I have a logging thermometer, a box of insulin syringes and no illusions about my worktop being sterile.
I can predict the outcome but I cannot explain it, which means I will get the next case wrong.
I would like to know how confident the field actually is about this.
What is actually going on here, physically?
The short version: U-100 only, smallest barrel that fits the dose, read the leading edge of the plunger stopper.
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.
On the detail: 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.
The 4 mm needle length is supported by injection-technique studies showing adequate subcutaneous delivery across body-mass ranges without increased intramuscular delivery.
One unit is 0.01 mL on U-100. That is the whole scale.
edited 30 Aug 2024 by thermal_mass — added the placebo-arm figures
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsMore usefully, a dose that lands below about five units on any barrel is being measured too coarsely and the reconstitution should be redone.
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.
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.
The caveat is that this describes an instrument, not a treatment, and research-use compounds are not approved for human use.
Read the leading edge of the stopper, every time.
The relevant precision limit is that you cannot reliably read half a graduation, so the barrel decides the smallest change you can make.
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.
To be exact about it, 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.
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.
In practice, fixed-needle syringes have far less dead space than detachable-needle ones, which matters more than any other design difference here.
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.
Fixed needle, not detachable, unless you have a specific reason.
The honest answer is that most dosing errors in this tag are reading errors rather than calculation errors.
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.
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.
Check the syringe is U-100 and not U-40 before doing any arithmetic with it.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.