BethLabQueen said:

Dead space is the answer to the missing dose.

I am going to disagree with reconstituting low as a general rule. More diluent, more punctures, more in-use days at room temperature, and the stability trade-off is real. Precision is not the only variable being optimised.

james_edin said:

Two of us with identical vials and identical diluent arrived at different unit counts, which means one of us has a conceptual error rather than a…

Reconstitution tip for anyone mixing dosing arithmetic from lyophilized powder: use bacteriostatic water, NOT sterile water. The benzyl alcohol in BAC water preserves multi-dose vials for up to 28 days.

Inject the water slowly down the side of the vial. NEVER shake — gentle swirling only until fully dissolved. Should be crystal clear with no particles.

labquiet_amy said:

I am going to disagree with reconstituting low as a general rule.

Compounding pharmacy dose accuracy check for dosing arithmetic: I always verify my first dose from a new vial by weighing it on a milligram scale. Draw up your dose, weigh the syringe, subtract empty syringe weight, compare to expected water weight at that volume.

For a 10-unit dose (0.1mL): expected weight ~100mg of water. If it's significantly off, your syringe may be inaccurate. Cheap insulin syringes can have ±5% volume accuracy. Spring for quality brands like BD or Terumo.

Following on from amsterdam_pete — and this may be the naive question:

How much material I am losing to dead space, and whether that explains why a 10mg vial gives me nine usable draws rather than ten?

Reporting back.

Resolved. I wrote mg, ml and mg/ml on the vial in marker and did the division in that order, and the two of us now get the same number every time.