Lessons · Pharmacy · reconstituting a powder
Reconstitution: the powder takes up room
The powder in the vial has a volume of its own, so the diluent added is less than the final volume, and the strength after mixing is the drug divided by that final volume.
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 label says add so many millilitres and the final volume will be so many more. A technician who adds the final volume as diluent makes a vial that is too weak, and nothing about the finished vial shows it.
How to think about it
Read two numbers off the vial's label: the diluent to add, and the final volume after mixing. The strength is the drug in the vial divided by the FINAL volume. The powder volume, if it is asked for, is the final volume less the diluent.
Worked example
Vial holds 1,000 mg of powder; final volume after mixing is 10 mLBoth numbers come off the label.
1,000 mg / 10 mL = 100 mg/mLThe strength to write on the vial once it is mixed.
Powder volume = 10 mL - 9.3 mL of diluent = 0.7 mLWhat the powder itself takes up.
A 400 mg dose off that vial: 400 / 100 = 4 mLEvery dose after this comes off the new strength.
Your turn
A vial holds 500 mg and the final volume after mixing is 2 mL. Write the strength.
500 / 2 = mg/mL
Solve one, graded on the server
The trap
Adding the final volume as diluent. The vial ends up bigger and weaker than the label says, and every dose drawn off it is short.