What is the dead-space loss per draw with a 30G needle at 5 mg/mL?

Concretely: a 30G needle · 5 mg/mL.

Please show the division. I want to check my own against yours.

I would like the general form as well as the specific number, so I can apply it again.

How many significant figures are actually justified here?

Specifically, this is arithmetic, so let us do the arithmetic and see where the losses actually are.

Low-dead-space syringe designs either have the needle bonded directly to the barrel — a fixed-needle syringe, which is the cheapest route — or add a moulded projection on the plunger tip that fills the luer cone.

Dead space quantified: a fixed-needle insulin syringe holds roughly 3 to 5 µL in the hub and needle after the plunger bottoms out. A luer-lock syringe with a detachable needle holds 35 to 100 µL depending on the hub design.

Published inter-laboratory comparisons of dead-space measurements on identical syringes show good agreement, suggesting the numbers are reliable.

If cost matters, this is the first thing to change, not the last.

In practice, dead space is the volume trapped in the syringe hub and needle after the plunger bottoms out, and it is the reason your 10 mg vial yields only 9.5 mg of usable draws.

The luer cone of the syringe plus the needle's own plastic hub accounts for the vast majority of the dead space.

Configuration A — 1 mL luer-lock plus detachable needle, dead space 84 µL: each draw removes 100 + 84 = 184 µL. Draws available from 2,000 µL = 2000 / 184 = 10.87, so 10 full draws.

Syringe residual volume has been measured properly, mainly in the infection-control literature, with a median residual of about 84 µL for a conventional 1 mL syringe with a detachable needle and roughly 2 µL for a fixed-needle low-dead-space design.

Buy the right syringe — a fixed-needle insulin syringe is cheap and solves the problem.

Put another way, the distinction that resolves most of these questions is understanding that dead space is a fixed volume — typically 3 to 5 µL in a fixed-needle syringe and 35 to 100 µL in a luer-lock — and its cost scales with how small your draws are.

Corollary that follows immediately: changing needle gauge or length barely changes your losses.

The limitation is that even with perfect technique, some loss is irreducible unless you switch to a low-dead-space syringe.

The switch nearly doubles your vial, which is better than most other optimisations combined.

The relevant detail is that dead space is irreducible with a high-dead-space syringe, which is why the hardware matters more than any technique.

Configuration B — 0.5 mL fixed-needle U-100 insulin syringe, dead space 2 µL: volume removed per draw = 100 + 2 = 102 µL.

The World Health Organisation guidance on injection equipment adopted the same high-versus-low dead-space distinction, using a low-dead-space threshold in the low single-digit microlitres.

If cost matters, this is the first thing to change, not the last.