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Year Five - Prescribing Safety Assessment > Drug Calculations > Flashcards

Drug Calculations Flashcards

1
Q

How do you convert from nanogram (ng) to micrograms (mcg)?

A

We divide by 1000

1000ng = 1mcg

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2
Q

How do you convert from micrograms (mcg) to milligrams (mg)?

A

We divide by 1000

1000mcg = 1mg

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3
Q

How do you convert from milligrams (mg) to grams (g)?

A

We divide by 1000

1000mg = 1g

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4
Q

In order to calculate the dose volume, what equation do we use?

A

Volume = Dose/Concentration

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5
Q

Volume Calculation Question Example: A 5 year old girl with post-operative nausea and vomiting requires treatment with cyclizine 0.5mg/kg by IV injection. Weight 18kg. Cyclizine is available in ampoules containing 50mg/1ml.

What volume (ml) of cyclizine is required?

A

Volume = Dose/Concentration

Dose = 0.5mg/kg = 0.5mg x 18kg = 9mg

Concentration = 0.5mg/kg

Volume = 9mg/50mg/1ml = 0.18ml

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6
Q

What is the drug dilution ratio, described as 1 part drug to 4 parts solution?

A

1:5 dilution

This is due to the fact that there is a total of five parts

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7
Q

What is the drug dilution ratio, described as 1 in 10 solution?

A

1:10 dilution

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8
Q

Dilution Calculation Question Example: A 60 year old woman is an inpatient with condition A. She requires treatment with drug B with IV infusion at a a dose of 10mg/kg. Her weight is 80kg. Drug B is initially available as a 2.5mg/1ml suspension but requires dilution with an equal volume of sodium chloride 0.9% prior to use.

What volume (ml) of diluted drug B should be given IV?

A

Volume = Dose/Concentration

Dose = 10mg/kg = 10mg x 80kg = 800mg

Initial Concentration = 2.5mg/1ml

Diluted Concentration = 1: 2 dilution, therefore multiply the volume by 2 = 2.5mg/2ml

Diluted Volume = 800mg/2.5mg/2ml = 640ml

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9
Q

Dilution Calculation Question Example: A 60 year old woman is an inpatient with condition A. She requires treatment with drug B with IV infusion at a a dose of 110mcg/kg. Weight 80kg. Drug B is initially available in 10ml ampoules of 50mg/25ml with each ampoule subsequently diluted with 90ml of sodium chloride 0.9% prior to use.

What volume (ml) of diluted drug B should be given IV?

A

Volume = Dose/Concentration

Dose = 110mcg/kg = 110mcg x 80kg = 8800mcg = 8.8mg

Initial Concentration = 50mg/25ml

Diluted Concentration = 1:10 dilution and therefore multiply volume by 10 = 50mg/250ml

Diluted Volume = 8.8mg/50mg/250ml = 250/50 x 8.8 = 44ml

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10
Q

Dilution Calculation Question Example: A 60 year old woman is an inpatient with condition A. She requires treatment with drug B with IV infusion. Weight 80kg. Drug B is initially available as 100mg/15ml which must be diluted with glucose 5% to a concentration of 2mg/1ml prior to use. The final volume to be administered is 50ml.

What volume (ml) of glucose 5% is required for this dilution?

A

Volume = Dose/Concentration

Diluted Volume = 50ml

Diluted Concentration = 2mg/1ml
Initial Concentration = 100mg/15ml

Work out the dose using the diluted concentration = 50ml x 2mg/1ml = 100mg

Work out the initial volume using the initial concentration = 100mg/100mg/15ml = 15ml

Volume of glucose 5% = Diluted Volume - Initial Volume = 50ml - 15ml = 35ml

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11
Q

In order to calculate the rate, which equation should be applied?

A

Rate (Volume -Per-Time) = Dose-Per-Time/Concentration

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12
Q

In order to calculate the minimum duration of an infusion (based on a maximal rate), which equation should be applied?

A

Rate = Dose/Time

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13
Q

Rate Calculation Example: A 60 year old woman is an inpatient with condition A. She requires treatment with drug B by IV infusion at a dose of 5mcg/kg/min. Weight 80kg. Drug B is available in 10ml ampoules containing 20mg each.

