A 5 mg vial and a 0.25 mg dose: the arithmetic in full — what changed since posts 61–90
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Coming back to post #60, because the follow-up matters more than the original answer.
Reconstituting the whole vial when you will use a quarter of it is a decision to store the rest in solution, which is the least stable state it will ever be in. Sometimes that is the right trade and it should be a decision.
The thing about 5 mg vial that took me longest to accept is that a plausible mechanism is not evidence of an effect. It is a reason to look, not a result.
Collapsed as off-topic by two members at trust level 3 or above
Confirming post #62 from a second method, which matters more than confirming it from a second person.
Whatever the answer on 5 mg vial turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, invite the correction.
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.
Filter needles are worth considering if you are drawing from a glass ampoule and are pointless overhead for a stoppered vial. The trade is dead volume against particulate risk.
If anyone can point at the primary source I would be grateful.
Two people can reconstitute the same vial to different concentrations and both be right. The dose is the same; only the volume drawn differs. This confuses more discussions here than any other single point.
One of those cases where knowing the mechanism does not help the decision.
Thank you — that answers what I came here to find out.
Everything in post #67 holds. The case it does not cover is the one I have.
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.
That is the version I would defend. It is not the version I started with.
Arithmetic step by step: a 5 mg vial with 2 mL of diluent gives (5 mg) / (2 mL) = 2.5 mg/mL. On a U-100 syringe at that concentration, 100 units = 1 mL = 2.5 mg, so each unit = 0.025 mg. A 0.25 mg dose = 0.25 / 0.025 = 10 units. Different concentration: different arithmetic, same principle.
I have deliberately not rounded that, because the rounding is where the argument starts.
Choosing a concentration on purpose rather than by accident: starting with "I want to draw 0.5 mL per dose" and working backward to the required concentration is more efficient than picking a diluent volume and hoping the math works out. State your target volume, then the required concentration follows.
I have separated what I observed from what I concluded, which does not always happen.
Post #74 answers the question as asked. The question underneath it is different.
Write the reconstitution date and the concentration on the vial. Not on a note, on the vial. Every account here of a dosing error involving the wrong concentration involves a vial with nothing written on it.
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.
I have no interest in any supplier named above.
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.
The general answer and the answer for your case may diverge here.
Second-hand on 5 mg vial, so weight it accordingly — someone whose method I trust told me this and I have not verified it myself.
Post #79 is right about the mechanism and I think understates the practical bit.
Over-dilution: if your target dose is 0.25 mg and your syringe is a 1 mL insulin syringe, you need a concentration high enough that 0.25 mg fits on the scale. A 0.25 mg/mL solution requires drawing the entire 1 mL syringe — not readable. A 5 mg/mL solution requires drawing 50 μL — also not practical on an insulin syringe.
Reconstituting the whole vial when you will use a quarter of it is a decision to store the rest in solution, which is the least stable state it will ever be in. Sometimes that is the right trade and it should be a decision.
I think the 5 mg vial question is answerable and has not been answered, which is a more optimistic position than most of this thread.
Collapsed as off-topic by two members at trust level 3 or above
Post #82 describes the usual case. This is about the unusual one.
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.
Adding this to the thread rather than to the wiki, because I am not confident enough for the wiki.
Post #83 is the version of this I will quote in future. One addition.
Two people can reconstitute the same vial to different concentrations and both be right. The dose is the same; only the volume drawn differs. This confuses more discussions here than any other single point.
That is the practical version. The rigorous version is longer and says the same thing.
A 10 mg vial reconstituted three different ways: 1 mL diluent gives 10 mg/mL, 2 mL gives 5 mg/mL, 4 mL gives roughly 2.5 mg/mL. The arithmetic is the same; the concentration determines which syringe graduations are legible.
Nothing above should be read as advice about what anyone else should do.
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.
The disagreement above is smaller than it looks once the terms are fixed.
The useful distinction on 5 mg vial is between what was measured and what was inferred from it. Both end up in the same sentence and only one of them has error bars.