Dead space: how much am I actually losing per injection?

Question that only matters at small volumes, which is most of what gets discussed here. How much solution stays in the hub and needle after the plunger bottoms out, and does it matter?

It matters much more than people expect, entirely because of the ratio.

A conventional syringe with a detachable needle can hold something in the region of 70 microlitres in the hub and needle. If you are administering 500 microlitres, that is noise. If you are administering 50, it is not noise, it is most of the dose.

Fixed-needle insulin syringes are built to minimise it and are dramatically better — the needle is bonded straight into the barrel with essentially no hub cavity.

So at small volumes, the syringe design changes what you actually administer, and it does so silently and consistently in the same direction.

Consistently in the same direction is the important part. A consistent bias is invisible — everything is self-consistent and nothing looks wrong.

And if you switch syringe types halfway through, your effective dose changes and nothing tells you.

Which is a decent argument for not changing equipment mid-vial for reasons unrelated to the equipment.