Module 8 of 14 · 14 min
Patient dosing & dispensing quantities
Every paediatric prescription, chemotherapy order and taper you check needs a weight- or BSA-based dose, a cap check and a quantity to dispense; exam cases hide the trap in the units (lb, per day vs per dose).
🧠 Kilos first, Day or dose?, Cap it, Convert, Count — "KDCCC".
Read the order twice: is the dose per day (to be divided) or per dose? $90\ \text{mg/kg/day}$ divided BID is $45\ \text{mg/kg}$ per dose, not $90$.
Convert the weight first: $1\ \text{kg}=2.2\ \text{lb}$. A child weighing 44 lb is 20 kg; using 44 as kilograms more than doubles the dose.
Body-surface-area dosing (chemotherapy, some paediatric drugs) uses Mosteller: $\text{BSA}(\text{m}^2)=\sqrt{\dfrac{\text{height (cm)}\times\text{weight (kg)}}{3600}}$.
Then cap and count: compare the calculated dose to the stated maximum, convert mg to mL with the product strength, and multiply by doses per day and days to get the quantity to dispense (or divide the package by daily use to get the days' supply).
🧭 The method
- 1Convert weight to kg (lb ÷ 2.2) or compute BSA if the dose is per m².
- 2Calculate the daily dose; decide whether the order is per day or per dose.
- 3Divide into doses and compare each dose (and the day total) with the maximum — use the lower value.
- 4Convert each dose to mL or tablets using the product strength (mg per mL).
- 5Quantity = amount per dose × doses per day × days (or days' supply = package ÷ daily use); round to whole packages if asked.
Before each reveal, predict the next step — then check.
A child weighing 44 lb is prescribed amoxicillin 90 mg/kg/day divided BID for 10 days (maximum 4000 mg/day). The suspension is 400 mg/5 mL. How many mL per dose, and how many mL should be dispensed?
- 1
Convert pounds to kilograms.
$$44\ \text{lb}\times\frac{1\ \text{kg}}{2.2\ \text{lb}}=20\ \text{kg}$$ Daily dose, then check the cap: 1800 mg is below 4000 mg/day, so no change.
$$90\ \frac{\text{mg}}{\text{kg}\cdot\text{day}}\times20\ \text{kg}=1800\ \text{mg/day}$$Divide into two doses (BID).
$$\frac{1800\ \text{mg/day}}{2\ \text{doses/day}}=900\ \text{mg/dose}$$Convert mg to mL with the product strength.
$$900\ \text{mg}\times\frac{5\ \text{mL}}{400\ \text{mg}}=11.25\ \text{mL}$$Total to dispense for the course.
$$11.25\ \frac{\text{mL}}{\text{dose}}\times2\ \frac{\text{doses}}{\text{day}}\times10\ \text{days}=225\ \text{mL}$$
Same method — the last steps are yours. Work on paper, then reveal.
A child weighing 66 lb is prescribed cephalexin 50 mg/kg/day divided q6h for 7 days. The suspension is 250 mg/5 mL. How many mL per dose and how many mL in total?
- 1
Convert pounds to kilograms.
$$66\ \text{lb}\div2.2=30\ \text{kg}$$ - 2
Daily dose.
$$50\ \text{mg/kg/day}\times30\ \text{kg}=1500\ \text{mg/day}$$ - 3
q6h means 4 doses a day.
$$1500\div4=375\ \text{mg/dose}$$ - 4
Your turn
Now you: convert 375 mg to mL of the 250 mg/5 mL suspension.
Work it out on paper, then reveal.
$$375\ \text{mg}\times\frac{5\ \text{mL}}{250\ \text{mg}}=7.5\ \text{mL}$$ - 5
Your turn
Now you: total volume for 7 days.
Work it out on paper, then reveal.
$$7.5\ \text{mL}\times4\times7=210\ \text{mL}$$
Type your number. Your first try decides when this module comes back for review.
A salbutamol inhaler contains 200 metered doses. The patient uses 2 puffs four times daily. How many days will one inhaler last?
Daily use = puffs per dose × doses per day.
= 25 days
Ibuprofen 10 mg/kg/dose is ordered for a 48 kg adolescent, maximum 400 mg per dose. The suspension is 100 mg/5 mL. How many mL per dose?
Calculate the dose, then compare it with the per-dose maximum before converting to mL.
