[PSA] mg/mL is not what most of this community thinks it is
mg/mL is not what most of this community thinks it is — with the reasoning, because a rule without a reason gets ignored.
The dead space question sent me down a rabbit hole for a week. It is a fraction of a unit and it is the same every draw.
Spent 17 weeks confused about this until somebody drew the table for me. 10mg, 3mL, 3.33mg/mL, 1mg is 30 units. That is the whole thing.
The whole calculation, once, with real numbers.
You have a 10mg vial. You add 2mL of bacteriostatic water. Concentration is mass over volume: 5mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 5mg you need 5 divided by 5 millilitres, which is 100 units on the barrel. Write "5mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
Corrections welcome, especially the pedantic ones. Pedantry is how this board earns its reputation.
best — the order this archive was captured in
Aim the stream down the wall of the vial rather than into the cake. Reconstitution is a solubility problem, not a mixing problem, and mechanical shear does nothing helpful.
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
Careful — that arithmetic is wrong. 10mg in 3mL is 3.33mg/mL, so 1.5mg draws to 45 units, not what you wrote.
If your calculation produces a fraction of a unit, the concentration is wrong for your dose. Change the water volume until the numbers land on whole units.