What I have: 15 units · 1 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.
Concretely, what should I do, and how would I know afterwards whether I did it right?
15 units is 0.15 mL, which at 1 mg/mL is 0.15 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.
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.
| Appearance | Interpretation | Action |
|---|---|---|
| Intact opaque puck, proud of base | Cycle ran correctly | Proceed |
| Slumped to one side | Shipped before fully dry, or vibration | Usually usable; note it |
| Glassy translucent film | Collapse above glass transition | Test before use |
| Melt-back ring at stopper | Thermal excursion in transit | Test before use |
| No visible cake at all | Very low fill, or nothing there | Weigh it; query the supplier |
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.
Read the leading edge of the stopper, every time.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsStart with the barrel size, because a 0.3 mL barrel graduated in single units and a 1 mL barrel graduated in twos are different instruments for this purpose.
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.
Mechanically, syringes are single-use. Reusing one blunts the tip against the stopper, degrades the silicone coating and makes the graduation reading no more accurate than it was the first time.
U-100 is a concentration standard meaning 100 units of insulin per millilitre; the syringe graduation is a volume scale expressed in those units.
Half-graduation estimation is not measurement. If the dose needs it, change the concentration instead.
Smallest barrel that holds the dose. Resolution is free and you should take it.
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.
Fixed-needle insulin syringes retain roughly 3 to 5 microlitres after the plunger bottoms out. Detachable-needle luer syringes retain 35 to 100 microlitres depending on hub design — an order of magnitude more.
Stated carefully, 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.
Published dead-space comparisons put low-dead-space fixed-needle designs under 2 microlitres and conventional detachable-needle designs at 35 microlitres or more.
Fixed needle, not detachable, unless you have a specific reason.
edited 12 Aug 2025 by tenth_of_a_unit — corrected a unit error in the worked example
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