= 20 mL
A patient is 165 cm tall and weighs 64 kg. A drug is dosed at 75 mg/m². Using Mosteller, what is the dose (mg)?
BSA $=\sqrt{\text{cm}\times\text{kg}/3600}$, then multiply by the mg/m² dose.
= 128.5 mg
One application of cream to one adult arm needs about 3 fingertip units (1 FTU = 0.5 g). The patient applies it to both arms twice daily for 14 days. How many grams are needed?
Grams per arm × number of arms × applications per day × days.
= 84 g
Prednisone taper: 40 mg daily for 3 days, then decrease by 10 mg every 3 days until stopped. Only 5 mg tablets are available. How many tablets should be dispensed?
List the steps: 40, 30, 20, 10 mg (the next step, 0 mg, means stop). Each lasts 3 days.
= 60 tablets
A course needs 9000 mg of amoxicillin as 250 mg/5 mL suspension. Bottles: 100 mL for CAD 6.00 or 150 mL for CAD 8.10 (bottles cannot be split). What is the lowest cost (CAD) that supplies the full course?
Find the total volume first, then price every combination that covers it.
= 12 CAD
⚠️ Tricky
Amoxicillin 90 mg/kg/day divided q12h is ordered for a child who weighs 33 lb (1 kg = 2.2 lb). The suspension is 400 mg/5 mL. How many mL per dose?
Convert lb to kg before anything else, then split the daily dose into 2 doses.
= 8.44 mL
⚠️ The trap: Using 33 (the weight in pounds) as kilograms gives 1485 mg per dose = 18.6 mL, more than double the correct dose.
⚠️ Tricky
Acetaminophen 15 mg/kg/dose q4h around the clock (6 doses/day) is ordered for a 62 kg adolescent. Limits: 75 mg/kg/day, and never more than 4000 mg/day. Using 160 mg/5 mL suspension, what is the largest volume per dose (mL) that keeps the daily total within both limits?
Check the per-dose order against each daily limit; the lowest limit wins, then divide it by 6 doses.
= 20.8 mL
⚠️ The trap: Giving the weight-based 930 mg (29.1 mL) breaches both limits, and stopping at the 75 mg/kg/day limit (24.2 mL) still breaches the 4000 mg/day cap.
⚠️ Tricky
A 72 kg patient is prescribed cephalexin 500 mg q6h for 10 days. Only 250 mg capsules are stocked (bottles of 100). How many capsules should be dispensed?
q6h means every 6 hours: how many doses fit in 24 hours? The weight and the bottle size are not needed.
= 80 capsules
⚠️ The trap: Reading q6h as six times a day gives 120 capsules (and the 72 kg weight is a distractor).
⚠️ Tricky
Capecitabine 1250 mg/m² is given twice daily on days 1–14 of each 21-day cycle. The patient's BSA is 1.6 m², and each dose is made up of 500 mg tablets. How many tablets are needed for 2 cycles?
Dosing runs only on days 1–14; days 15–21 are a rest week.
= 224 tablets
⚠️ The trap: Counting all 21 days of each cycle instead of the 14 dosing days gives 336 tablets.
⚠️ Tricky
Azithromycin for an 18 kg child: 10 mg/kg on day 1, then 5 mg/kg once daily on days 2–5. The suspension is 200 mg/5 mL. What total volume (mL) is needed for the course (ignore overfill)?
Day 1 and days 2–5 use different doses; add them separately.
= 13.5 mL
⚠️ The trap: Applying the day-1 dose of 10 mg/kg to all 5 days gives 22.5 mL.
🕵️ Spot the mistake
Acetaminophen 15 mg/kg/dose for a child weighing 33 lb; suspension 160 mg/5 mL. How many mL per dose?
A student wrote
⚠️ Traps examiners set
- Using pounds as kilograms (always divide lb by 2.2 first).
- Treating a mg/kg/DAY order as mg/kg/DOSE (or the reverse).
- Forgetting to apply the maximum single or daily dose cap.
- Counting a taper step of 0 mg, or missing a step, when totalling tablets.
- Rounding volumes to whole bottles before choosing the cheapest combination.
💬 Explain it in your own words
Why must you check the maximum dose after multiplying by the weight, and what does it tell you when a child's calculated dose exceeds the adult dose?