What is the arithmetic to convert 30 mg in 2 mL into units on a U-100 scale?

What I have: 30 mg · 2 mL.

This should be a straightforward calculation and I keep getting two different answers.

The numbers are arbitrary; the method is what I am after.

Can someone walk through the arithmetic step by step?

15 mg/mL, so one unit carries 0.15 mg. 30 ÷ 2 = 15 mg/mL; one unit on a U-100 barrel is 0.01 mL; 15 × 0.01 = 0.15 mg per unit. To go the other way, divide your intended dose by 0.15: a 1.5 mg dose is 10 units, and a 3 mg dose is 20. Write both the concentration and the milligrams per unit on the vial.

Write the units at every step, because units errors are the failure mode that catches everyone eventually.

The rounding error accumulates if you round too many times — rounding concentration to 5.0, rounding the dose volume to 0.1 mL, rounding the unit reading to 10 — and the safest approach is to work the full precision and round only the final answer.

Reading a lyophilised cake

The concentration you actually work with is label claim times content fraction divided by actual diluent volume, which is usually not the same as the nominal concentration because content is usually not 100 per cent and you rarely measure the diluent volume to 0.1 mL precision.

The Arrhenius relationship for drawing kinetics means that cold solution takes noticeably longer to draw than room-temperature solution.

Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.

Stated carefully, the arithmetic only stops being confusing once you work it through once and see that it is straightforward.

Breaking it down further: if a 10 mg vial has 96.5 per cent content, you have 9.65 mg of peptide. Divide that by 2.00 mL and your concentration is 4.825 mg/mL, not 5.00 mg/mL, which is a 3.5 per cent systematic error in every dose calculation.

Do not use the same needle to pierce the stopper and to administer. The tip is blunted by the stopper, and the hub now contains a dose you are about to lose to dead space anyway.

Published data on syringe dead space quantifies low-dead-space designs as retaining under 2 µL against 35 µL or more for conventional detachable-needle syringes.

Do the arithmetic twice, ideally with someone else doing it independently.

Concretely, this is arithmetic, so let us do the arithmetic rather than argue about it.

On filtration: a 0.22 µm syringe filter will remove particulates and organisms, and it will also adsorb a fraction of your peptide onto the membrane — with a low-binding PVDF or PES membrane the loss is typically a few per cent.

Room temperature before drawing is worth the ten minutes. Cold solution is more viscous, draws slower, and condensation on a cold barrel makes it harder to read the meniscus.

The insulin-unit standard U-100 means 100 units per millilitre, so one unit is 0.01 mL — this is the conversion that trips up more people here than any other single piece of arithmetic.

Worth noting: the concentration after reconstitution is not the same as the label claim, and most people do not account for the difference.

If in doubt, use more diluent and accept the shorter usable window.

The common error is getting the concentration right but then misreading the syringe scale, which is why checking the barrel marking rather than your memory matters.

Rotation of injection site is a tolerability measure, not a pharmacokinetic one, but if you are going to do it you might as well do it right.

The limitation is that technique reduces risk, it does not remove it, and nothing you can do outside a controlled environment makes a non-sterile preparation sterile.

Write the arithmetic on the vial label. It costs nothing and it removes the step where you reconstruct it from memory at an inconvenient moment.

edited 23 Dec 2024 by tenth_of_a_unit — added the method parameters

Two people working through the same arithmetic independently should get the same answer, and if they do not, someone has made a unit error.

Air bubbles at these volumes are a measurement problem rather than a safety one. A 2 mm bubble in a 0.3 mL syringe is roughly 4 µL, which at 10 units drawn is a four per cent error.

The content assay results from major testing services show that nominal vial claim and measured content differ by one to ten per cent, making content a driver of dose error.

The practical summary: fine gauge, gentle swirl, diluent down the wall, room temperature before drawing, and check the syringe scale against the barrel rather than against your assumption.