[PSA] units are not what most of this community thinks it is
units are not what most of this community thinks it is — with the reasoning, because a rule without a reason gets ignored.
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.
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.
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.
I will update this if the picture changes rather than quietly leaving it up.
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.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
Yes, and the dead-space point is worth repeating: it is small, it is real, and it is the same for every draw.
The arithmetic in full: concentration is mass over volume, so 15mg into 3mL gives 5mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 1.5mg is 1.5 divided by 5 millilitres, which is 30 units.
bacteriostatic water has the preservative, sterile water does not
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.
Right — the volume you add is the only variable you control here, and the cake does not change it meaningfully.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
Spent 19 weeks confused about this until somebody drew the table for me. 10mg, 1mL, 10mg/mL, 2.5mg is 25 units. That is the whole thing.
Disagree with the "more water is safer" framing. It is not safer, it is just a different number to divide by.
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.
label the vial with the date as well as the concentration
I would not round that figure. At 10mg/mL a rounding error of two units is a real fraction of the intended amount.
Pushback on the syringe advice: a U-100 barrel reads units of volume. Calling them "doses" is how the confusion starts.
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.
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.
Working has to be shown on arithmetic posts. Added a note asking for it rather than removing this one.
[removed by moderator]
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.
Correction: that gives 10 units, not what you have written. 10mg into 2mL is 5mg/mL and the division follows from there.
Asked QYB what volume they reconstitute to for their own stability testing and they answered in one line. Useful thing to know.
Asked QYB what volume they reconstitute to for their own stability testing and they answered in one line.
This is the sentence that makes the rest unnecessary. Write it on the vial and the problem is gone.
reconstituting at a round concentration makes every later dose easy
the number you write down is the number you will trust at 6am
do the arithmetic before you reconstitute, not while holding a syringe
Yes. Writing 10mg/mL on the vial takes four seconds and removes the entire class of error this board exists to fix.
Cosigning the round-number approach. Reconstituting to something awkward is how you end up drawing 27.5 units at six in the morning.
- 1Adding more water does not change how much peptide is in the vial. It…9 comments in this branch · started by u/swirl_dont_shake