units — 21 things I got wrong before I got it right
Thinking out loud about this: units — 21 things I got wrong before I got it right.
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.
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
Wrote the date on the vial as well as the concentration after losing track of which one was opened first.
I will update this if the picture changes rather than quietly leaving it up.
best — the order this archive was captured in
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.
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.
I would not round that figure. At 10mg/mL a rounding error of two units is a real fraction of the intended amount.
do the arithmetic before you reconstitute, not while holding a syringe
Right — the volume you add is the only variable you control here, and the cake does not change it meaningfully.
How many mg in the vial and how many mL are you adding?
Yes. Writing 2.5mg/mL on the vial takes four seconds and removes the entire class of error this board exists to fix.
Yes.
Disagreeing with this line only: adding more water is not a safety margin, it is a different divisor.
the powder cake sits at the bottom, the volume you add is the volume that counts
What is written on the vial right now?
Right — the volume you add is the only variable you control here, and the cake does not change it meaningfully.
Adding one thing — put the date on the label too. Concentration plus date and you can never mix two vials up.
units are a volume on a U-100 syringe, not a dose
Corrected the concentration in the title, which contradicted the body. Nothing else touched.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
Correction: that gives 10 units, not what you have written. 30mg into 2mL is 15mg/mL and the division follows from there.
Did the maths give you a whole number of units, or a fraction?
The whole calculation, once, with real numbers.
You have a 10mg vial. You add 1mL of bacteriostatic water. Concentration is mass over volume: 10mg/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 10 millilitres, which is 50 units on the barrel. Write "10mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
What concentration did you actually reconstitute at, rather than what did you intend to?
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
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.
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.
Spent 10 weeks confused about this until somebody drew the table for me. 10mg, 2mL, 5mg/mL, 0.5mg is 10 units. That is the whole thing.
Asked SSA what volume they reconstitute to for their own stability testing and they answered in one line. Useful thing to know.
if the maths gives you a fraction of a unit, change the concentration
Pushback on the syringe advice: a U-100 barrel reads units of volume. Calling them "doses" is how the confusion starts.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
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.
Not quite. Bacteriostatic and sterile water are not interchangeable for a multi-draw vial, and the difference is the preservative.
- 1Right — the volume you add is the only variable you control here, and the…8 comments in this branch · started by u/clara_weiss
- 2Bacteriostatic water contains benzyl alcohol as a preservative, which is…8 comments in this branch · started by u/karl_fischer_kev