Details up front: a 31G needle · 10 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.
Can someone walk through the arithmetic step by step?
At 10 mg/mL every microlitre left behind is 10 µg, so a 50 µL hub costs 0.5 mg per draw and a 5 µL fixed-needle barrel costs 0.05 mg. Multiply by the draws, not by the doses: ten draws through a 50 µL dead space is 5 mg gone, which at 10 mg/mL is 0.5 mL of solution you paid for and never administered. Against a 2 mg dose that 50 µL is 25 per cent; against a 0.25 mg dose it is 200 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.
Dead space is irreducible with a high-dead-space syringe, which is why the hardware matters more than any technique.
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.
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.
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.
I would not underestimate the dead-space cost when calculating your true cost per dose.
Buy the right syringe — a fixed-needle insulin syringe is cheap and solves the problem.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemThe 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.
The needle lumen volume is under a microlitre in a typical fine-gauge configuration, so the needle is not the problem.
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.
If cost matters, this is the first thing to change, not the last.
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.
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 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.
The switch nearly doubles your vial, which is better than most other optimisations combined.
edited 31 May 2024 by tenth_of_a_unit — added the placebo-arm figures
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.
Published inter-laboratory comparisons of dead-space measurements on identical syringes show good agreement, suggesting the numbers are reliable.
The limitation is that even with perfect technique, some loss is irreducible unless you switch to a low-dead-space syringe.
Write the arithmetic on the vial label. It costs nothing and it removes the step where you reconstruct it from memory at an inconvenient moment.
This is arithmetic, so let us do the arithmetic and see where the losses actually 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.
The practical summary: fine gauge, gentle swirl, diluent down the wall, room temperature before drawing, and check the syringe scale against the barrel rather than against your assumption.
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