My setup is a refrigerator with a logger and a small work area I wipe down, nothing more.
Please show the division. I want to check my own against yours.
I would like the general form as well as the specific number, so I can apply it again.
How many significant figures are actually justified here?
The part that matters: 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.
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.
| 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.
Corollary that follows immediately: changing needle gauge or length barely changes your losses.
Published inter-laboratory comparisons of dead-space measurements on identical syringes show good agreement, suggesting the numbers are reliable.
One qualification: the dead space does not affect the dose accuracy if the hub was full of solution at the start of the draw.
If cost matters, this is the first thing to change, not the last.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsMechanically, before anything else: understand that dead space is a property of the syringe architecture, not of the needle gauge.
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.
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.
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.
Worth noting: draw size matters enormously — the smaller your draws, the more the syringe architecture matters.
Buy the right syringe — a fixed-needle insulin syringe is cheap and solves the problem.
The relevant detail is that start from the worked arithmetic: a 10 mg vial reconstituted with 2 mL gives 5 mg/mL. One hundred microliter draws are 0.5 mg each. At 84 µL dead space per draw, half your vial disappears into the hub.
The luer cone of the syringe plus the needle's own plastic hub accounts for the vast majority of the dead space.
The underlying point is that configuration A — 1 mL luer-lock plus detachable needle, dead space 84 µL: each draw removes 100 + 84 = 184 µL. Draws available from 2,000 µL = 2000 / 184 = 10.87, so 10 full draws.
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.
I would not underestimate the dead-space cost when calculating your true cost per dose.
The switch nearly doubles your vial, which is better than most other optimisations combined.
edited 1 Sept 2024 by tobias_maartens — added the citation requested in comments
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