Accepted answer
Both, and that is the trap: a U-40 barrel is 40 units per millilitre, so one unit is 1 ÷ 40 = 0.025 mL against the 0.01 mL of a U-100 — a factor of 2.5. Draw a dose you worked out for a U-100 barrel on a U-40 one and you have drawn 2.5 times the volume, and therefore 2.5 times the dose. Worked the other way: 20 units on U-100 is 0.2 mL, which is 8 units on U-40. The markings changed and the arithmetic changed with them, because the markings are the arithmetic — a unit is a volume with a scale attached, not a quantity of drug. The safe habit is to stop thinking in units entirely at the point of calculation: go from milligrams to millilitres through your concentration, then convert millilitres to marks using the scale printed on the barrel you are actually holding. U-40 barrels exist because U-40 insulin does, and a box that arrives without you having chosen it is the usual way one ends up in a drawer.
The short version: U-100 only, smallest barrel that fits the dose, read the leading edge of the plunger stopper.
Barrel sizes and graduations: 0.3 mL barrels are marked in single units to 30, 0.5 mL barrels in single units to 50, and 1 mL barrels usually in two-unit increments to 100. The smallest barrel that holds the dose gives the finest reading.
Fixed-needle insulin syringes retain roughly 3 to 5 microlitres after the plunger bottoms out. Detachable-needle luer syringes retain 35 to 100 microlitres depending on hub design — an order of magnitude more.
Published dead-space comparisons put low-dead-space fixed-needle designs under 2 microlitres and conventional detachable-needle designs at 35 microlitres or more.
Nothing here is medical advice.
Smallest barrel that holds the dose. Resolution is free and you should take it.
8Worth flagging that the U-40 syringes still exist and this arithmetic does not apply to them. – tenth_of_a_unit 7 months ago 7Adding a vote because this deserves more of them. – marta_okonkwo 5 months ago add a comment