u/bac_water_bandit

you avoided it because the person who sold it to you sent the right one. that is the whole mechanism and it is not reassuring.

Fasting or not, and what time of day?

baseline before you start is worth more than any later panel

Tracked eGFR against the weight trend for a year. The correlation was much weaker than I expected and that was worth knowing.

Left up and flaired Explainer — the numbers in this one check out.

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.

Working has to be shown on arithmetic posts. Added a note asking for it rather than removing this one.

The arithmetic in full: concentration is mass over volume, so 5mg into 2mL gives 2.5mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 1mg is 1 divided by 2.5 millilitres, which is 40 units.

Corrected the concentration in the title, which contradicted the body. Nothing else touched.

Reminder: this board does maths, not dosing. Nobody here tells another member what amount to take.

units are a volume on a U-100 syringe, not a dose

Bacteriostatic or plain sterile water?

Corrected the concentration in the title, which contradicted the body. Nothing else touched.

Yes — same lab, same fasting state, same rough time of day, or the comparison is doing nothing.

the trend line matters more than any single value

the powder cake sits at the bottom, the volume you add is the volume that counts

Working has to be shown on arithmetic posts. Added a note asking for it rather than removing this one.

more water is not more drug, it is the same drug in a bigger volume

I would not round that figure. At 2.5mg/mL a rounding error of two units is a real fraction of the intended amount.

Corrected the concentration in the title, which contradicted the body. Nothing else touched.

more water is not more drug, it is the same drug in a bigger volume

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.

Same.

Adding one thing — put the date on the label too. Concentration plus date and you can never mix two vials up.

Corrected the concentration in the title, which contradicted the body. Nothing else touched.

the number you write down is the number you will trust at 6am

Left up and flaired Explainer — the numbers in this one check out.

Retitled — the original asserted a causal claim the post does not support.

Agreed — and standardising the draw conditions is the cheapest improvement available.

Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.

The dead space question sent me down a rabbit hole for a week. It is a fraction of a unit and it is the same every draw.

The whole calculation, once, with real numbers.

You have a 10mg vial. You add 2mL of bacteriostatic water. Concentration is mass over volume: 5mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.

To draw 0.5mg you need 0.5 divided by 5 millilitres, which is 10 units on the barrel. Write "5mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.

Panicked about a single result, posted here, got told to repeat it, repeated it, and it was fine.

Brought the whole set to my clinician rather than one flagged value. Completely different conversation.

Panicked about a single result, posted here, got told to repeat it, repeated it, and it was fine.

Left up and flaired Explainer — the numbers in this one check out.

Reminder: this board does maths, not dosing. Nobody here tells another member what amount to take.

Left up and flaired Explainer — the numbers in this one check out.

Corrected the concentration in the title, which contradicted the body. Nothing else touched.

Correcting my own comment: the two draws were 8 weeks apart, not the figure I gave earlier.