At what rate (ml/h) should the drug B infusion be delivered?

A

Rate (Volume -Per-Time) = Dose-Per-Time/Concentration

Rate = 5mcg/min x 80kg = 400mcg/min = 0.4mg/min = 24mg/h

Concentration = 20mg/10ml

Rate = 24mg/h/20mg/10ml = 12ml/h

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14
Q

Rate Calculation Example: An 85 year old man on the coronary care unit is being treated for angina.

He requires treatment with isosorbide denigrate 0.1% solution by IV infusion via an infusion at a rate of 2.5mg/h

At what rate of delivery (mL/h) should the infusion pump be set?

A

Rate of Isosorbide Dinitrate Infusion = 2.5mg/h

Concentration of Solution = 0.1% = 0.1g per 100mL = 100mg per 100mL = 1mg/mL

Rate of Infusion = 2.5mg/h = 2.5/1 = 2.5mL/h

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15
Q

Rate Calculation Example: A 19 year old woman presents to the emergency department having taken a staggered paracetamol overdose. Weight 80kg

She is treated with a first IV infusion of acetylcysteine

One hour later, she requires a second IV infusion of acetylcysteine to be initiated, at a dose of 50mg/kg. The IV infusion is prepared by adding the requisite amount of acetylcysteine concentrate for IV infusion to 500 mL of glucose 5% and infused over 4 hours.

Acetylcysteine concentrate for IV infusion is available in 10mL ampoules containing 200mg/mL

At what rate (mL/h) should the second infusion be delivered?

A

Dose = 50mg x 88 = 4400mg

Concentration = 220mg/mL

Acetylcysteine Volume = 4400/220 = 22mL

Total Volume = 500mL of glucose 5% infusion fluid + 22mL of acetylcysteine infusion fluid

Rate = 522mL/4 hours = 130.5mL/h

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16
Q

Rate Calculation Example: An 80 year old man with terminal prostate cancer has been taking morphine sulphate MR 80mg twice daily supplemented with oromorph PRN. Over the last 24 hours he has required 40mg of oromorph

He would like this to eb switched to a syringe driver which is delivered subcutaneously. 10mg morphine sulphate orally is equivalent to 5mg morphine subcutaneously. The concentration of morphine to be administered subcutaneously 10mg/10ml

At what rate (ml/h) should the morphine sulphate subcutaneous syringe driver be set?

A

Morphine Total Dose = 160mg + 40mg = 200mg

Subcutaneous Morphine Dose = 200/2 = 100mg

10mg = 10ml
100mg = 100ml

100ml/24 hours = 4.17ml/h

17
Q

Infusion Rate Calculation Example: A 60 year old woman is an inpatient with condition A. She requires treatment with drug B by IV infusion at a dose of 5mg/kg. Weight 80kg. The infusion should be at a rate not exceeding 3mg/kg/hr.

What is the minimum duration (mins) that drug B can be safely infused?

A

Rate = Dose/Time
Time = Dose/Rate

Dose = 5mg/kg
Rate = 3mg/kg/h

Time = 5mg/kg/3mg/kg/h = 1.67mg/h = 100 minutes

18
Q

What does ‘x % w/v cream’ mean?

A

It means that there is 1g of the drug in 100ml

Therefore, 1000mg in 100ml

Therefore, 10mg in 1ml

19
Q

Percentage Calculation Example: A 60 year old woman has received 10ml of lidocaine hydrochloride 0.5% solution.

What amount (mg) of lidocaine hydrochloride has the patient received?

A

0.5g in 100mls = 500mg in 100mls = 5mg in 1ml

Concentration = 5mg/1ml
Volume = 10ml

Dose = Volume x Concentration = 10ml x 5mg/1ml = 50mg

20
Q

Percentage Calculation Example: A 65 year old woman develops AF with an increased ventricular response.

She requires an emergency loading dose of digoxin, to be given as 750mcg over 2 hours. Digoxin is available as 0.025% 0.5ml vials

How many vials are needed to prepare the infusion?

A

Dose = 750mcg = 0.75mg

Concentration = 0.025% = 0.025g per 1000ml = 25mg in 1000ml = 0.25mg per 1ml

0.25mg = 1ml
0.75mg = 3ml

0.5ml = 1 Vial
3ml = 3 Vials

21
Q

In ratio questions, what does 1 in a 1000 mean?

