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 follows the opening post rather than contradicting it.

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.

I read post #2 twice before replying, because I had assumed the opposite.

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.

Good question, well framed, and I would like to see it answered properly.

Picking up post #4: that is the part I would want checked first.

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.

Worth separating two things that post #7 runs together.

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.

I read post #9 twice before replying, because I had assumed the opposite.

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 #9 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.

Nothing to add, except that this is the answer I would give if asked.

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.

I would rather be precise about what I do not know than vague about what I do.