the insulin syringe question that gets asked weekly, answered properly
Posting this as a discussion rather than a claim: the insulin syringe question that gets asked weekly, answered properly.
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 20mg vial. You add 1mL of bacteriostatic water. Concentration is mass over volume: 20mg/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 20 millilitres, which is 25 units on the barrel. Write "20mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
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.
That is everything I have. The rest is opinion and I have tried to keep it out.
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.
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.
Yes, and the dead-space point is worth repeating: it is small, it is real, and it is the same for every draw.
Yes. Writing 5mg/mL on the vial takes four seconds and removes the entire class of error this board exists to fix.
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Right — the volume you add is the only variable you control here, and the cake does not change it meaningfully.
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.
Agreed — units are volume. Once that lands, the rest of the arithmetic is one division.
How many mg in the vial and how many mL are you adding?
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
- 1Yes. Writing 5mg/mL on the vial takes four seconds and removes the entire…7 comments in this branch · started by u/aleksi_lehtinen