Accepted answer
Your friend understated it. At 100 µL draws, a 1 mL luer syringe with a detachable needle costs you closer to half the vial, and the dead space is almost entirely in the hub, not the needle. Full arithmetic below.
The published figures
Syringe residual volume has been measured properly, mainly in the infection-control literature where it matters for a different reason. The widely cited figures are a median residual of about 84 µL for a conventional 1 mL syringe with a detachable needle, against roughly 2 µL for a fixed-needle low-dead-space design [1]. The World Health Organization 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.
That is a forty-fold difference between two things that look almost identical on a shelf.
Where the volume actually is
Compute the needle lumen and see for yourself. Lumen volume is cross-sectional area times length.
- A 27 G needle has an internal diameter of roughly 0.21 mm, so radius 0.105 mm. Area = π x 0.105² = 0.0346 mm².
- At 13 mm length: 0.0346 x 13 = 0.45 mm³ = 0.45 µL.
- A 21 G needle, ID about 0.51 mm, radius 0.255 mm. Area = π x 0.255² = 0.204 mm². At 38 mm: 7.8 mm³ = 7.8 µL.
- A 30 G, ID about 0.16 mm, at 8 mm: π x 0.08² x 8 = 0.16 mm³ = 0.16 µL.
So the needle contributes under a microlitre in a typical fine-gauge configuration. The remaining 83 of those 84 µL are in the luer cone of the syringe plus the needle's own plastic hub — the conical void between where the plunger tip stops and where the lumen begins. That is why a fixed-needle syringe is transformative: it has no luer cone and no needle hub, because the needle is bonded directly into the barrel and the plunger tip is shaped to occupy what little void remains.
Corollary that follows immediately: changing needle gauge or length barely changes your losses. Changing the syringe architecture changes everything. People fret over 30 G versus 31 G and ignore a variable forty times larger.
Your two configurations, in mg
Concentration: 10 mg / 2.00 mL = 5.00 mg/mL = 0.005 mg/µL.
Configuration A — 1 mL luer-lock plus detachable needle, dead space 84 µL. Each draw removes the dose plus the dead space from the vial, because the hub fills from the vial and its contents are never delivered:
- Volume removed per draw = 100 + 84 = 184 µL.
- Draws available from 2,000 µL = 2000 / 184 = 10.87, so 10 full draws.
- 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 = 5.0 / 10 = 50 %.
Configuration B — 0.5 mL fixed-needle U-100 insulin syringe, dead space 2 µL.
- Volume removed per draw = 100 + 2 = 102 µL.
- 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 = 9.5 / 10 = 95 %.
The switch nearly doubles your vial. Nine extra draws of 0.5 mg, for a syringe that costs about the same. If a vial costs 40 units of currency, configuration A is effectively paying 80 for it.
Summary table across configurations
| Configuration | Reported dead space | Loss per 100 µL draw | Full draws from 2.00 mL | Delivered from a 10 mg vial |
|---|
| Fixed-needle insulin syringe, U-100 | about 2 µL | 0.010 mg | 19 | 9.5 mg |
| Low-dead-space detachable-needle design | about 3–5 µL | 0.015–0.025 mg | 19 | 9.5 mg |
| 1 mL luer syringe + detachable needle | about 84 µL | 0.42 mg | 10 | 5.0 mg |
| 3 mL luer-lock syringe + detachable needle | about 100 µL or more | 0.50 mg or more | 9 | 4.5 mg |
Is low dead space a real category?
Real, and specified. Two distinct designs: the fixed-needle syringe, where the needle is bonded into the barrel — a standard U-100 insulin syringe, and the cheapest route to low dead space — and the low-dead-space detachable, which keeps the luer fitting but adds a moulded projection on the plunger tip that fills the luer cone when fully depressed. 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 regardless of the packaging. Five seconds to check, once per product.
What the fixed-needle route costs you: the needle cannot be changed, so the same needle does the stopper entry and the delivery, and a needle blunted by a puncture is measurably duller. Hence the standard split — a separate cheap detachable-needle syringe for the diluent addition, where dead space is irrelevant because it only holds water.
edited 25 Dec 2024 by amara_nwachukwu — added the placebo-arm figures
4The needle-lumen calculation showing under a microlitre is the part that reframes the whole problem. The hub is the enemy. – bac_or_bust 30 days ago 3Using a cheap luer syringe for the diluent addition and fixed-needle syringes for every draw is the obvious split and I had never thought of it. – helena_vidmar 9 months ago 7Half a vial. I have been doing this wrong for a year. – fiadh_cronin 4 months ago add a comment