Why does a U-100 insulin syringe read differently from the volume I thought I drew at 2.5 mg/mL?

Details up front: a U-100 insulin syringe · 2.5 mg/mL.

I have a result I cannot explain, and I would rather diagnose it than guess.

I have checked the obvious explanations and eliminated the two easiest ones.

What is the most likely explanation, and how would I confirm it?

Concretely, dose arithmetic has three parts: concentration from vial content and diluent, volume from dose and concentration, and units from volume and syringe scale.

The concentration you actually work with is label claim times content fraction divided by actual diluent volume, which is usually not the same as the nominal concentration because content is usually not 100 per cent and you rarely measure the diluent volume to 0.1 mL precision.

On filtration: a 0.22 µm syringe filter will remove particulates and organisms, and it will also adsorb a fraction of your peptide onto the membrane — with a low-binding PVDF or PES membrane the loss is typically a few per cent.

Worth noting: the concentration after reconstitution is not the same as the label claim, and most people do not account for the difference.

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

edited 27 Jul 2026 by tare_weight — removed a claim I could not source