[Question] mg/mL — what am I missing here
Slightly embarrassed to be asking this, but: mg/mL — what am I missing here.
Bacteriostatic water contains benzyl alcohol as a preservative, which is what makes repeated entry into the same vial reasonable. Plain sterile water has no preservative.
If your calculation produces a fraction of a unit, the concentration is wrong for your dose. Change the water volume until the numbers land on whole units.
Aim the stream down the wall of the vial rather than into the cake. Reconstitution is a solubility problem, not a mixing problem, and mechanical shear does nothing helpful.
Corrections welcome, especially the pedantic ones. Pedantry is how this board earns its reputation.
best — the order this archive was captured in
The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests. It is a fraction of a percent and it is dwarfed by fill tolerance.
Reminder: this board does maths, not dosing. Nobody here tells another member what amount to take.
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a U-100 syringe reads 100 units to the millilitre, every time
I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume.
dead space in the needle hub is real and it is small
The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests.
This is the sentence that makes the rest unnecessary. Write it on the vial and the problem is gone.
the powder cake sits at the bottom, the volume you add is the volume that counts
10mg into 1mL is 10mg/mL, and 1mg of that is 10 units
Adding more water does not change how much peptide is in the vial. It changes the concentration, which changes the volume you draw for the same amount. That is the entire relationship.
This. Showing the working is the fastest way for somebody else to spot the mistake.
do the arithmetic before you reconstitute, not while holding a syringe
Research-use-only material is not approved for human use, so treat everything in this thread as arithmetic about solutions rather than instructions for a person.
Agreed on the swirl. There is no reason to shake it and a good reason not to.
Disagree with the "more water is safer" framing. It is not safer, it is just a different number to divide by.
Pushback on the syringe advice: a U-100 barrel reads units of volume. Calling them "doses" is how the confusion starts.
Cosigning the round-number approach. Reconstituting to something awkward is how you end up drawing 27.5 units at six in the morning.
aim the stream at the glass wall, not at the cake
Are you accounting for the powder volume, or is that where the confusion is?
write the concentration on the vial in marker, that is the whole post
Wrote the date on the vial as well as the concentration after losing track of which one was opened first.
Reconstituted at 1mL when I meant to use a different volume. Caught it because the units looked wrong on the barrel. Now I write 5mg/mL on the glass before I put the cap back.
Careful — that arithmetic is wrong. 15mg in 3mL is 5mg/mL, so 2.5mg draws to 50 units, not what you wrote.
What concentration did you actually reconstitute at, rather than what did you intend to?
The arithmetic in full: concentration is mass over volume, so 10mg into 2mL gives 5mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 0.75mg is 0.75 divided by 5 millilitres, which is 15 units.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
label the vial with the date as well as the concentration
- 1The lyophilised cake displaces a small volume, so the true concentration is…15 comments in this branch · started by u/arne_restrepo
- 2Agreed on the swirl. There is no reason to shake it and a good reason not to.11 comments in this branch · started by u/ines_lindqvist