Reconstitution arithmetic for peptide dosing: units, volumes and a worked check

A short methods note on the unit conversions behind lyophilised peptide reconstitution, and on keeping a dose log that can be audited after the fact.

1. Why the arithmetic goes wrong

Reconstitution itself is trivial: add solvent to a lyophilised vial, swirl, store cold. The errors happen in the units. A vial is labelled in milligrams, bacteriostatic water is measured in millilitres, syringes are graduated in insulin units, and the intended dose is usually quoted in milligrams or micrograms. Four unit systems, three of them implicit, and the conversion between them depends on a number that is not printed anywhere: the volume of solvent that the person doing the mixing chose to add.

2. The single relation that matters

Everything follows from concentration:

concentration (mg/mL) = vial content (mg) / solvent added (mL)

And then, because a U-100 syringe puts 100 units to the millilitre:

units to draw = dose (mg) / concentration (mg/mL) * 100

The 100 is the whole reason insulin syringes are convenient here. One unit is 0.01 mL, so a concentration expressed in mg/mL converts to "mg per 100 units" by inspection.

3. Worked example

A 5 mg vial, reconstituted with 2 mL of bacteriostatic water, intended dose 0.25 mg.

Change the solvent volume to 1 mL and the same 0.25 mg dose becomes 5 units, not 10. Nothing about the vial changed; only the dilution did. This is why a written record of the solvent volume belongs with the vial and not in memory, and why sharing a "units" figure between two people is meaningless unless both reconstituted identically.

4. Two checks worth running every time

1. Doses per vial. Divide vial content by dose. If the answer is not close to a whole number, either the titration schedule or the vial size is being mismatched, and the last dose in the vial will be short.
2. Draw volume in range. A draw below about 4 units on a U-100 syringe sits where graduation error is largest. If the arithmetic lands there, reconstitute the next vial with more solvent rather than trying to read a finer mark. Weekly titration schedules that start very low are the usual cause.

I use the reconstitution calculator to do both checks in one pass, mostly because it forces the solvent volume to be entered explicitly instead of assumed.

5. Logging so the record is auditable later

A dose log that only records "10 units" is not reconstructable six weeks later. The minimum useful record per injection is: date, vial lot or opening date, solvent volume for that vial, computed concentration, dose in mg, and volume drawn. With those fields, any past injection can be recomputed and any transcription error found. Without the solvent volume, none of it can.

Terminology in this area is inconsistent between suppliers and forums; the compound and unit definitions in this peptide and GLP-1 glossary are what I use to keep notes comparable. The logging schedule above is roughly what Peptide Tracker & Calculator records per injection, which is where the field list came from.

6. Limitations

This note covers arithmetic only. It says nothing about whether a given compound, dose or schedule is appropriate, which is a clinical question and not an arithmetic one. Sterility, solvent choice, storage temperature and expiry after reconstitution are all outside its scope and all matter more than the arithmetic does.