The particulars: a 31G needle · 1 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?
At 1 mg/mL every microlitre left behind is 1 µg, so a 50 µL hub costs 0.05 mg per draw and a 5 µL fixed-needle barrel costs 0.005 mg. Multiply by the draws, not by the doses: ten draws through a 50 µL dead space is 0.5 mg gone, which at 1 mg/mL is 0.5 mL of solution you paid for and never administered. Against a 2 mg dose that 50 µL is 2.5 per cent; against a 0.25 mg dose it is 20 per cent, which is why the loss matters most at exactly the doses where you can least afford it. a 31G 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.
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.
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.
| Vial | Diluent | Concentration | 0.25 mg | 0.5 mg | 1 mg | 2.5 mg |
|---|---|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 5 mg | 2 mL | 2.5 mg/mL | 10 u | 20 u | 40 u | 100 u |
| 10 mg | 1 mL | 10 mg/mL | 2.5 u | 5 u | 10 u | 25 u |
| 10 mg | 2 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 u | 15 u | 30 u | 75 u |
Units are U-100 insulin units, where 1 unit = 0.01 mL. Divide dose by concentration for millilitres, then multiply by 100.
Configuration B — 0.5 mL fixed-needle U-100 insulin syringe, dead space 2 µL: volume removed per draw = 100 + 2 = 102 µL.
Published inter-laboratory comparisons of dead-space measurements on identical syringes show good agreement, suggesting the numbers are reliable.
Buy the right syringe — a fixed-needle insulin syringe is cheap and solves the problem.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsIn practice, at 100 µL draws the dead-space penalty with a luer-lock is 84 per cent per draw — the cost is genuinely catastrophic.
The needle lumen volume is under a microlitre in a typical fine-gauge configuration, so the needle is not the problem.
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.
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.
edited 21 Jan 2025 by dead_volume — added the method parameters
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