Calculating Quantities
Calculating quantities needed to prescribe:
Tablets or capsules: This is fairly straightforward as you know how many tablets the patient needs to take and how long you need to give them. It can get more tricky however with variable dosage. Here are 2 examples of more complicated dosage quantity calculations:
1. A patient with brittle asthma requires a reducing course of prednisolone back to maintenance dose of 5mg following an acute exacerbation. You prescribe the following regimen, calculate how many prednisolone 5mg tablets are required to get back to maintenance dose.
40mg daily for 5 days then
30mg daily for 5 days
25mg daily for 3 days
20mg daily for 3 days
15mg daily for 2 days
10mg daily for 2 days
then 5mg maintenance as before
2. A patient is on warfarin for atrial fibrillation. You prescribe a new dosage regimen based on their latest INR result: Take 4mg daily Mon-Fri and 5mg daily Sat/Sun. Warfarin comes as 1mg, 3mg and 5mg tablets. Calculate the quantity you would need for a 28 day supply.
Inhalers and sprays: You would need to know how many dosage units are in each inhaler or spray and know the strength of the dosage unit, the frequency and duration of treatment.
3. A patient requires a 200 dose beclometasone nasal spray 50 micrograms for hay fever. The dose is 2 sprays in each nostril twice a day, how long should the spray last?
4. How long will the eye drops for glaucoma in the following prescription last?
Bimatoprost ophthalmic solution
1 drop once daily in both eyes at night
Supply 5ml
Answers
1 = 107
2 = 1mg + 3mg = 4mg daily dose, 5 days a week, = 20 x 1mg, 20 x 3mg. 5mg daily dose 2 days per week = 8 x 5mg
(Note: in practice patients would probably have a surplus of tablets supplied as a 28 day pack so if dosage changes they can easily adjust)
3 = 25 days (2 sprays x 2 each time = 4 x 2 (twice daily) = 8 then 200/8 = 25 days)
4 = The patient will use 2 drops per day.
Assume 1 mL = 20 drops and, then, 5 mL = 100 drops
Then, the total days supply will be 100 drops divided by 2 drops which is 50 days.
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