Lessons · Nursing · reading a label: strength per volume
Reading a label: how much drug in how much liquid
A label states a strength as an amount of drug in an amount of liquid, or in one tablet: 125 mg per 5 mL means every 5 mL holds 125 mg, so every 1 mL holds 25 mg.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
The vial in your hand says 10 mg/mL and holds 2 mL. That is 20 mg in the vial, not 10, and a nurse who reads the strength as the whole vial draws up double. The label is the second number in every dose problem, and it has to be read as a rate, not a total.
How to think about it
Find the two numbers on the label: the amount of drug and the amount it is in. Write them as one over the other, with units. If it helps, divide to get the amount in one millilitre. Then look separately for the total volume in the container; that is a different number for a different question.
Worked example
Label: 125 mg per 5 mLThe strength. Every 5 mL of this liquid carries 125 mg.
Per mL: 125 / 5 = 25 mg/mLThe same strength said per millilitre. Either form works in the dose formula.
Vial: 10 mg/mL, 2 mL in the vial, so 10 × 2 = 20 mg in the whole vialStrength times volume is the total. The total is what the vial holds, not what any one dose is.
Tablet: 500 mg, scored, so half a tablet is 250 mgA scored tablet has a line to break it on. An unscored one does not, and is not halved.
Your turn
A label reads 80 mg per 2 mL. Write the strength per millilitre.
80 / 2 = mg/mL
Solve one, graded on the server
The trap
Reading the total in the vial as the strength. A 2 mL vial labelled 10 mg/mL holds 20 mg; draw up the whole vial for a 10 mg order and you have given twice the dose. Strength and total are two different numbers on the same label.