25mg/kg/day in divided doses. On
admission, Mrs X says she is on three 250mg capsules twice daily. If
the dose is correct, what is Mrs X approximate weight?
A
50kg
B
55kg
C
60kg
D
65kg
E
70kg
750mg bd = 1500mg daily
1500/25 = 60kg
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Q16
You have on your dispensary shelf 1 litre stock bottle which is exactly
half full of a 4 molar aqueous solution of potassium chloride (KCl). You
receive a written order, which requires you to dilute 50ml of the stock
solution with 100ml of water. What strength of KCl solution was
written on the order?
(Relative molecular mass KCL = 75g)
A 15%
B 5%
C 10%
D 20%
E 40%
The fact that the bottle is hall full is irrelevant.
4 molar solution = 4 moles/per litre
1 mole = MW
therefore = 75 x 4 = 300g/litre = 30%w/v
Method 1:
50ml : 100ml = 1:2 dilution or 1 in 3 dilution
30% / 3 = 10%
Method 2:
50ml to 100ml = halving the concentration
30g in 100ml halve concentration = 15g in 100ml, but making 50ml to
100ml, therefore 15g in 150ml = 10%
Method 3:
C1V1 = C2V2
15 x 100 = C2 x 150
1500 = C2
150
C2 = 10%
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Q17
You are required to produce 250ml of Potassium Citrate Mixture BP,
what is the volume of concentrated chloroform water required?
Potassium citrate
Citric acid monohydrate
Lemon spirit
Quillaia tincture
Syrup
Double strength chloroform water
Water
A
1.5ml
B
3.75ml
C
37.5ml
D
4.5ml
E
15ml
300g
50g
5ml
10ml
250ml
300ml
to 1000ml
300ml/4 = 75ml double strength water
75/20 = 3.75ml (conversion 1:20), answer b) is correct
Q18
Mrs Smith has been taking 400mg carbamazepine (Tegretol retard)
once a day to control her seizures. She is currently on ITU and has a
nasogatric tube. The nurse has requested a liquid preparation and asks
whether the dose needs to be adjusted. Calculate the dose of the liquid
preparation required. Carmazepine liquid is available as 100mg/5ml.
Tegretol retard
Carbamazepine liquid
A
360ml
B
14ml
C
28ml
D
18ml
E
3.6ml
400mg = 100g/5ml
F= 0.9
F= 1.0
5ml x 4 = 20ml
9/10 of 20ml = 180/10ml = 18ml
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)
Q19
A coated tablet has a dry weight of coating of 10mg/tablet. The coating
solution is prepared to contain 10% w/v of coating material. How long
is needed to coat a batch of 1 milllion tablets at a spray rate of
250ml/min, given that coating efficiency is 100%?
A
25 min
B
40 min
C
100 min
D
250 min
E
400 min
1 million x 10mg = 10 million mg coating material = 10,000g
10g/100ml = 10,000 = 100L
250ml/min = 0.25L/min
So 100/0.25 = 400 min
Q20
You receive a prescription that requires you to supply 150g of a 1.5%
drug Y in cetomacrogol cream. You only have a 2.5% drug Y in
cetomacragol cream preparation. What quantity of the 2.5% cream do
you need to dilute to 150g to prepare a 1.5% drug Y cream?
A 30g
B 45g
C 60g
D 75g
E 90g
You want to produce 150g of a 1.5% cream of drug Y.
1.5% = 1.5g in 100g
Quantity for 150g would be 1.5 x 1.5 = 2.25g
Therefore you can use ratios:
=
2.5g
2.25g
in 100g
in x
2.25/2.5 = 9/10, therefore 9/10 of 100 = 90g and the answer is E
Tip if you find it hard to find the ration between 2.25: 2.5, double the figures 4.5: 5
to make it easier or even double it again to get 9:10
LPE&T 2013
(Questions taken from Drug Calculations 2 compiled by Roy Sinclair and LNR regional training programme)