Stated plainly: 20 mg · 1.5 mL.
The units are where I keep going wrong, so please be explicit about them.
I have sanity-checked the order of magnitude and it seems right, which is not the same as being right.
Is my approach right even if my number is wrong?
13.33 mg/mL, so one unit carries 0.133 mg. 20 ÷ 1.5 = 13.33 mg/mL; one unit on a U-100 barrel is 0.01 mL; 13.33 × 0.01 = 0.133 mg per unit. To go the other way, divide your intended dose by 0.133: a 1.33 mg dose is 10 units, and a 2.67 mg dose is 20. Write both the concentration and the milligrams per unit on the vial.
Rounding to the nearest whole syringe unit is usually the right error to make, but understanding which direction it is and why matters.
Breaking it down further: if a 10 mg vial has 96.5 per cent content, you have 9.65 mg of peptide. Divide that by 2.00 mL and your concentration is 4.825 mg/mL, not 5.00 mg/mL, which is a 3.5 per cent systematic error in every dose calculation.
On filtration: a 0.22 µm syringe filter will remove particulates and organisms, and it will also adsorb a fraction of your peptide onto the membrane — with a low-binding PVDF or PES membrane the loss is typically a few per cent.
One qualification: if your arithmetic and someone else's disagree by a factor of ten, one of you has made a unit error, and writing out the units at every step is the diagnostic.
If in doubt, use more diluent and accept the shorter usable window.
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Browse resultsThe answer depends on exactly which dose and which vial you are asking about, but the method is always the same.
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. 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.
Do not use the same needle to pierce the stopper and to administer. The tip is blunted by the stopper, and the hub now contains a dose you are about to lose to dead space anyway.
The Arrhenius relationship for drawing kinetics means that cold solution takes noticeably longer to draw than room-temperature solution.
Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.
Write the units at every step, because units errors are the failure mode that catches everyone eventually.
Room temperature before drawing is worth the ten minutes. Cold solution is more viscous, draws slower, and condensation on a cold barrel makes it harder to read the meniscus.
Worked example, because the general form is easier to trust once you have seen it once. Take a 10 mg vial and add 2 mL of diluent: the concentration is 10 ÷ 2 = 5 mg/mL. A 0.5 mg dose is 0.5 ÷ 5 = 0.1 mL. On a U-100 syringe, where 1 unit = 0.01 mL, that is 0.1 ÷ 0.01 = 10 units. Change the diluent to 1 mL and the same dose becomes 5 units — same dose, half the resolution.
Do the arithmetic twice, ideally with someone else doing it independently.
The underlying point is that this is one of those calculations where checking your work takes two minutes and prevents a very consequential error.
The rounding error accumulates if you round too many times — rounding concentration to 5.0, rounding the dose volume to 0.1 mL, rounding the unit reading to 10 — and the safest approach is to work the full precision and round only the final answer.
The limitation is that technique reduces risk, it does not remove it, and nothing you can do outside a controlled environment makes a non-sterile preparation sterile.
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.
More usefully, the single most useful thing to do is write the arithmetic on the vial label, because you will reconstruct it from memory at an inconvenient moment if you do not.
The concentration you actually work with is label claim times content fraction divided by actual diluent volume, which is usually not the same as the nominal concentration because content is usually not 100 per cent and you rarely measure the diluent volume to 0.1 mL precision.
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.
edited 24 Oct 2024 by kwn_analytical — added a caveat about sampling
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