n.norgaard, post #4: Understood, and I withdraw the assumption I opened with. Go to post
Following this. I have the same question and no better information than the first post.
Practice · Reconstitution · continued
Second pass at: Choosing a concentration on purpose rather than by accident posts 31–55
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
c.draganov, post #11: 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. Go to post
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.
Confirming post #32 from a second method, which matters more than confirming it from a second person.
I disagree with the framing of Choosing a concentration 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.
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I had written a reply contradicting post #32 and deleted it. Here is what survived.
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.
The general case is well covered; this is the awkward specific one.
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l.aaltonen, post #9: 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. Go to post
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.
If anyone can point at the primary source I would be grateful.
Collapsed as off-topic by two members at trust level 3 or above
Coming back to post #34, because the follow-up matters more than the original answer.
Preservative effectiveness is tested against a defined microbial challenge under defined conditions. It is not a licence to treat an entered vial as sterile indefinitely, and no supplier claims otherwise.
This follows post #36 rather than contradicting it.
Worth separating Choosing a concentration as a question about the compound from Choosing a concentration as a question about the documentation. They get answered by different people and only one of them is answerable here.
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e.halonen, post #22: 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. Go to post
Useful. I have added it to my own notes with the date on it.
b.nwosu, post #2: 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. Go to post
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 would call that likely rather than established.
PharmNotes_Whitfield, post #34: I had written a reply contradicting post #32 and deleted it. Here is what survived. 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. The general… Go to post
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.
That reframing is the whole thing. The facts I already had.
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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.
Correct me on the arithmetic if it is wrong; I would rather know.
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r.scholten, post #7: 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… Go to post
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.
Narrowing post #44, because the general version has more than one answer.
Answering the Choosing a concentration question as asked, then the question I think is meant. As asked: yes, with the qualification below. As meant: it depends on how the first measurement was taken.
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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.
Adding it in case it saves somebody the afternoon it cost me.
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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.
For what it is worth, the same held on the two occasions I checked.
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a.cabrera, post #12: Choosing a concentration is worth one more sentence than it usually gets, and the sentence is the one about how the number was arrived at. Go to post
On post #47 — agreed on the reasoning, with one qualification.
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.
Picking up post #47: that is the part I would want checked first.
Reading back through the Choosing a concentration threads from last year, the same three questions come up every time and only one of them has ever been answered properly. That seems like a documentation gap rather than a knowledge gap.
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I read post #48 twice before replying, because I had assumed the opposite.
A note on how Choosing a concentration gets discussed rather than on Choosing a concentration itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.
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Preservative effectiveness is tested against a defined microbial challenge under defined conditions. It is not a licence to treat an entered vial as sterile indefinitely, and no supplier claims otherwise.
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j.erdogan, post #45: 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. Go to post
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.
nl_translator, post #15: 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. Go to post
Narrowing post #54, because the general version has more than one answer.
The arithmetic on Choosing a concentration is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.
This topic was referenced in
- A 5 mg vial and a 0.25 mg dose: the arithmetic in fullPractice › Reconstitution · 141 replies
- Reconstituting a multi-strength kit without mixing yourself up — what changed sincePractice › Reconstitution · 46 replies
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