I reconstituted a 10mg vial with 2ml of bacteriostatic water and I want someone to check my unit arithmetic before I draw anything, because I keep getting two different answers.
The three numbers to write on the vial: total mg, total ml, and mg per ml. Everything else is division. And the sanity check is that dose volume times number of doses should be less than the volume you put in, because dead space takes the difference.
The question I want answered is how to check the arithmetic without trusting a website calculator, since three calculators gave me three answers.
Practical detail welcome, however dull — the duller the better.
Answering the narrow version, because the broad one does not have a single answer. Do it in two steps and it stops being confusing. First concentration: 10mg into 2ml is 5mg/ml. Then volume: a 0.5mg dose is 0.5 ÷ 5 = 0.1ml. Then units, and this is where people go wrong — a U-100 syringe is graduated in hundredths of a millilitre, so 0.1ml is 10 units. The word "units" has nothing to do with milligrams; it is a volume marking that exists because insulin happens to come at 100 units per ml.
Happy to go further on any of that.
InsuranceTom said:Do it in two steps and it stops being confusing.
InsuranceTom has the substance of this right. The condition it depends on is worth stating. The arithmetic is right and one caveat matters: a U-40 syringe changes the markings, not the volumes. If someone hands you U-40 and you read it as U-100 you will be 2.5 times out, and that error has a direction — it is always an overdose.
Ask again with the specifics and you will get a better answer than this one.
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Shop Reference Standardspete_manc_UK said:I reconstituted a 10mg vial with 2ml of bacteriostatic water and I want someone to check my unit arithmetic before I draw anything, because I keep…
This matches mine closely enough to be worth saying so. Concentration choice is a precision decision, not a preference. Reconstitute high and every dose is a tiny volume where one unit of syringe error is a large fraction of the dose. Reconstitute low and you get more graduations per dose, so the same hand tremor costs proportionally less. Against that, more diluent means more benzyl alcohol and a shorter comfortable in-use window.
Adding the clinical framing, because it changes how the question reads.
Regarding dosing arithmetic multi-dose vial contamination prevention: after reconstituting, I wipe the vial stopper with a fresh alcohol swab before EVERY single draw. I also never touch the stopper with my fingers and use a new needle for each draw.
Even with bacteriostatic water preservative, contamination is cumulative. One sloppy draw can introduce bacteria that multiply over days. Aseptic technique isn't optional — it's essential for injectable medications.