If I reconstitute a 10 mg vial with 5 mL of 0.9% sodium chloride, what concentration do I end up with?

Concretely: 10 mg · 5 mL · 0.9% sodium chloride.

I want the working, not the result — I need to be able to redo it with different numbers.

I care about the precision as well as the value — I want to know how many figures are real.

Is my approach right even if my number is wrong?

10 ÷ 5 = 2 mg/mL. Concentration is vial content divided by diluent volume, so 10 mg of peptide in 5 mL of 0.9% sodium chloride gives 2 mg/mL. On a U-100 barrel one unit is 0.01 mL, so one unit of this solution carries 0.02 mg — 20 µg. That is the number to write on the label, because you will not reconstruct it from memory at an awkward moment.

The answer depends on what you want your measurement resolution to be, and that is a real trade-off rather than a preference. More diluent gives you more syringe marks per dose and therefore less rounding error; it also gives you a larger volume to keep cold and a longer period over which the solution has to remain within specification.

If the material arrived warm and it was lyophilised, test it and proceed on the result.

Check the barrel marking, not your memory of it.

The 28-day beyond-use convention for a multiple-withdrawal preserved preparation derives from USP compounding chapters, which set it on microbiological risk rather than chemical stability.

Do the arithmetic twice, ideally with someone else doing it independently.

Before anything else: a kitchen counter with an alcohol wipe is not an aseptic environment, and it is worth being honest about that rather than pretending the procedure is something it is not. What you are doing is reducing bioburden, not achieving sterility.

Room temperature before drawing is worth the ten minutes. Cold solution is more viscous, draws slower, and is more likely to pull a bubble past the plunger seal.

Number of stopper piercings matters less than the gauge doing the piercing.

Coring of elastomeric closures is a well-characterised failure mode in the parenteral packaging literature.

None of this is exotic. It is just the difference between doing it deliberately and doing it approximately.

edited 21 May 2025 by tabular_nums — added the placebo-arm figures

If the supplier documentation specifies a diluent, there is usually a reason, and if it specifies nothing, water for injection is the conservative default.

Photograph the vial against a matte black card with a single point light source off to one side, not with a flash from the front.

The practical summary: fine gauge, gentle swirl, diluent down the wall, room temperature before drawing.

The caveat on all of this is that it assumes the vial contains what the label says.

The practical summary: fine gauge, gentle swirl, diluent down the wall, room temperature before drawing, and check the syringe scale.

Mechanically, reconstitution is the step where most hands-on errors enter the system, which is why spending time on technique here pays off more than anywhere else.

Do not use the same needle to pierce the stopper and to administer.

The general principle — that peptides adsorb and denature at air–liquid and solid–liquid interfaces — is standard formulation science.

Write the arithmetic on the vial label. It costs nothing and it removes the step where you reconstruct it from memory at an inconvenient moment.

The part that matters: add the diluent down the vial wall rather than directly onto the cake. Peptides are surface-active and shear at an air–liquid interface, so a jet of water into a lyophilised puck generates foam, and foam is aggregated protein at the interface, not just air.

On re-freezing something that thawed in transit: if it arrived as a lyophilised solid that warmed but never got wet, re-freezing costs you nothing except the thermal cycle.

Published data on syringe dead space in the context of injection-equipment programmes quantifies low-dead-space designs as retaining under 2 µL against 35 µL or more.

One limitation: technique reduces risk, it does not remove it.

The practical summary: fine gauge, gentle swirl, diluent down the wall, room temperature before drawing, and check the syringe scale against the barrel rather than against your assumption.