What is the dead-space loss per draw with an 18G drawing needle at 10 mg/mL?

Details up front: an 18G drawing needle · 10 mg/mL.

I can do the algebra. I am not confident about the conversion factors.

If there is a standard way to lay this out, I would rather learn that than invent one.

Can someone walk through the arithmetic step by step?

At 10 mg/mL every microlitre left behind is 10 µg, so a 50 µL hub costs 0.5 mg per draw and a 5 µL fixed-needle barrel costs 0.05 mg. Multiply by the draws, not by the doses: ten draws through a 50 µL dead space is 5 mg gone, which at 10 mg/mL is 0.5 mL of solution you paid for and never administered. Against a 2 mg dose that 50 µL is 25 per cent; against a 0.25 mg dose it is 200 per cent, which is why the loss matters most at exactly the doses where you can least afford it. an 18G drawing needle has a bore, a hub and a length, and the hub dominates: a fixed-needle insulin barrel has almost none, a luer connection has a measurable one before the needle even starts.

Put another way, 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.

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.

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

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

The relevant detail is that the switch to a low-dead-space syringe nearly doubles your usable vial, which is better than switching suppliers if you are looking for cost savings.

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

Specifically, the needle lumen volume is under a microlitre in a typical fine-gauge configuration, so the needle is not the problem.

The caveat is that dead space is a yield loss and not a dose-accuracy loss, so the person feeling this loss most is the person with the most total draws.

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

Concretely, 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.

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.

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

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

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

edited 9 Dec 2025 by u100_marks — added the placebo-arm figures

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

The complete rule: fix the syringe architecture first, and then the reconstitution volume becomes a free choice you can make on stability grounds rather than on economics.

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

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

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.

At 5 mg/mL that is 15 to 25 µg lost per draw on the insulin syringe and 175 to 500 µg on the luer-lock — which over ten draws is the difference between losing a rounding error and losing half a milligram.

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.

The practical summary: fine gauge, gentle swirl, diluent down the wall, room temperature before drawing, and check the syringe scale against the barrel rather than against your assumption.