The case in front of me: 8 units · 2 mg/mL.
I have done this once and I suspect I got away with it rather than got it right.
For context: I keep records of every batch, every lot number and every result, so an answer that requires me to track something is fine.
What is the correct sequence, and where is the step that people usually skip?
Stated carefully, 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.
Number of stopper piercings matters less than the gauge doing the piercing. A 30G or 31G needle through a butyl stopper leaves a track that reseals; a 21G or 18G drawing needle punches a core and can drop it into the solution.
Specifically, air bubbles at these volumes are a measurement problem rather than a safety one. A 2 mm bubble in a 0.3 mL syringe is roughly 4 µL, which at 10 units drawn is a four per cent error.
Published data on syringe dead space quantifies low-dead-space designs as retaining under 2 µL against 35 µL or more for conventional detachable-needle syringes.
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.
edited 10 Sept 2024 by Dr_Aoife_Brennan — reworded for clarity after a comment
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Browse resultsThe common error is getting the concentration right but then misreading the syringe scale, which is why checking the barrel marking rather than your memory matters.
Rotation of injection site is a tolerability measure, not a pharmacokinetic one, but if you are going to do it you might as well do it right.
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.
Write the arithmetic on the vial label. It costs nothing and removes the step where you reconstruct it from memory.
This is one of those calculations where checking your work takes two minutes and prevents a very consequential error.
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.
Worth being precise here: 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.
Do the arithmetic twice, ideally with someone else doing it independently.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.