Reading a syringe: U-100 units versus mL.

Almost every dosing error that reaches a forum post is a units error, and most of those come from reading a syringe as though it were marked in millilitres. It is worth ten minutes to make this unambiguous.

What U-100 means.

U-100 describes the scale printed on the barrel, not the syringe's capacity and not the contents. On a U-100 syringe the barrel is divided so that 100 units equals exactly 1 mL. From that single fact everything else follows: 10 units is 0.1 mL, 25 units is 0.25 mL, and 50 units is 0.5 mL.

Capacity is a separate number.

Insulin syringes commonly come in 0.3 mL, 0.5 mL, and 1 mL barrels, which hold 30, 50, and 100 units respectively. All three use the same U-100 scale, so 20 units means the same volume in each. The difference is how far apart the marks are printed. A 0.3 mL barrel spreads 30 units over the same physical length that a 1 mL barrel uses for 100, which makes small volumes far easier to read accurately.

U-100 versus U-40.

U-40 syringes exist, mostly in veterinary supply, and on them 40 units equals 1 mL. A volume drawn to the same numeric mark on a U-40 syringe is two and a half times what it would be on a U-100 syringe. The barrels look alike, so check the printed scale rather than the shape, and do not mix scales within a protocol.

Reading the mark.

1. 01Hold the syringe at eye level so the plunger seal and the mark line up without parallax.
2. 02Read from the leading edge of the plunger seal, the edge nearer the needle.
3. 03Expel air bubbles before reading, since a bubble occupies volume the scale is counting.
4. 04If your target consistently lands between marks, consider reconstituting to a different concentration so it lands on one.

The PepCue calculators return results in U-100 units and draw them on a syringe illustration for exactly this reason: the number on screen should match the mark you are looking at, with no conversion left for you to do at the counter.