4 replies·
Using the calculator and I want to check I understand what it is telling me rather than just trusting the number.
If a vial is 5 mg and I add 2 mL, that is 2.5 mg per mL. An insulin syringe marked to 100 units is 1 mL, so 100 units is 2.5 mg and 10 units is 250 mcg. Is that the whole of it or am I missing something?
Arithmetically that is correct and you have understood it.
The things that make it not the whole of it are all physical rather than mathematical:
The vial does not necessarily contain what the label says. Fill accuracy varies, and for anything not made under pharmaceutical quality systems it can vary a great deal. Your 5 mg is a claim.
The powder has volume. Small, usually negligible, but "2 mL of water" and "2 mL of solution" are not identical.
A U100 syringe is calibrated for insulin. The unit markings are a volume scale that happens to be labelled in insulin units. Reading them as volume is right; thinking they mean anything about your compound is not.
Losses. Dead space in the needle hub, a drop left in the barrel, a bit on the stopper.
None of that changes your maths. It changes how much confidence the number deserves, which is a different quantity and usually a smaller one.
On the syringe point — pick the barrel to fit the dose, not the other way round.
If your dose lands at 4 units, measuring it in a 1 mL barrel means reading a mark down at the very bottom of the scale where the graduations are closest together and the meniscus is hardest to judge. Same dose in a 0.3 mL barrel sits in the readable middle.
The needles and syringes guide covers this and it is the single most useful thing in it. Barrel size causes more dosing error than anything else I have seen.
That is a genuinely useful reframe, thank you. I had been treating the syringe as fixed and the dose as the variable.
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