Syringe measuring mistakes

The mistakes at a glance

1. Units and mL confused

A U-100 unit is 0.01 mL, so reading “10 units” as “10 mL” is a 100-fold error. ISMP lists “U” or “u” for units as error-prone: it can be read as a zero or a four, or as “cc”, leading to a dose in volume instead of units. “cc” itself can be mistaken for “u”. ISMP's advice is to write “units” and “mL”.^[1]

Packaging adds to the risk. In 2024 ISMP reported a close call in which a 1 mL U-100 insulin syringe and a 1 mL syringe with mL markings came in nearly identical packaging.^[2] ISMP also recommends expressing a single drug's dose in metric weight, not volume alone.^[3] Our calculators therefore show mg, mL and units together.

2. The wrong syringe scale

A unit line holds 0.025 mL on a U-40 syringe and 0.01 mL on a U-100 syringe. Merck Animal Health warns that a U-100 syringe with U-40 veterinary insulin gives 2½ times less insulin than needed, and a U-40 syringe with U-100 insulin 2½ times too much, which could be fatal.^[5] Measuring U-500 insulin on U-100 scales led to what ISMP calls five-fold dosing errors.^[4] See U-40 vs U-100 and U-500 syringes.

3. Drawing insulin out of a pen

ISMP reported a patient who used a U-300 pen cartridge as a vial, drew up 100 units on a U-100 syringe and received 300 units, leading to hospital admission for low blood sugar.^[3] ISMP's guidance is that a pen cartridge is never used as a vial; drawing from one can also let air in and cause later under-doses.^[4] Concentrated-insulin labels say the same: do not transfer the insulin from the pen to a syringe.^[6][7]

4. Decimal points, mg and mcg

ISMP's list warns against a trailing zero (“1.0 mg” misread as 10 mg) and requires a leading zero before a decimal point (“0.5 mg”, not “.5 mg”). It lists “µg” as error-prone because it can be read as “mg”, a 1,000-fold slip; write “mcg”.^[1] The mcg to units calculator accepts both units so you do not have to convert by hand.

5. Assuming what one line is worth

Insulin syringes are made with ½-unit, 1-unit or 2-unit lines, depending on size and model; 1 mL syringes exist with either 1-unit or 2-unit lines.^[8] Counting lines on a 2-unit syringe as if each were 1 unit halves the dose. Check the numbers printed on your barrel first. See how to read an insulin syringe.

6. Air bubbles

A bubble takes up space in the barrel, so the liquid is less than the reading. MedlinePlus and embecta both describe tapping the bubbles to the top, pushing them back into the vial and drawing again to the right line.^[9][11] If there are many bubbles, redraw and recheck.^[10]

7. Not checking again

MedlinePlus advises looking at the syringe again after drawing up to make sure the dose is right.^[9] If the markings are hard to see, clip-on magnifiers are available.^[9]

8. Reusing or sharing

MedlinePlus says never to reuse needles or syringes.^[9] The CDC's rule is “One Needle, One Syringe, Only One Time”, and a syringe is never used for more than one person, even if the needle is changed.^[12]