PILLS - Drug calculations

Drug calculations

Drug Calculations

Here are 10 typical situations where a dose needs to be calculated. None are too difficult. Think carefully as it can be easy to make mistakes. Click 'Answers' when you're finished so you can check how you did.

1. The dose of tinzaparin for the treatment of pulmonary embolism is 175units/kg. You have a patient who weighs 56kg. The tinzaparin preparation is 20,000 units/ml.

What is the dose?
What volume should be administered?

2. A 25kg child with tuberculosis requires the following oral medicines

What doses should be prescribed?
What volumes should be administered?
Rifampicin 10mg/kg (liquid 100mg/5ml)
Isoniazid 10mg/kg (liquid 50mg/5ml)
Pyrazinamide 35mg/kg (liquid 500mg/5ml)
Ethambutol 15mg/kg (liquid 200mg/5ml)

3. A patient who is nil-by-mouth requires 200 micrograms of digoxin intravenously. You have 0.5mg in 2ml injection available.

What volume should be administered?

4. A 62kg, 80 year-old patient requires a 5mg/kg loading dose of intravenous aminophylline for severe chronic obstructive pulmonary disease. You have 250mg in 10ml ampoules available

What is the dose?
What volume is required?

5. A 68.5kg patient with pneumonia requires gentamicin 7mg/kg. You have gentamicin injection 80mg/2ml available.

What is the dose?
What volume of gentamicin injection is needed?

6. A 70kg patient requires intravenous lorazepam for a severe, acute panic attack at a dose of 30 micrograms/kg. You have lorazepam injection 4mg/ml available.

What is the dose?
What volume should be administered?

7. A 65kg patient is admitted with a paracetamol overdose and requires an infusion of acetylcysteine at 100mg/kg. You have ampoules of acetylcysteine 2g/10ml available. The infusion needs to be given in 1 litre of glucose 5% over 16 hrs.

What is the dose?
What volume of acetylcysteine is needed?
How would you prepare and administer the infusion?

8. A 70kg patinet requires dopaine 4 micrograms/kg/min. The pharmacy supplies pre-made syringes containing 200mg/50ml.

What is the dose?
What rate in ml/hr needs to be administered?

9. A 25kg child with severe kyperkalaemia requires salbutamol 4 micorgrams/kg by intravenous injection. You have 500micrograms/ml injection available. The injection needs to be diluted with glucose 5% to a concenrtaion of 50 micrograms/ml prior to administration.

What is the dose?
How would you prepare the injection?

10. A 2.8kg neonate requires meropenem intravenous infusion 20mg/kg for septacemia. You have meropenem 500mg injection (dry powder) available. You speak to the on-call pharmacist who says to reconstitute the meropenem with water for injection to make 10ml and that meropenem has a displacement value of 0.4ml. Then you need to dilute further with sodium chloride 0.9% to obtain a 10mg/ml solution.

What is the dose
How would you prepare the infusion?
What volume of the meropenem 10mg/ml needs to be adminstered?

Answer
1. Dose = 9,800 units, Volume = 0.49ml (would round up to 0.5ml)

2. Rifampicin 250mg = 12.5ml of 100mg/5ml; Isoniazid 250mg = 25ml of 50mg/5ml; Pyrazinamide 875mg = 8.75ml of 500mg/5ml; Ethambutol 375mg = 9.4ml of 200mg/5ml

3. Volume = 0.5mg in 2ml = 500 micrograms in 2ml, 200micrograms/500micrograms x 2ml = 0.8ml

4. Dose = 62kg x 5mg = 310mg, Volume = 310mg/250mg x 10ml = 12.4ml

5. Dose = 62kg x 7mg = 479.5mg (round up to 480mg), Volume = 480mg/80mg x 2ml = 12ml

6. Dose = 70kg x 30micrograms = 2100 micrograms, 2100micrograms/1000 = 2.1mg, Volume = 2.1mg/4mg x 1ml = 0.53ml

7. Dose = 65kg x 100mg = 6500mg = 6.5g, Volume of acetylecysteine = 6.5g/2g x 10ml = 32.5ml.
Preparation/administration = remove 32.5ml from 1L gluocse 5% bag and add 32,5ml acetylcysteine 2g/10ml. Run the infusion at 62.5ml/hour (1000ml/16).

8. Rate = 4micrograms/kg/min = 4 x 70mivrograms/min = 280 micrograms/min
Want rate per hour so 280micrograms/min = 280 x 60 = 16800micrograms/hr = 16800/1000 = 16.8mg/hr, Have 200mg/50ml, want 16.8mg/hr so (16.8/200) x 50ml = 4.2ml/hour.

9. Dose = 100micrograms (25kg x 4 micrograms), 100 micrograms/500 micrograms x 1ml = 0.2ml of injection needed.

10. Dose = 20mg x 2.8kg = 56mg
Infusion preparation = 9.6ml water for injection to the 500mg vial of meropenem. This gives 500mg in 10ml (as displacement value is 0.4ml) i,e, 50mg/ml. Therefore, this needs diluting by a factor of five to obtain a 10mg/ml solution.
Take 10ml of the 500mg/10ml solution and add to 40ml of sodium chloride 0.9%. This gives 500mg in 50ml i.e. 10mg/ml. Need 56mg dose so 56mg/10mg x 1ml = 5.6ml. Give 5.6ml over 15-30 min (could also give over 5 min as a bolus)