Units, millilitres and micrograms: reading an insulin syringe

A unit is a volume, not an amount of compound

An insulin syringe is a U-100 syringe. U-100 means it was designed for insulin at a concentration of 100 units per millilitre, so the barrel is marked in 100 units per mL: one unit is 0.01 mL, ten units is 0.1 mL, and a full 1 mL syringe reads 100 units. Those marks measure liquid. They say nothing at all about how much compound is dissolved in it — that depends entirely on the water you added.

The three conversions worth knowing by heart

1. 1Units to millilitres: divide by 100. 25 units is 0.25 mL.
2. 2Milligrams to micrograms: multiply by 1,000. 0.25 mg is 250 mcg.
3. 3Concentration: vial content divided by water added. A 5 mg vial in 2 mL is 2.5 mg/mL, which is 2,500 mcg per mL, or 25 mcg per unit.

Run those in order and a dose becomes a mark on the barrel: at 25 mcg per unit, a 250 mcg draw is 10 units. Skip one and the answer is wrong by whole powers of ten, which is how the two classic errors happen: reading 0.25 mL as 25 mL is a hundredfold slip, and using a figure in milligrams as though it were micrograms is a thousandfold one.

Barrel size changes the resolution, not the scale

A 0.3 mL syringe holds 30 units, a 0.5 mL holds 50, a 1 mL holds 100 — all still U-100, all still 0.01 mL per unit. The smaller barrels simply spread those units over the same physical length, so each mark is further apart and a small draw is easier to read. If a draw comes out at four units, a 0.3 mL barrel will measure it far better than a 1 mL one.

Pens count in clicks, not units

A reusable cartridge pen does not have a barrel to read. It dials in fixed steps, and a requested volume that falls between two clicks has to land on one of them. That is a different kind of rounding from a syringe, where any mark can be split by eye — which is why the calculator reports what a pen can actually be dialled to rather than the exact figure.