Insulin syringe (revision 7)

An insulin syringe is a small-volume syringe graduated in insulin units rather than in millilitres. The unit is a measure of insulin activity, and the graduation scale is derived from the U-100 concentration convention in which 100 units of insulin occupy 1 mL.^[1]

For any substance that is not U-100 insulin, the unit markings are therefore simply a volume scale in which one mark equals 0.01 mL. This is the whole of the conversion, and it is the source of most of the confusion surrounding these devices when they are used for other preparations. See Insulin syringe unit conversion.^[2]

Their prevalence in this context follows from the volumes involved. A dose of a few hundred micrograms of a peptide at a few milligrams per millilitre is a volume of tens of microlitres, which a 1 mL syringe measures poorly and a 0.3 mL syringe measures adequately.^[3]

A 0.3 mL syringe is graduated in single units, sometimes in half units; a 1.0 mL syringe usually in two-unit increments. Finer graduations on a smaller barrel give better resolution for small volumes, which is why barrel size is chosen to suit the volume rather than for convenience.^[3]

Accuracy standards for syringes specify tolerances as a percentage of nominal volume, so absolute error is smaller on a smaller syringe. Measuring 5 units in a 100-unit barrel is measuring at 5% of full scale, where both the tolerance and the reading error are large relative to the quantity.

Dead space — the volume retained in the hub and needle after the plunger is fully depressed — is a separate consideration and is proportionally larger for small doses. Low-dead-space designs reduce it substantially.^[3]