A 5 mg vial and a 0.25 mg dose: the arithmetic in full — the long version posts 151–166

The decimal-point error: computing 5 mg / 2 mL as 0.25 mg/mL instead of 2.5 mg/mL is the most common arithmetic error in this subcategory. The habit that catches it: writing the units in every step of the calculation.

This is where my knowledge stops and I would rather mark the edge than blur it.

Sensible. I would want the same detail before I acted on it either.

Post #150 answers the question as asked. The question underneath it is different.

Add the diluent down the side of the vial rather than directly onto the cake. It is slower and it avoids the foaming that makes people think something has gone wrong.

Do not shake. Swirl, or leave it. Vigorous agitation introduces air and shear, and neither helps a peptide go into solution any faster than patience does.

Reading it back, the second half matters more than the first.

Adding what did not work for me on 5 mg vial, since the failures never get written up and they are half the useful information.

Why "add 2 mL" is not an instruction: the powder in the vial takes up space. "Add 2 mL to a 10 mL vial" and "add 2 mL of diluent so the final volume is approximately 2 mL" are different instructions. Stating the final target volume is clearer than stating the diluent added.

Where I would look next, rather than where I would stop.

Narrowing post #154, because the general version has more than one answer.

Swirling until fully clear before drawing is worth the extra minute. A partially dissolved preparation is not uniform and the first dose out of it is not the same as the last.

Everything in post #156 holds. The case it does not cover is the one I have.

Worked example, since the arithmetic is the whole question. Five milligrams into one millilitre is 5 mg/mL. A 0.25 mg dose is 0.05 mL, which is five units on a U-100 syringe. Check that against your own numbers rather than taking mine.

Understood. Thank you for being specific about the limits of it.

The arithmetic on 5 mg vial is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.

Post #157 put the caveat in the right place and I want to underline it.

Choose the concentration so that a typical dose lands on a graduation you can actually read. A dose that falls between two marks on a 1 mL barrel is a dose you will get wrong sooner or later.

I would hold that lightly until someone with a larger sample weighs in.

If your arithmetic gives a volume smaller than one graduation on your syringe, the answer is a lower concentration rather than a more careful hand.

Take the reasoning and check the arithmetic; I do not always get it right.

The best check on any reconstitution calculation is to do it twice by two different routes — mass per volume, then volume per dose — and see whether they agree. They should, and when they do not it is nearly always the concentration step.