I keep a written log of every draw with date, volume and syringe type.
Something has gone wrong and I would like to know how badly before I decide what to do.
Nothing else in the setup changed, which is what makes this puzzling.
What is the differential here, and which test discriminates between the options?
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.
| Configuration | Dead volume | Loss at 5 mg/mL | Over 20 draws |
|---|---|---|---|
| Fixed-needle insulin syringe | 3–5 µL | 15–25 µg | 0.3–0.5 mg |
| Low-dead-space, detachable | <2 µL | <10 µg | <0.2 mg |
| Standard luer-lock + 30G | 35–60 µL | 175–300 µg | 3.5–6 mg |
| Luer-lock + 21G drawing needle | 70–100 µL | 350–500 µg | 7–10 mg |
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.
The 4 mm needle length is supported by injection-technique studies showing adequate subcutaneous delivery across body-mass ranges without increased intramuscular delivery.
Half-graduation estimation is not measurement. If the dose needs it, change the concentration instead.
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 standardsAnswering this needs to know whether the syringe is U-100 or U-40, since a U-40 syringe used with the U-100 conversion produces a two-and-a-half-fold error.
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.
It helps to be literal here: 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.
Graduation intervals by barrel size are printed on the packaging and are the practical limit on dosing resolution.
Nothing here is medical advice.
Check the syringe is U-100 and not U-40 before doing any arithmetic with it.
edited 12 Nov 2024 by shear_at_the_front — fixed an arithmetic slip in the third paragraph
The short version: U-100 only, smallest barrel that fits the dose, read the leading edge of the plunger stopper.
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.
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.
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.
The relevant precision limit is that you cannot reliably read half a graduation, so the barrel decides the smallest change you can make.
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.
Fixed needle, not detachable, unless you have a specific reason.
Start 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.
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.
Published dead-space comparisons put low-dead-space fixed-needle designs under 2 microlitres and conventional detachable-needle designs at 35 microlitres or more.
Smallest barrel that holds the dose. Resolution is free and you should take it.
edited 14 Oct 2024 by Dr_Ilse_Vandenberg — reworded for clarity after a comment
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