Second pass at: Choosing a concentration on purpose rather than by accident Writing it up because I had to work it out twice and would rather nobody else did.
Posting a small dataset on Choosing a concentration. It is mine, it is uncontrolled, and the method is stated so it can be discounted appropriately.
What I would like is not agreement but a second dataset collected by someone with no stake in mine. If one exists I would rather read it than argue for this one.
Two claims get bundled together under Choosing a concentration and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.
Almost every disagreement in threads like this one dissolves once you say which of the two you are making.
Everything in the opening post holds. The case it does not cover is the one I have.
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.
Understood, and I withdraw the assumption I opened with.
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.
Two people can read the same figure differently here and both be reasonable.
Post #5 is the version of this I will quote in future. One addition.
Working out the concentration after the fact from the volume you added is fine. Working it out from the volume you meant to add is where the errors come from — read the barrel, not the plan.
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.
Written quickly, so the reasoning may be tighter than the wording.
A vial that will not fully dissolve: check in order: is the diluent genuinely room temperature (some preservatives crystallise in cold), is the vial being warmed gently rather than shaken hard, is the injection technique clean, is the vial integrity intact. Work through that checklist before concluding the powder is insoluble.
Noting that the question and the thing people usually mean by it are different.
I had written a reply contradicting post #5 and deleted it. Here is what survived.
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.
Confirming post #8 from a second method, which matters more than confirming it from a second person.
A cake that takes a few minutes to dissolve is ordinary. A cake that will not dissolve after gentle swirling and standing at room temperature is worth raising with the supplier rather than working around.
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.
Worth saying I have only my own numbers here, and n is small.
Building on post #10 rather than restating it.
Trying to state the Choosing a concentration position in a way that someone who disagrees would recognise as fair, because I do not think the version in this thread passes that test.
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.
I would want the raw data before agreeing with my own summary of it.
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.
The right answer here may simply be that it has not been measured.
Taking post #14 at face value and following it one step further.
Where I would push back on the Choosing a concentration consensus is the confidence, not the direction. The direction looks right. The confidence is borrowed.
Post #16 and I disagree about the size of the effect, not about the direction.
Having read the whole Choosing a concentration thread before replying: the question in the first post has not actually been answered yet, and three of us have answered a nearby one instead.
Picking up post #18: that is the part I would want checked first.
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.
Reconstituting a multi-strength kit: if a kit contains 5 mg, 10 mg, 15 mg vials and you are reconstituting all of them, writing the concentration on each vial in permanent marker as you go is the single most useful thing you can do to avoid dose errors later.
How much of the diluent volume the powder itself displaces: for a small peptide vial, the powder volume is negligible. For a larger vial or a kit with multiple compounds, the displacement can be a few tenths of a millilitre. If precision matters to you, account for it by targeting a final weight rather than a final volume.
On Choosing a concentration the community has more anecdote than the confidence in this thread implies, and I include my own contribution in that.
Post #22 is right about the mechanism and I think understates the practical bit.
Two questions I would want answered before drawing anything from the Choosing a concentration data above: how were the cases selected, and what happened to the ones that dropped out.
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.
Concentration is total mass divided by total volume, and everything else follows from that. Ten milligrams into two millilitres is five milligrams per millilitre, and it does not matter how the vial was labelled before you added anything.
I have separated what I observed from what I concluded, which does not always happen.
I read post #25 twice before replying, because I had assumed the opposite.
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.
That is the version I would defend. It is not the version I started with.
Post #25 put the caveat in the right place and I want to underline it.
An honest declaration on Choosing a concentration: I have a prior here and it is strong enough that you should weight what I say downward. Stating it rather than hiding it.
Building on post #29 rather than restating it.
Choosing a concentration sits at the boundary between what this community can usefully discuss and what it cannot, and I think it falls on the discussable side, narrowly.