W3 Pharmaceutical calculations

Question 1

• od
• tds, tid

Answer 1

once daily
three times a day

Question 2

bd, bds, bid
qds, qid

Answer 2

twice a day
four times a day

Question 3

ii qds prn mdu

Answer 3

two tablets, four times a day, when required, as directed

Question 4

i, ii and iii
Examples?

Answer 4

1,2,3 tablet (s) Eye drops, inhalations etc

Question 5

om, mane, am
on, nocte, pm

Answer 5

In the morning
At night

Question 6

Prn,sos

Answer 6

When required/if necessary

Question 7

Mdu, ud

Answer 7

As directed/use as directed

Question 8

qld
qod, alt
Mitt. Mitte, send

Answer 8

Once daily
On alternate days
Total quantity

Question 9

qh, q1h
q2h
q4h or qqh
q12h
stat.

Answer 9

Hourly
Every two hours
Every four hours
Every twelve hours
Immediately

Question 10

1 kilogram in grams?

Answer 10

1000 grams (g)

Question 11

1 gram in milligrams?

Answer 11

1000 milligrams (mg)

Question 12

1mg in mcg?

Answer 12

1000 micrograms (mcg)

Question 13

1mcg in nanograms?

Answer 13

1000 (ng)

Question 14

Which are bigger?
Nanograms
milligrams
micrograms

Answer 14

Milligrams > Micrograms > Nanograms
10- 3 > 10-6 > 10-9

Question 15

1 litre in millilitres
1mL in microlitres?

microlitres is NOT mL (millilitres)

Answer 15

1L= 1000 millilitres
1mL=1000 microlitres

Question 16

What are solid dosage forms measured in?

Answer 16

grams, milligrams,micrograms

Question 17

What are non-solid dosage forms measured in?

Answer 17

milligrams/mL
e.g. 100mg/5ml or 10mg/ml

Question 18

Non solid dosage forms:

What do (% w/v) (% v/v) (%w/w) mean??

Answer 18

• weight in volume (% w/v): % = g per 100ml [e.g. 2% = 2g per 100ml]
• volume in volume (% v/v): % = ml per 100ml [e.g. 2% = 2ml per 100ml]
• weight in weight (% w/w): %= g per 100g

Question 19

Non solid dosage forms:

What do (% w/v) (% v/v) (%w/w) mean??

Answer 19

• weight in volume (% w/v): % = g per 100ml [e.g. 2% = 2g per 100ml]
• volume in volume (% v/v): % = ml per 100ml [e.g. 2% = 2ml per 100ml]
• weight in weight (% w/w): % = g per 100g [e.g. 2% = 2g per 100g]

Question 20

Ratios:
What is; 1:1000 and 1:10,000

Answer 20

1:1000= 1g in 1000ml
1:10,000= 1g in 10,000ml

Question 21

What is the definition of dose?

Answer 21

The amount of medicine taken by/administered to a patient in order to achieve a therapeutic outcome.

Question 22

1)A patient is prescribed amoxicillin capsules 250mg 1 tds x 7 days.

a. How many capsules should we dispense?
b. How many grams of amoxicillin do they take if they complete the course?

The same patient needs amoxicillin suspension 250mg/5ml instead of capsules.
a. How many millilitres of suspension would we dispense to cover the full course?
b. What strength (%w/v) is this suspension?

Answer 22

a: 21 capsules
b: 21x250= 5,250mg= 5.25g

a. 5ml x 3 x 7 =105mL
b. 250mg/5mL
100/5= 20
250x20= 5,000
5000mg/100mL
so, 5g/100mL so 5% w/v

Question 23

Doses based on renal function:
When does a dose need to be decreased?
What can renal function be expressed as? (2)

Answer 23

When there is a reduction in kidney function
eGFR- estimated Glomerular Filtration Rate
Creatinine clearance

Question 24

Creatinine clearance; things to remember (3):

Answer 24

Creatinine clearance (ml/min) = (140-age) x weight x constant/ serum creatinine

Do brackets first
Constant is different for males and females:
1.23 for male and 1.04 for female (micro mol/l)
Age in years, weight in kg,

Question 25

Mrs Mitra is a 67 year old patient who weighs 70kg. Her recent bloods show a serum creatinine of 90 micromol/L constant= Work out her creatinine clearance: (use GpHC approved calculator) Creatinine clearance (ml/min) = (140-age) x weight x constant/ serum creatinine

Answer 25

59.05

Question 26

“Parts” in formulae and dilutions: (for info)

Answer 26

* Industrial methylated spirit (IMS), BP * spirit (L) 95 parts * wood naphtha (L) 5 parts For IMS above, the formula adds up to 100 parts. To make 200L of IMS, you would use * 95L x 2 = 190L of spirit * 5L x 2 = 10L of wood naphtha

Question 27

Differentiate between “parts” and “to parts” Example 1 * calamine powder 1 part * aqueous cream to 10 parts Example 2 * calamine powder 1 part * aqueous cream 10 parts

Answer 27

Example 1: Equivalent to: 1g 9g Total: 10g and (1g/10g) x 100% = 10% w/w Example 2: Equivalent to: 1g 10g Total: 11g and (1g/11g) x 100% = 9.09% w/w

Question 28

A prescription calls for 10ml qds x 7 days. How many millilitres do you supply?

