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Hong KongWeight-based dosing
Convert the weight to kilograms first, multiply by the dose per kilogram, then convert the resulting mass to a volume using the product concentration. The conversion is where errors happen.
Three steps in a fixed order. Doing them out of order is how a correct method produces a wrong answer.
The order
- Weight to kilograms
- Kilograms times the dose per kilogram, giving a mass
- Mass to volume, using the concentration
Never skip step one, even when the weight looks like it is already in kilograms. Read the unit rather than assuming it.
Worked
A child weighs 44 pounds. The order is 15 mg per kg per day, and the suspension is 250 mg per 5 mL.
Forty-four divided by 2.2 is 20 kg. Twenty times 15 is 300 mg per day. Three hundred divided by 250, times 5, is 6 mL per day.
If the order is 15 mg per kg per day divided into three doses, the answer to "how much per dose" is 2 mL, not 6. The stem will say which it wants and the distractors will include both. Reading the question is worth more here than the arithmetic.
The distractors are predictable
| Wrong answer | The error behind it |
|---|---|
| Multiplied by 2.2 instead of dividing | Conversion inverted |
| Used the pound figure directly | Conversion skipped |
| Daily dose given when a single dose was asked | Frequency ignored |
| Out by a factor of ten | Decimal place lost in the concentration step |
Knowing the four failure modes means you can often spot your own error by seeing your answer sitting in the options.
Checking
Sanity-check the scale. A 20 kg child receiving 6 mL of a suspension is plausible; 600 mL is not. That check costs two seconds and catches the decimal errors.
Maximum dose questions
A variant gives you a calculated dose and a published maximum and asks whether the order is safe. The calculation is the same; the extra step is the comparison, and the answer is sometimes that the dose exceeds the maximum.
Do not assume the order in the stem is correct. Some of these questions exist precisely because it is not.
Common questions
How do I do a weight-based dose calculation?
Convert weight to kilograms, multiply by the dose per kilogram, then convert the mass to a volume using the concentration.
What is the most common error?
Inverting or skipping the pound-to-kilogram conversion.
What is the difference between per day and per dose?
A daily dose divided into three gives a per-dose figure a third of the size. The stem says which it wants.
How do I check my answer?
Sanity-check the scale. 6 mL for a 20 kg child is plausible; 600 mL is not.
What is a maximum dose question?
One where you calculate the dose and compare it to a published maximum. Sometimes the order in the stem exceeds it.