[Meta] the mg/mL rule is doing its job and people should stop complaining
the mg/mL rule is doing its job and people should stop complaining. Short post, long comment section, probably.
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
The whole calculation, once, with real numbers.
You have a 10mg vial. You add 3mL of bacteriostatic water. Concentration is mass over volume: 3.33mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 1mg you need 1 divided by 3.33 millilitres, which is 30 units on the barrel. Write "3.33mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume.
If two or three other people have done the same thing we might actually learn something. Alone it is an anecdote.
best — the order this archive was captured in
The arithmetic in full: concentration is mass over volume, so 5mg into 3mL gives 1.67mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 0.5mg is 0.5 divided by 1.67 millilitres, which is 30 units.
Reminder: this board does maths, not dosing. Nobody here tells another member what amount to take.
do the arithmetic before you reconstitute, not while holding a syringe
The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests. It is a fraction of a percent and it is dwarfed by fill tolerance.
The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests.
This is the sentence that makes the rest unnecessary. Write it on the vial and the problem is gone.
Pushback on the syringe advice: a U-100 barrel reads units of volume. Calling them "doses" is how the confusion starts.
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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.
What concentration did you actually reconstitute at, rather than what did you intend to?
What is written on the vial right now?
the number you write down is the number you will trust at 6am
dead space in the needle hub is real and it is small
What is written on the vial right now?
Right, and the arithmetic behind it: 15mg in 3mL is 5mg/mL, so 0.75mg is 15 units.
I would not round that figure. At 5mg/mL a rounding error of two units is a real fraction of the intended amount.
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.
5mg into 1mL is 5mg/mL, and 0.5mg of that is 10 units
- 1do the arithmetic before you reconstitute, not while holding a syringe7 comments in this branch · started by u/marta_weiss