Answer 28

=280 ml 10x4x7

Question 29

A patient takes digoxin tablets 250 micrograms od. How many milligrams of digoxin do they take in a month (28 days)?

Answer 29

= 7 milligrams 250 x 28= 7000mcg / 1000= 7mg

Question 30

A patient uses chloramphenicol eye drops 1% (w/v), 1 drop q2h (during waking hours – for this example 16 hours per day) for two days and 1 drop qds for the next three days. Each drop is 0.05ml. i) How many millilitres has the patient used? ii) How many milligrams has the patient used?

Answer 30

i) (9x2) +(4x3)= 30 eye drops x0.05ml = 1.5ml ii) 1.5ml 1% w/v= 1g in 100ml 1.5/100= 0.015g so 15 milligrams 8 can be used instead of 9

Question 31

How many grams of active ingredient are contained in 1L of a 5% w/v solution?

Answer 31

=50g * 5%w/v = 5g/100mL * (1000mL/100mL) x 5g = 50g

Question 32

What strength, expressed as w/v, is paracetamol suspension 120mg/5ml?

Answer 32

= 2.4% w/v * w/v means g/100mL * (100mL/5mL) x 120mg = 2400mg = 2.4g * If there are 2.4g in 100mL, strength is 2.4%w/v

Question 33

How many grams of amoxicillin would a patient take in total if they were prescribed: Amoxicillin Capsules 500mg tds x 5 days?

Answer 33

=7.5g * 500mg tds means 500mg three times a day = 1500mg daily * Given for 5 days = 7500mg = 7.5g

Question 34

What volume of adrenaline 1 in 1000 would need to be administered to provide a 300mcg dose?

Answer 34

=0.3ml * 1 in 1000 means 1g in 1000ml * 1g in 1000ml = 1000mg in 1000ml = 1mg/ml = 1000mcg/ml * Want 300mcg, so (300mcg/1000mcg) x 1ml = 0.3ml

Question 35

How much of each ingredient is required to make 250ml of Chalk Mixture, Paediatric BP? * chalk 20g * tragacanth powder 2g * cinnamon water, conc 4ml * syrup 100ml * chloroform water, double strength 500ml * water to 1000ml

Answer 35

Need 250ml/1000ml = 0.25 times (a quarter) of each ingredient. * chalk 50g * tragacanth powder 0.5g or 500mg * cinnamon water, conc 1ml * syrup 25ml * chloroform water, double strength 125ml * water to 250ml

Question 36

When may we need to change to strength of a medicine? Reasons for decreasing and increasing?

Answer 36

Decrease – e.g. for administration to a child or an older patient or for preparation of a medicine to give to a patient from a stock solution. This is done by dilution. Increase – e.g. to make a product that is stronger than the one we have available. This is done by admixing (the action of adding one ingredient to something else)

Question 37

When we dilute: the amount of active ingredient inc/dec ..? the concentration inc/dec ..?

Answer 37

Stays the same Decreases

Question 38

We have a 20% w/v of stock solution and want to make 200ml of a 5% w/v solution. -How much stock solution and how much diluent do we need? 2 methods to work out; C1V1=C2V2 or First principles

Answer 38

Stock- 20% w/v = 20g in 100ml Product- 5% w/v= 5g in 100ml (200ml) 10g active ingredient in 200ml x2 so 50ml of stock (half) is needed for 10g and 150ml diluent to make 200ml

Question 39

You have a 25% stock solution and need to make 1L of a 1% preparation. How much stock solution and how much diluent do you need?

Answer 39

40ml stock and 960ml of diluent Using the equation C1V1 = C2V2 25% x V1 = 1% x 1000ml So, V1 = (1%/25%) x 1000ml =40ml

Question 40

You need to make 50ml of a 7.5% w/v solution. You have a 12% w/v solution in stock. How much stock and how much diluent do you need?

Answer 40

31.25ml stock and 18.75ml of diluent Using the equation C1V1 = C2V2 12% x V1 = 7.5% x 50ml So, V1 = (7.5%/12%) x 50ml = 31.25ml 18.75 diluent

Question 41

You need to make a 150ml of a 1.25mg/5ml solution. You have a 1.5% w/v solution in stock. How much stock and how much diluent do you need?

