[Discussion] can we stop arguing about bacteriostatic water until somebody posts a number
Slightly embarrassed to be asking this, but: can we stop arguing about bacteriostatic water until somebody posts a number.
Spent 2 weeks confused about this until somebody drew the table for me. 30mg, 3mL, 10mg/mL, 1mg is 10 units. That is the whole thing.
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.
Ask me anything specific. Anything general I will probably get wrong.
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.
Why "more water" keeps confusing people, including me for a month.
The vial contains a fixed amount of peptide. Water changes nothing about that amount. What it changes is concentration, and concentration is the divisor between the dose you want and the volume you draw.
So doubling the water halves the concentration and doubles the units for the same dose. Nothing is diluted in any sense that matters; you are simply reading a bigger number off the same barrel. Pick the water volume that puts your usual dose on a whole number of units and life gets easier.
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Correcting myself above: I wrote the concentration the wrong way up. Mass over volume, not volume over mass.
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.
I would not round that figure. At 2.5mg/mL a rounding error of two units is a real fraction of the intended amount.
I would not round that figure.
Adding one thing — put the date on the label too. Concentration plus date and you can never mix two vials up.
Working has to be shown on arithmetic posts. Added a note asking for it rather than removing this one.
dead space in the needle hub is real and it is small
aim the stream at the glass wall, not at the cake
aim the stream at the glass wall, not at the cake
This is the sentence that makes the rest unnecessary. Write it on the vial and the problem is gone.
Did the maths give you a whole number of units, or a fraction?
5mg into 2mL is 2.5mg/mL, and 2.5mg of that is 100 units
- 1Working has to be shown on arithmetic posts. Added a note asking for it…7 comments in this branch · started by u/units_not_mg