The particulars: a 29G needle · 1 mg/mL.
I would rather understand the derivation than memorise the outcome.
Two people I asked gave two answers that differ by a factor of ten, which is suggestive.
Can someone walk through the arithmetic step by step?
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 29G 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.
Mechanically, 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.
The needle lumen volume is under a microlitre in a typical fine-gauge configuration, so the needle is not the problem.
| Appearance | Interpretation | Action |
|---|---|---|
| Intact opaque puck, proud of base | Cycle ran correctly | Proceed |
| Slumped to one side | Shipped before fully dry, or vibration | Usually usable; note it |
| Glassy translucent film | Collapse above glass transition | Test before use |
| Melt-back ring at stopper | Thermal excursion in transit | Test before use |
| No visible cake at all | Very low fill, or nothing there | Weigh it; query the supplier |
To be exact about it, 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.
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.
edited 15 Nov 2024 by dead_volume — fixed an arithmetic slip in the third paragraph
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe 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.
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.
Mechanically, 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.
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 switch nearly doubles your vial, which is better than most other optimisations combined.
edited 15 Nov 2024 by dead_volume — added the placebo-arm figures
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