Details up front: an 18G drawing needle · 2.5 mg/mL.
This should be a straightforward calculation and I keep getting two different answers.
The numbers are arbitrary; the method is what I am after.
What is the general form of this calculation?
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.
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.
| Configuration | Dead volume | Loss at 5 mg/mL | Over 20 draws |
|---|---|---|---|
| Fixed-needle insulin syringe | 3–5 µL | 15–25 µg | 0.3–0.5 mg |
| Low-dead-space, detachable | <2 µL | <10 µg | <0.2 mg |
| Standard luer-lock + 30G | 35–60 µL | 175–300 µg | 3.5–6 mg |
| Luer-lock + 21G drawing needle | 70–100 µL | 350–500 µg | 7–10 mg |
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.
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.
Buy the right syringe — a fixed-needle insulin syringe is cheap and solves the problem.
edited 24 May 2025 by Dr_Yusuf_Adeyemi — removed a claim I could not source
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Browse resultsThis is arithmetic, so let us do the arithmetic and see where the losses actually are.
Corollary that follows immediately: changing needle gauge or length barely changes your losses.
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.
Worth noting: draw size matters enormously — the smaller your draws, the more the syringe architecture matters.
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 needle lumen volume is under a microlitre in a typical fine-gauge configuration, so the needle is not the problem.
The relevant detail is that 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.
I would not underestimate the dead-space cost when calculating your true cost per dose.
If cost matters, this is the first thing to change, not the last.
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.
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.
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.
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.
One qualification: the dead space does not affect the dose accuracy if the hub was full of solution at the start of the draw.
The switch nearly doubles your vial, which is better than most other optimisations combined.
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