Insulin and Heparin Units: Safe Calculation for the CBT

Insulin and heparin are dosed in units, a measure of biological activity that never converts to milligrams. The arithmetic in this chapter is the same want-over-have you’ve used since the tablets chapter. What’s different is that the errors here are the ones that reach coroner’s courts, so the exam tests the safety conventions as hard as the numbers.

Three facts anchor everything:

• Standard insulin is 100 units per mL.
• Heparin ampoules come in several strengths: 1,000, 5,000 and 25,000 units/mL are all common.
• A unit of one drug has no relationship to a unit of another.

Drug	Common strength	Safety note
Standard insulin	100 units/mL	Give from an insulin syringe graduated in units, not a general-purpose syringe
Heparin	1,000 / 5,000 / 25,000 units/mL	Always read the ampoule — same dose, very different volumes

Insulin: the syringe is the answer

For subcutaneous insulin doses, the calculation question has a trick ending: the safest answer isn’t a volume at all. Insulin syringes are graduated in units. You dial 24 units, you give 24 units, and no conversion happens anywhere.

The CBT exploits this. “How do you draw up 24 units from a 100 units/mL vial?” offers 0.24 mL, which is arithmetically correct, alongside “24 units on an insulin syringe”, which is the right answer. Converting insulin to millilitres in a general-purpose syringe is itself the unsafe act; done with the wrong mental arithmetic it produces the ten-fold errors insulin is notorious for.

Insulin infusions are built around the same safety idea: 50 units in 50 mL gives 1 unit per mL, so the prescribed units per hour and the pump’s mL per hour are the same number. When you see that concentration in a question, the “calculation” is recognising there isn’t one.

Heparin: read the ampoule

Heparin’s danger is stock variety. Prescribed 5,000 units SC, the volume is 1 mL from a 5,000 units/mL ampoule and 0.2 mL from a 25,000 units/mL ampoule. Same dose, five-fold different volumes, both ampoules in circulation. The exam mirrors real incidents by putting the assumed-strength answer in the options.

Heparin infusions are the dose-calibrated pump calculation from the infusion chapter: concentration first (25,000 units ÷ 50 mL = 500 units/mL), then prescribed rate over concentration (1,400 ÷ 500 = 2.8 mL/h).

The abbreviation rule

Write “units” in full. Always. The abbreviation U, handwritten, reads as a zero: 10U becomes 100 units. IU (international units) fails the same way, with the I read as a 1. This isn’t style guidance; ten-fold insulin overdoses from misread abbreviations recur in NHS incident data, and UK prescribing standards ban the abbreviations outright. The CBT asks about this directly, as a safety question wearing numeracy clothing.

Five questions below cover the syringe rule, both heparin shapes and the abbreviation trap. Next: infusion durations, the calculation you’ll run every time a pump alarm is due.