units are the most under-discussed thing on this board
Something I keep coming back to: units are the most under-discussed thing on this board.
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.
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.
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.
Happy to answer the boring questions. Those are usually the ones worth asking.
best — the order this archive was captured in
Left up and flaired Explainer — the numbers in this one check out.
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.
This. Showing the working is the fastest way for somebody else to spot the mistake.
the powder cake sits at the bottom, the volume you add is the volume that counts
The whole calculation, once, with real numbers.
You have a 15mg vial. You add 3mL of bacteriostatic water. Concentration is mass over volume: 5mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 0.25mg you need 0.25 divided by 5 millilitres, which is 5 units on the barrel. Write "5mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
Adding one thing — put the date on the label too. Concentration plus date and you can never mix two vials up.
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Show the arithmetic — where did the unit figure come from?
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.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
anya_antonsen has the units point exactly right. They are a volume on the barrel and nothing else.
Is that a U-100 syringe? The barrel markings change the answer completely.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
reconstituting at a round concentration makes every later dose easy
Disagree with the "more water is safer" framing. It is not safer, it is just a different number to divide by.
Correcting myself above: I wrote the concentration the wrong way up. Mass over volume, not volume over mass.
units are a volume on a U-100 syringe, not a dose
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.
bacteriostatic water has the preservative, sterile water does not
Correction: that gives 100 units, not what you have written. 15mg into 3mL is 5mg/mL and the division follows from there.
Yes, and the dead-space point is worth repeating: it is small, it is real, and it is the same for every draw.
Careful — that arithmetic is wrong. 30mg in 3mL is 10mg/mL, so 1mg draws to 10 units, not what you wrote.
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
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.
Same. I got this wrong once, caught it because the units looked absurd, and have written on every vial since.
Are you accounting for the powder volume, or is that where the confusion is?
I would not round that figure. At 2.5mg/mL a rounding error of two units is a real fraction of the intended amount.
Reconstituted at 3mL when I meant to use a different volume. Caught it because the units looked wrong on the barrel. Now I write 3.33mg/mL on the glass before I put the cap back.
5mg into 3mL is 1.67mg/mL, and 0.5mg of that is 30 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.
- 1The whole calculation, once, with real numbers. You have a 15mg vial. You…7 comments in this branch · started by u/ledger_mod
- 2Is that a U-100 syringe? The barrel markings change the answer completely.6 comments in this branch · started by u/india_generic_g