Answer 41

2.5ml stock, 147.5ml diluent 1.25mg/5ml = 25mg/100ml = 0.025g/100ml = 0.025% w/v Then, V1 = (0.025%/1.5%) x 150ml = 2.5ml

Question 42

You have a 10ml of a 4% solution. How many 1ml ampoules of a 1 in 1000 solution could you make?

Answer 42

1 in 1000 = 1g in 1000ml = 0.1g/100ml = 0.1% w/v 4% x 10ml = 0.1% x V2 So, V2 = (4%/0.1%) x 10ml = 400ml 400 x 1ml ampoules If we can make 400ml of 1 in 1000 solution, we can fill 400 x 1ml amps

Question 43

Multiple/Serial Dilutions: What weight of active ingredient is required to produce 1000ml of a solution such that, when 2.5ml is diluted to 50ml with water, it gives a 0.25% w/v solution?

Answer 43

= 50g Stock Solution Product 5% w/v 0.25% w/v = 0.25g per 100ml = 5g per 100ml 20x dilution * 0.25% = 0.25g/100ml, so final product 50ml contains 0.125g (as it did 2.5ml before dilution) * So: if there is 0.125g in 2.5ml of original solution, we need: (1000ml/2.5ml) x 0.125g = 50g to make 1000ml of stock method 2: * Diluted 50ml/2.5ml = 20 times * Original solution must therefore be 20 x 0.25% w/v = 5% w/v * 5% w/v = 5g/100ml * So, need: (1000ml/100ml) x 5g = 50g active ingredient to make 1000ml

Question 44

What weight of active ingredient is required to produce 250ml of a solution such that, when 10ml is diluted to 200ml, we have a 1% w/v solution?

Answer 44

Solution is diluted 200ml/10ml = 20 times * So, original solution is 20 x 1% w/v = 20% w/v * 20% w/v = 20g in 100ml * To make 250ml, need (250ml/100ml) x 20g = 50g or 20g x 2.5=50g

Question 45

What are the different types of dose?

Answer 45

Single dose, Daily dose, Daily divided dose, Weekly dose

Question 46

Examples of doses based on weight: (for info)

Answer 46

* Can often be for children, but also for specific medicines in adults. * Expressed as mg/kg or mcg/kg for weight e.g. Paracetamol * Expressed as mg/m2 or mcg/m2 for BSA * Often there is an accompanying maximum dose that should not be exceeded.

Question 47

For infusions: the dose if often expressed as:

Answer 47

mg/kg/min or mcg/kg/min

Question 48

A patient, weight 68kg, has been diagnosed with viral encephalitis and requires aciclovir at a dose of 10mg/kg every 8 hours. What total dose is required?

Answer 48

68kg x 10mg/kg = 680mg every 8 hours

Question 49

A child aged 2, weight 12kg, has asthma and requires a 3 day course of steroids. The dose of prednisolone would be 1-2mg/kg od. What dose range could be administered?

Answer 49

12-24 mg od x 3 days

Question 50

The maintenance dose of sodium valproate for epilepsy in children is 30mg/kg daily in two divided doses. i) What is the dose in mg for a 12kg child? ii) Sodium valproate is available as a 200mg/5ml oral solution. What is the dose in ml?

Answer 50

1. 12kg x 30mg/kg = 360mg per day In two divided doses, this is 180mg bd 2. (180mg/200mg) x 5ml = 4.5ml bd

Question 51

Example (BSA) - Aciclovir [Refer to BNF] A 9-year-old child with a BSA calculated at 1.2m2 requires an I/V infusion of aciclovir. What is the 8-hourly dose?

Answer 51

1.2m2 x 500mg/m2 = 600mg every 8 hours

Question 52

Refer to BNF: Dopamine Hydrochloride - What is the maximum dose of dopamine that can be given to a 70kg adult patient? -Dopamine infusion vials contain 200mg/5ml. What volume of dopamine should be used to provide a 6-hour infusion?

Answer 52

a. 70 x 5mcg = 350mcg/min b. 350mcg x 60 (minutes) x 6 (hours) = 126,000 mcg = 126mg Need: (126mg/200mg) x 5ml = 3.15ml

Question 53

A 75kg patient requires an atenolol intravenous infusion after admission for an arrythmia [Refer to BNF Hypertension] 1. What dose of atenolol is required initially? 2. Atenolol is available as 5mg/10ml solution for injection ampoules. How much should be added to the infusion solution for a single dose?

Answer 53

1. 75kg x 150mcg = 11,250mcg (11.25mg) over 20 minutes 2. (11.25mg/5mg) x 10ml = 22.5ml