Short answer first, then the reasoning. Dead space is the answer to the missing dose. A fixed-needle insulin syringe holds a few microlitres in the hub and needle after the plunger bottoms out, and on small draws that is a measurable percentage of every dose. Across ten draws it adds up to most of an eleventh, which is exactly the "nine draws from a ten-dose vial" complaint. Luer-lock syringes are worse; low-dead-space fixed-needle designs are better.

That is the short version; the long version is somebody else's post.

COA_Karl said:

Dead space is the answer to the missing dose.

Agreed, and it is worth saying that rounding to the nearest whole unit is usually acceptable at maintenance doses and is not acceptable at the bottom of a ladder, where one unit can be a fifth of the intended dose.

That is the short version; the long version is somebody else's post.

InsuranceTom said:

I reconstituted a 10mg vial with 2ml of bacteriostatic water and I want someone to check my unit arithmetic before I draw anything, because I keep…

This matches mine closely enough to be worth saying so. Do it in two steps and it stops being confusing. First concentration: 10mg into 2ml is 5mg/ml. Then volume: a 0.5mg dose is 0.5 ÷ 5 = 0.1ml. Then units, and this is where people go wrong — a U-100 syringe is graduated in hundredths of a millilitre, so 0.1ml is 10 units. The word "units" has nothing to do with milligrams; it is a volume marking that exists because insulin happens to come at 100 units per ml.

Clinical perspective, offered as context rather than as advice.

Quick syringe math for dosing arithmetic dosing: if your vial is reconstituted at 5mg/2mL, that's 2.5mg/mL. For a 0.25mg dose you need 0.1mL = 10 units on an insulin syringe.

I keep a chart on my fridge:

• 0.25mg = 10 units
• 0.5mg = 20 units
• 1.0mg = 40 units
• 1.25mg = 50 units

Always double-check your concentration before drawing up!