genuine question about concentration that I am slightly embarrassed to ask
genuine question about concentration that I am slightly embarrassed to ask. If this has been answered properly somewhere, link me and I will delete.
The failure modes I have actually seen posted here, in order of frequency.
One: reconstituting at a different volume than intended and not writing it down, so the next draw is based on a remembered concentration. Two: treating units as a dose rather than a volume, which breaks the moment the concentration changes. Three: assuming a syringe is U-100 when it is not.
All three are solved by the same two habits — do the arithmetic before the water goes in, and write the result on the glass. Neither costs anything and between them they cover almost every question this board gets.
Asked SWB what volume they reconstitute to for their own stability testing and they answered in one line. Useful thing to know.
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
Not medical advice, obviously, and nothing here is approved for human use. One person with a spreadsheet.
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.
Small fix — a U-100 syringe is 100 units per mL. If yours is marked differently it is not U-100 and the whole calculation changes.
How many mg in the vial and how many mL are you adding?
Show the arithmetic — where did the unit figure come from?
I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume.
Corrected the concentration in the title, which contradicted the body. Nothing else touched.
Agreed — units are volume. Once that lands, the rest of the arithmetic is one division.
What concentration did you actually reconstitute at, rather than what did you intend to?
Careful — that arithmetic is wrong. 5mg in 1mL is 5mg/mL, so 5mg draws to 100 units, not what you wrote.
Correction: that gives 50 units, not what you have written. 10mg into 1mL is 10mg/mL and the division follows from there.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
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.
Research-use-only material is not approved for human use, so treat everything in this thread as arithmetic about solutions rather than instructions fo
This is the sentence that makes the rest unnecessary. Write it on the vial and the problem is gone.
Bacteriostatic or plain sterile water?
I would not round that figure. At 2.5mg/mL a rounding error of two units is a real fraction of the intended amount.
Spent 9 weeks confused about this until somebody drew the table for me. 20mg, 1mL, 20mg/mL, 2mg is 10 units. That is the whole thing.
the powder cake sits at the bottom, the volume you add is the volume that counts
Disagree with the "more water is safer" framing. It is not safer, it is just a different number to divide by.
dead space in the needle hub is real and it is small
Right — the volume you add is the only variable you control here, and the cake does not change it meaningfully.
Right — the volume you add is the only variable you control here, and the cake does not change it meaningfully.
Disagreeing with this line only: adding more water is not a safety margin, it is a different divisor.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
Did the maths give you a whole number of units, or a fraction?
Pushback on the syringe advice: a U-100 barrel reads units of volume. Calling them "doses" is how the confusion starts.
- 1the powder cake sits at the bottom, the volume you add is the volume that…9 comments in this branch · started by u/lyophile_lou
- 2Corrected the concentration in the title, which contradicted the body.…6 comments in this branch · started by u/bac_water_bandit