Coming back to: Checking someone else's arithmetic: a worked example thread

Posting this under the heading it deserves: Checking someone else's arithmetic: a worked example thread Everything below is what sits behind that.

Practical question with the units stated, because I have seen how quickly these go wrong without them.

I have a 5 mg vial of tirzepatide and I am working to a 0.5 mg step. My syringes are U-100 insulin syringes, 1.0 mL barrel.

I can do the arithmetic and I have done it twice, getting the same answer both times, but I would like someone to check the reasoning rather than the number — specifically whether I have thought about the residual volume correctly, and whether the graduation I am landing on is one a person can actually read.

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.

Happy to expand any of that if it is the useful part.

This is the sort of exchange that makes the archive worth searching.

Post #2 describes the usual case. This is about the unusual one.

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.

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.

Posting it because the silence on this was starting to look like agreement.

Where I part company with post #2, and it is a narrow parting.

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 arithmetic in post #4 is right; the assumption feeding it is the part to check.

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.

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.

Second-hand, so weight it accordingly.

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.

Post #6 put the caveat in the right place and I want to underline 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.

Everything in post #9 holds. The case it does not cover is the one I have.

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.

Narrowing post #9, because the general version has more than one answer.

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.

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.

Filing this under things that are true until someone shows me otherwise.

Following this. I have the same question and no better information than the first post.