genuine question about units that I am slightly embarrassed to ask
genuine question about units 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 KP 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.
Corrections welcome, especially the pedantic ones. Pedantry is how this board earns its reputation.
best — the order this archive was captured in
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.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume.
I keep a laminated card in the drawer with the arithmetic on it.
Right, and the arithmetic behind it: 5mg in 2mL is 2.5mg/mL, so 0.25mg is 10 units.
How many mg in the vial and how many mL are you adding?
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 — units are volume. Once that lands, the rest of the arithmetic is one division.
Agreed — units are volume.
This is the sentence that makes the rest unnecessary. Write it on the vial and the problem is gone.
Yes, and the dead-space point is worth repeating: it is small, it is real, and it is the same for every draw.
Agreed — units are volume.
Disagreeing with this line only: adding more water is not a safety margin, it is a different divisor.
Reminder: this board does maths, not dosing. Nobody here tells another member what amount to take.
Correction: that gives 30 units, not what you have written. 5mg into 1mL is 5mg/mL and the division follows from there.
do the arithmetic before you reconstitute, not while holding a syringe
the number you write down is the number you will trust at 6am
The arithmetic in full: concentration is mass over volume, so 10mg into 3mL gives 3.33mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 1mg is 1 divided by 3.33 millilitres, which is 30 units.
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.
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.
Spent 9 weeks confused about this until somebody drew the table for me. 5mg, 3mL, 1.67mg/mL, 1mg is 60 units. That is the whole thing.
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.
I would not round that figure. At 10mg/mL a rounding error of two units is a real fraction of the intended amount.
- 1The arithmetic in full: concentration is mass over volume, so 10mg into 3mL…6 comments in this branch · started by u/vito_beaulieu