Reconstitution calculator (revision 63)

The reconstitution calculator relates the four quantities involved in dissolving a lyophilised peptide and drawing a dose from it: the mass of solid in the vial, the volume of diluent added, the intended dose, and the reading on the syringe barrel. All four are connected by one division, and almost every error reported in the literature and in community accounts is a confusion between two of them rather than a failure of the arithmetic itself.^[1]

The calculation is a routine unit conversion: concentration is mass divided by volume, dose volume is dose divided by concentration, and a syringe unit is one hundredth of a millilitre on the U-100 scale. The tool below performs those operations and shows each intermediate result, because the intermediate is where a mistake becomes visible.

This page is a reference tool and not a clinical decision aid. It converts numbers a reader enters; it does not select a dose, and it has no knowledge of the vial, the label claim, or the person. See Project:Medical disclaimer.

Enter the label mass of the vial, the volume of diluent, and the intended dose. The tool reports the resulting concentration, the volume that dose occupies, the reading on the selected syringe scale, and how many doses the vial contains.

The tool's defaults correspond to the first row below. Each row is a division; none of them requires a calculator, and reproducing one by hand is a useful check that the inputs were entered as intended.

The last row illustrates the most common unit confusion: a dose expressed in micrograms against a concentration expressed in milligrams per millilitre. One thousand micrograms is one milligram, so 100 µg of a 1 mg/mL solution is 0.1 mL — ten units on a U-100 syringe, not one hundred.

Two rows are worth noticing for a different reason. A 150-unit dose does not fit in a 1 mL insulin syringe, and the tool warns when a computed volume exceeds a plausible barrel. A 5-unit dose sits at the very bottom of a U-100 barrel, where the graduation interval is a large fraction of the dose and precision is correspondingly poor; a lower concentration would place the same dose further up the scale.^[2]

The calculation above treats the vial's label mass as the mass of peptide present. That is an assumption, and on a certificate of analysis it is frequently not true. A lyophilised solid contains water (typically 3–8% by mass), a counterion (2–12%), and residual salts, all of which occupy the labelled mass unless the label claim is explicitly a net peptide figure.^[3]

A vial labelled 10 mg with 5.2% water and 6.4% trifluoroacetate contains approximately

10 × (1 − 0.052 − 0.064) ≈ 8.7\,mg

of peptide-plus-related-substances, before any correction for chromatographic purity. Entering 10 mg into the calculator therefore yields a concentration about 13% higher than the peptide concentration actually achieved.

Two systematic effects are worth stating because the calculator does not model the first and only optionally models the second.

Displacement. Adding 2 mL of diluent to a vial containing 5 mg of solid produces slightly more than 2 mL of solution, because the dissolved solid occupies volume. For peptide masses of a few milligrams in a millilitre or more of diluent the effect is on the order of a few thousandths of a millilitre and is negligible against the graduation interval of the syringe. It ceases to be negligible when the solid mass approaches the diluent volume, which does not arise in the cases this tool is used for.

Dead space. Volume retained in the syringe hub and needle after the plunger is fully depressed is not delivered. Reported figures range from roughly 0.02 mL for a low-dead-space insulin syringe with a fixed needle to 0.07 mL or more for a detachable-needle arrangement.^[4] Against a 0.10 mL dose that is a 20–70% loss if the syringe is not primed, which is why the dead-space field exists: entering a figure shows its effect on the number of doses the vial yields, not on the delivered dose, since a primed syringe delivers its nominal volume.

The tool assumes complete dissolution, an accurate label mass, an accurately measured diluent volume, and a syringe manufactured to its nominal scale. Each of those is an approximation:

• incomplete dissolution leaves a concentration lower than calculated and a suspension of unknown uniformity;
• diluent volume drawn on the same syringe used for dosing carries that syringe's own error;
• syringe volume tolerance under ISO 7886 is a few percent of nominal capacity, which at the bottom of the barrel is a large fraction of a small dose;^[2]
• aggregated or partly degraded material has a peptide content lower than the certificate reports, and no arithmetic detects that.

It performs no clinical reasoning of any kind. It does not know what compound is in the vial, whether the compound is approved for human use, or whether the entered dose has ever been studied. Compounds distributed for research are not approved for human use in any major jurisdiction.

• Reconstitution of lyophilised peptides
• Insulin syringe unit conversion
• Bacteriostatic water
• Syringe dead space
• Peptide content
• Karl Fischer titration

• International Council for Harmonisation, Q6A: Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products.