Coming back to: Over-dilution: when your dose falls below one readable graduation posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
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.
This is the answer, and the reason it is the answer is the more useful part.
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.
Post #37 is right about the mechanism and I think understates the practical bit.
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.
Adding it because I spent an afternoon working it out and nobody should have to twice.
Coming back to post #38, because the follow-up matters more than the original answer.
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.
For what it is worth, the same held on the two occasions I checked.
I read post #39 twice before replying, because I had assumed the opposite.
On Over-dilution I would separate what is worth knowing from what is worth acting on. The first list is long and the second is short, and conflating them is how threads get heated.
Post hidden by community flags
Staff rationale: Hidden by community flags. The claim about a named supplier was not accompanied by a batch, a date, a method or a document, which R6 requires.
This settles it for me, at least until somebody posts a reason it should not.
The corresponding entry is in the public moderation log. Hidden posts are never deleted.
Taking post #42 at face value and following it one step further.
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.
Two people can read the same figure differently here and both be reasonable.
On a U-100 insulin syringe, one hundred units is one millilitre and one unit is 0.01 millilitres. Units are volume marks. They mean nothing until you know the concentration.
The honest answer on Over-dilution is that it depends, and the useful part is the list of what it depends on. Four items, in rough order of how much they matter.
Most people get the first two right and then argue about the fourth.
Worth separating two things that post #44 runs together.
I read the earlier replies on Over-dilution twice before writing this, because I had assumed the opposite and wanted to be sure I was disagreeing with what was said rather than what I expected.
This follows post #47 rather than contradicting it.
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.
Not a conclusion. A place to stand while looking for one.
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.
Confirming post #47 from a second method, which matters more than confirming it from a second person.
Dead volume is the part nobody mentions until it costs them a dose. A fixed-needle insulin syringe holds very little; a detachable-needle luer configuration can hold enough to matter at small doses.
Not a strong opinion, just a consistent one.
Coming back to post #48, because the follow-up matters more than the original answer.
The confident answers on Over-dilution and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
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.
Filing this under things that are true until someone shows me otherwise.
Research-use-only material reconstituted at home is not a compounded sterile preparation and nothing about the procedure makes it one. Members describing what they do are not describing a pharmaceutical process.
Collapsed as off-topic by two members at trust level 3 or above
Post #54 answers the question as asked. The question underneath it is different.
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.
The literature is thinner on this than the confidence in the thread implies.
I read post #52 twice before replying, because I had assumed the opposite.
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 would rather be precise about what I do not know than vague about what I do.
I would put moderate confidence on the mainstream reading of Over-dilution and no more. That is not scepticism for its own sake; it is where the sourcing actually stops.
On Over-dilution: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.
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.
Flagging that the sources on this are thinner than the confidence in the thread suggests.
I disagree with the framing of Over-dilution above, and I think it is a substantive disagreement rather than a terminological one. Setting out why, so it can be checked.
The reasoning depends on an assumption that is doing a lot of work and is never stated. If the assumption holds, the conclusion follows. I do not think it holds generally.