What I have: 10 units · 10 mg/mL.
I would like to understand the steps well enough to explain them to someone else.
I have access to a refrigerator with a logger and a freezer without one, which may be relevant.
So: what is the actual procedure, and which steps matter as opposed to being ritual?
10 units is 0.1 mL, which at 10 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.
Answer first: on a U-100 syringe one unit is 0.01 mL, so units are simply volume in millilitres multiplied by one hundred. Nothing else about the scale is doing any work.
The conversion in full: volume in mL = dose in mg ÷ concentration in mg/mL; units = volume × 100. At 5 mg/mL, a 0.25 mg dose is 0.05 mL, which is 5 units. At the same concentration a 1 mg dose is 0.2 mL, which is 20 units.
Read the leading edge of the rubber plunger stopper against the graduation, not the tip of the conical shoulder. The difference is roughly one unit on a 0.3 mL barrel, which is a four per cent error on a 25-unit dose.
U-100 is a concentration standard meaning 100 units of insulin per millilitre; the syringe graduation is a volume scale expressed in those units.
The caveat is that this describes an instrument, not a treatment, and research-use compounds are not approved for human use.
One unit is 0.01 mL on U-100. That is the whole scale.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe relevant detail is that a dose that lands below about five units on any barrel is being measured too coarsely and the reconstitution should be redone.
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.
Put another way, 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.
The 4 mm needle length is supported by injection-technique studies showing adequate subcutaneous delivery across body-mass ranges without increased intramuscular delivery.
Smallest barrel that holds the dose. Resolution is free and you should take it.
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