[Math] someone check my arithmetic before i draw this up
Thinking out loud about this: someone check my arithmetic before i draw this up.
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.
The arithmetic in full: concentration is mass over volume, so 5mg into 1mL gives 5mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 1.5mg is 1.5 divided by 5 millilitres, which is 30 units.
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.
Ask me anything specific. Anything general I will probably get wrong.
best — the order this archive was captured in
The whole calculation, once, with real numbers.
You have a 10mg vial. You add 1mL of bacteriostatic water. Concentration is mass over volume: 10mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 2mg you need 2 divided by 10 millilitres, which is 20 units on the barrel. Write "10mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
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That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
twice as much water halves the concentration and doubles the units for the same dose
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.
Did the maths give you a whole number of units, or a fraction?
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.
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
label the vial with the date as well as the concentration
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.
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.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
Working has to be shown on arithmetic posts. Added a note asking for it rather than removing this one.
What is written on the vial right now?
more water is not more drug, it is the same drug in a bigger volume
reconstituting at a round concentration makes every later dose easy
a U-100 syringe reads 100 units to the millilitre, every time
Are you accounting for the powder volume, or is that where the confusion is?
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.
units are a volume on a U-100 syringe, not a dose
write the concentration on the vial in marker, that is the whole post
the powder cake sits at the bottom, the volume you add is the volume that counts
bacteriostatic water has the preservative, sterile water does not
- 1If your calculation produces a fraction of a unit, the concentration is…6 comments in this branch · started by u/cesar_ivaturi
- 2Shook the first vial because nobody told me not to. It was fine, but I have…6 comments in this branch · started by u/georgi_adebayo