A

1g in 1000ml

22
Q

Ratio Calculation Example: A 60 year old woman has received 1.5ml of adrenaline 1:1000 intramuscularly during anaphylaxis resuscitation.

What amount (mg) of adrenaline has the patient received?

A

1g in 1000mls = 1000mg in 1000mls = 1mg in 1ml

Concentration = 1mg/1ml
Volume = 1.5ml

Dose = Volume x Concentration = 1.5ml x 1mg/1ml = 1.5mg

23
Q

How Much More Calculation Example: A 60 year old woman has received 22ml of lidocaine hydrochloride 0.5% as a peripheral nerve block. She can still feel some pain and is requesting additional local anaesthesia. The maximal dose for lidocaine hydrochloride 0.5% infiltration is 200mg.

What additional volume (ml) of lidocaine hydrochloride can the patient receive before reaching the maximal dose?

A

Volume = Dose/Concentration

Max Dose = 100mg

Concentration = 0.5% = 0.5g in 100mls = 5mg in 1ml

Maximum Volume = 200mg/5mg/1ml = 40ml

Remaining Volume = Maximum Volume - Received Volume = 40ml - 22ml = 18ml

24
Q

How Many Calculation Example: A 60 year old woman is an inpatient with condition A. She requires treatment with drug B by IV infusion at a dose of 15mg/kg twice daily. Weight 80kg. Drug B is available in 250mg vials which cannot be reused and must be discarded. Her next clinic appointment is in 7 days.

How many 250mg vials of drug B does she require to last her 7 days?

A

Dose = 15mg/kg = 15mg x 80kg = 1200mg per dose

250mg = 1 Vial
1200mg = 4.8 Vials (5 Vials)

Twice Daily = 10 Vials Per Day

7 Days = 70 Vials

25
Q

Conversion Calculation Example: A 60 year old woman with iron deficiency anaemia has been taking ferrous fumerate 210mg PO twice daily. Due to difficulty swallowing, she has requested conversion to FerroEss syrup. FerroEss syrup contains ferrous fumerate at a concentration of 140mg/5ml with each 5ml of syrup equivalent to 45mg of elemental iron. It is measurable to the nearest 5ml. Ferrous fumerate 210mg contains 70mg of elemental iron.

What volume of FerroEss syrup (to the nearest 5ml) is required per day to meet the elemental iron content of her ferrous fumerate tablets?

A

Dose of Ferrous Fumerate 210mg Tablets = 70mg elemental iron = 140mg elemental iron per day

Concentration of FerroEss syrup = 45mg/5ml

Volume = Dose/Concentration = 140mg/45mg/5ml = 15.6ml = 15ml

26
Q

Conversion Calculation Example: A 60 year old woman with terminal breast cancer has been taking Zomorph (morphine sulphate MR) 60mg twice daily supplemented with oromorph (morphine sulphate oral solution) PO PRN. Over the last 24 hours, she has required 20mg oromorph. She would like this to be switched to a syringe driver which is delivered subcutaneously. 10mg morphine sulphate orally is equivalent to 5mg morphine subcutaneously. The concentration of morphne to be administered subcutaneously is 10mg/10ml.

At what rate (ml/hr) should the morphine sulphate subcutaneous syringe driver be set?

A

Zomorph = 60mg x 2 = 120mg
Oromorph = 20mg
Total Oral Dose = 140mg/Day

Oral morphine dose is equivalent to 1/2 subcutaneous morphine dose. Therefore, 140/2 = 70mg

Concentration = 10mg/10ml

Volume = 70mg/10mg/10ml = 70mg/24 hr = 2.92ml/h

27
Q

Weight Calculation Example: 4.8ml of 3% lidocaine solution is administered to provide local anaesthetic to a C-section wound post-operatively. Lidocaine is administered at a dose of 2mg/kg

What does this patient weigh (kg)?

A

3% Solution = 3g in 100ml, 3000mg in 100ml, 30mg in 1 ml

4.8ml = 30 x 4.8 = 144mg

2mg = 1kg
144mg = 72kg

Year Five - Prescribing Safety Assessment (8 decks)

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