three years of insulin syringe threads, summarised so you do not have to read them
Posting this as a discussion rather than a claim: three years of insulin syringe threads, summarised so you do not have to read them.
Why "more water" keeps confusing people, including me for a month.
The vial contains a fixed amount of peptide. Water changes nothing about that amount. What it changes is concentration, and concentration is the divisor between the dose you want and the volume you draw.
So doubling the water halves the concentration and doubles the units for the same dose. Nothing is diluted in any sense that matters; you are simply reading a bigger number off the same barrel. Pick the water volume that puts your usual dose on a whole number of units and life gets easier.
The whole calculation, once, with real numbers.
You have a 5mg vial. You add 3mL of bacteriostatic water. Concentration is mass over volume: 1.67mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 0.75mg you need 0.75 divided by 1.67 millilitres, which is 45 units on the barrel. Write "1.67mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
Wrote the date on the vial as well as the concentration after losing track of which one was opened first.
Tell me where this is wrong. That is the useful part of posting it.
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.
Show the arithmetic — where did the unit figure come from?
The failure modes I have actually seen posted here, in order of frequency.
One: reconstituting at a different volume than intended and not writing it down, so the next draw is based on a remembered concentration. Two: treating units as a dose rather than a volume, which breaks the moment the concentration changes. Three: assuming a syringe is U-100 when it is not.
All three are solved by the same two habits — do the arithmetic before the water goes in, and write the result on the glass. Neither costs anything and between them they cover almost every question this board gets.
Small fix — a U-100 syringe is 100 units per mL. If yours is marked differently it is not U-100 and the whole calculation changes.
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.
I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume.
Bacteriostatic water contains benzyl alcohol as a preservative, which is what makes repeated entry into the same vial reasonable. Plain sterile water has no preservative.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
What is written on the vial right now?
Correction: that gives 90 units, not what you have written. 5mg into 3mL is 1.67mg/mL and the division follows from there.