Bacteriostatic water sourcing and sterility considerations — July 2024

This one has a reasonably settled answer, so here it is. 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.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

DebRD_ATL said:

Do it in two steps and it stops being confusing.

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.

Dr.GastroMayo said:

Two of us with identical vials and identical diluent arrived at different unit counts, which means one of us has a conceptual error rather than a…

This matches mine closely enough to be worth saying so. Concentration choice is a precision decision, not a preference. Reconstitute high and every dose is a tiny volume where one unit of syringe error is a large fraction of the dose. Reconstitute low and you get more graduations per dose, so the same hand tremor costs proportionally less. Against that, more diluent means more benzyl alcohol and a shorter comfortable in-use window.

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!