U-100, U-50
and U-40.
The number on an insulin syringe is units per millilitre. One unit on a U-100 syringe is 0.01 ml; on a U-40 it is 0.025 ml. Draw a number of units worked out for a U-100 syringe on a U-40 barrel and you have drawn two and a half times the volume you meant to.
Insulin syringes are not interchangeable, and nothing on the barrel warns you. The scale is printed for a particular concentration of insulin, and using it for anything else means the units are only a ruler — one whose spacing depends on which syringe you happen to be holding.
What the number means
U-100 means 100 units per millilitre. U-40 means 40. The syringe is otherwise the same object: the same barrel, the same plunger, a different scale printed down the side.
So one unit is 0.01 ml on a U-100 syringe and 0.025 ml on a U-40. A U-50 sits between them at 0.02 ml, and is usually a 0.5 ml barrel with a finer scale rather than a different calibration.
Why the mix-up is the expensive one
The error does not look like an error. Ten units drawn on the wrong syringe is still ten units on the scale — the plunger simply sits somewhere else, and 0.1 ml becomes 0.25 ml.
That is two and a half times the intended volume, and it is reported as the single most common source of dosing mistakes in sub-milligram protocols. Nothing about the result looks wrong at the moment it happens.
Which one to reach for
U-100 is the usual choice, and for small volumes it is the more precise: the marks are closer together, so a given amount of liquid spans more of them and is easier to hit.
Whichever you use, work the arithmetic for that syringe rather than converting afterwards. A conversion done in your head at the vial is one more place for the factor of 2.5 to enter.
What is the difference between a U-100 and a U-40 syringe?
The number is units per millilitre. One unit on a U-100 syringe is 0.01 ml; on a U-40 it is 0.025 ml. The barrel is otherwise identical — only the printed scale differs.
What happens if you use a U-40 syringe instead of a U-100?
You draw two and a half times the volume you intended. The number of units on the scale is the same, so nothing looks wrong; the plunger simply stops at a different place and 0.1 ml becomes 0.25 ml.
How many ml is one unit on an insulin syringe?
0.01 ml on a U-100, 0.02 ml on a U-50 and 0.025 ml on a U-40. Multiply your volume in millilitres by 100, 50 or 40 respectively to get units.
Which insulin syringe is best for small doses?
U-100, because its marks are closest together. A small volume spans more of them, so it is easier to draw the amount you meant and easier to see that you did.
For informational purposes only — not medical advice