What is the dead-space loss per draw with a 18G drawing needle at 6.67 mg/mL?

The specifics, since they change the answer: an 18G drawing needle · 6.67 mg/mL.

I would like the arithmetic checked rather than the conclusion asserted.

I have deliberately not used an online calculator because I want to be able to check the result.

Can someone show the working rather than just the answer?

Changing syringe architecture changes everything, while changing needle gauge changes almost nothing.

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

Be sceptical of anything advertised as low dead space that retains a conventional plunger tip: if you can look into the fitting with the plunger fully forward and see an open conical void, that void is your dead space.

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.

edited 8 Sept 2024 by plate_count_9k — added the placebo-arm figures

Mechanically, the short answer is that dead space is small in absolute terms and huge as a fraction of a small dose, which is why it feels like a rounding error and behaves like a systematic loss.

Draws available = 2000 / 102 = 19.6, so 19 full draws. Delivered peptide = 19 x 0.5 mg = 9.5 mg. Lost to dead space = 19 x 2 µL = 38 µL x 0.005 = 0.19 mg. Yield = 95 per cent.

More usefully, 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.

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.

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

The single most important fact about dead space is that it is almost entirely in the hub cone, not in the needle, which is why changing needle gauge or length barely changes your losses.

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

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

One qualification: the dead space does not affect the dose accuracy if the hub was full of solution at the start of the draw.

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

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.

Delivered peptide = 10 x 0.5 mg = 5.0 mg. Lost to dead space = 10 x 84 µL = 840 µL x 0.005 = 4.2 mg. Yield = 50 per cent.

Worth noting: draw size matters enormously — the smaller your draws, the more the syringe architecture matters.

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