How to read an insulin syringe

This is the sentence to internalise. The markings on an insulin syringe describe how much liquid is in the barrel. They say nothing whatsoever about how much compound that liquid contains.

The scale was designed for insulin, where the concentration is standardised — U-100 insulin contains 100 international units of insulin per millilitre, so the syringe marks and the drug amount happen to coincide. That coincidence is what makes the syringe convenient for insulin and confusing for everything else.

Reconstitute a peptide and you have created your own concentration, chosen when you decided how much water to add. How much compound sits in one unit is entirely a consequence of that choice. Two people drawing the same number of units from differently reconstituted vials are drawing very different amounts.

The number after the U is the syringe's graduation: units per millilitre. U-100 is by far the most common; U-40 syringes exist mainly for veterinary insulin and turn up occasionally.

The consequence is that identical volumes read as different numbers. Drawing to 20 on a U-100 gives 0.2 mL; drawing to 20 on a U-40 gives 0.5 mL — two and a half times as much liquid, and therefore two and a half times as much compound. Any calculation has to know which syringe you are holding.

The plunger's rubber stopper has a flat top edge and a conical tip below it. Read to the flat top edge — the wide part where the rubber meets the barrel wall. Reading to the cone's point overstates the volume, consistently, by an amount that matters most when you are drawing small quantities.

Read at eye level with the syringe vertical. Looking from above or below introduces parallax, and on a barrel where a single unit is a fraction of a millimetre that is a real error.

Barrel capacity varies — 0.3 mL, 0.5 mL and 1 mL are all common — and so does what the markings represent. On a 1 mL U-100 syringe every line is typically one unit. On a 0.3 mL syringe the lines may be half-units, giving finer resolution. Count the lines between two numbered marks before assuming.

Air bubbles occupy volume, so a bubble in the barrel means less liquid than the marking suggests. With small volumes the proportional error can be significant.

Draw slightly past your target, hold the syringe needle-up, tap the barrel so bubbles rise to the top, then push the plunger gently to expel the air and settle at your mark. A small amount of foam near the top is common with peptide solutions and is not the same as a trapped bubble in the liquid column.