The particulars: 40 units · 2.5 mg/mL.
I want a method I can write down and repeat, not a rule of thumb.
I would rather over-engineer this than discover a problem later, within reason.
So: what is the actual procedure, and which steps matter as opposed to being ritual?
40 units is 0.4 mL, which at 2.5 mg/mL is 1 mg. Work it in that order and there is nothing to guess at: units to volume on the U-100 scale, volume to dose through the concentration. Draw with the barrel vertical, expel the air before you read, and read the leading edge of the plunger stopper rather than the shoulder — the difference between the two is about a unit on a 0.3 mL barrel.
The short version: U-100 only, smallest barrel that fits the dose, read the leading edge of the plunger stopper.
Needle length for subcutaneous administration is conventionally 4 to 8 mm; the 4 mm length is sufficient for essentially all adults and reduces the chance of an intramuscular injection.
Stated carefully, u-40 syringes still exist, mostly in veterinary supply. A U-40 barrel marked "20 units" holds 0.5 mL, not 0.2 mL. Using one with U-100 arithmetic overdoses by a factor of two and a half.
U-100 is a concentration standard meaning 100 units of insulin per millilitre; the syringe graduation is a volume scale expressed in those units.
Read the leading edge of the stopper, every time.
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Browse resultsThe relevant precision limit is that you cannot reliably read half a graduation, so the barrel decides the smallest change you can make.
Barrel sizes and graduations: 0.3 mL barrels are marked in single units to 30, 0.5 mL barrels in single units to 50, and 1 mL barrels usually in two-unit increments to 100. The smallest barrel that holds the dose gives the finest reading.
A 2 mm air bubble in a 0.3 mL barrel is about 4 microlitres, which against a 10-unit (100 microlitre) draw is a four per cent error. Flick it to the hub and expel it before reading.
Half-graduation estimation is not measurement. If the dose needs it, change the concentration instead.
Check the syringe is U-100 and not U-40 before doing any arithmetic with it.
edited 10 Dec 2024 by h_pergande — reworded for clarity after a comment
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