Bug report: the reconstitution calculator and a rounding edge case posts 61–73

Nothing a calculator produces is medical advice, and a correct calculation on inappropriate inputs is still an inappropriate answer.

Where a calculator and your own arithmetic disagree, the productive move is to find which step differs rather than to decide which is right.

On post #59 — agreed on the reasoning, with one qualification.

Reconstitution calculator: enter vial mass, final volume target, and the calculator tells you concentration and syringe mark for a given dose. Useful for double-checking arithmetic when you are doing it by hand.

One of those cases where knowing the mechanism does not help the decision.

Nothing to add on the substance. Thank you for taking the question at face value.

Where I part company with post #63, and it is a narrow parting.

The reconstitution calculation is total mass divided by total volume for concentration, then dose divided by concentration for volume, then volume times one hundred for units on a U-100 barrel. Three steps, and every error is in one of them.

Noting that the question and the thing people usually mean by it are different.

Do the arithmetic by hand the first few times, whatever tool you use afterwards. A calculator with a wrong input gives a confident wrong answer and there is nothing in the interface to tell you.

Tools that run entirely in the browser send nothing anywhere, which matters more to some people here than others and is worth stating either way.

I have written this out at length because the short version keeps being misread.

Adding the measurement that post #67 says would settle it.

A saved calculation is worth more than a remembered one. Write down the inputs alongside the result, because the result alone is unverifiable a month later.

Written quickly, so the reasoning may be tighter than the wording.

Noted, and I have changed what I was going to do on the strength of it.

Building on post #67 rather than restating it.

Unit-conversion tools are only as good as the assumption they make about the syringe. Ask what barrel and what graduation interval it assumed before trusting the units figure.

Not a conclusion. A place to stand while looking for one.

Storage lookup: which conditions (temperature, time) are you considering storing in, and how much stability loss is expected? The calculator gives a rough estimate based on typical stability data. Not a substitute for your compound's specific stability data.

Post #68 put the caveat in the right place and I want to underline it.

Significant figures in the output should not exceed what the inputs support. A concentration calculated from a nominal vial mass is not known to four decimal places.

Mass error checking: enter your observed mass and theoretical mass and check whether the difference in ppm is within reasonable bounds for your instrument. Useful sanity check on LC-MS results.

The mechanism is plausible, which is not the same as established.