how much of what we believe about mg/mL actually comes from insulin syringe threads
how much of what we believe about mg/mL actually comes from insulin syringe threads, and I want the answer with the reasoning attached rather than just the conclusion.
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 arithmetic in full: concentration is mass over volume, so 20mg into 2mL gives 10mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 2.5mg is 2.5 divided by 10 millilitres, which is 25 units.
Research-use-only material is not approved for human use, so treat everything in this thread as arithmetic about solutions rather than instructions for a person.
Happy to answer the boring questions. Those are usually the ones worth asking.
best — the order this archive was captured in
Adding more water does not change how much peptide is in the vial. It changes the concentration, which changes the volume you draw for the same amount. That is the entire relationship.
This. Showing the working is the fastest way for somebody else to spot the mistake.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
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.
bacteriostatic water has the preservative, sterile water does not
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.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
What is written on the vial right now?
Not quite. Bacteriostatic and sterile water are not interchangeable for a multi-draw vial, and the difference is the preservative.
Cosigning the round-number approach. Reconstituting to something awkward is how you end up drawing 27.5 units at six in the morning.
Same. I got this wrong once, caught it because the units looked absurd, and have written on every vial since.
aim the stream at the glass wall, not at the cake
Did the maths give you a whole number of units, or a fraction?
Is that a U-100 syringe? The barrel markings change the answer completely.
Correction: that gives 40 units, not what you have written. 10mg into 2mL is 5mg/mL and the division follows from there.
twice as much water halves the concentration and doubles the units for the same dose
Disagree with the "more water is safer" framing. It is not safer, it is just a different number to divide by.
label the vial with the date as well as the concentration
I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume.
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.
- 1bacteriostatic water has the preservative, sterile water does not8 comments in this branch · started by u/priya_weiss