Hi Sally.
I'm sorry that Lily's trial didn't work but I'm now glad to see that you've managed to get a Lantus Rx for her.
Great job on drawing the vet's attention to Lily's prior very low dose of Vetsulin.
Do you still have the vial of Vetsulin, Sally? If yes, then you would have the option to pick up where you left off with the 0.25IU dose of Vetsulin to tide Lily over till you get the Lantus from Marks Marine. Obviously give the vet a call to confirm it's OK to do so.
If it's OK to do this, I'd recommend monitoring closely for the first few cycles - AM and PM - to make sure that it doesn't take Lily too low mid-cycle. It's been a little while since she's had any insulin and her system may need a little time to adjust to it again (as she did when she first started treatment).
When the Lantus does arrive you should be able to move seamlessly onto it from Vetsulin because the latter is an in-out insulin and doesn't need a 'washing out' period. Needless to say you'll be able to get guidance from your vet and also you can post here as usual for support.
In the meantime, some homework!

Here's a link to the FDMB guide to Lantus dosing methods:
Lantus Dosing Methods: 'Start Low, Go Slow' (SLGS) and 'Tight Regulation' (TR)
I'd suggest starting off on the SLGS method while you're learning the Lantus ropes and getting a feel for how Lily responds to it. You always have the option of switching to TR later if you wish.
The normal procedure for cats switching from another insulin onto Lantus is to keep the dose the same* (assuming that the dose for the previous insulin was safe).
Vetsulin is an 'in-out' insulin: a dose is given and it all gets used up in a single cycle - often hours before the next dose is due. By comparison, Lantus is a
depot insulin, i.e. it builds up a little 'tank' of insulin in the body (a bit like a 'chemical pancreas'). The depot gradually releases insulin into the body and this helps the BG lowering effect to last longer than the likes of Vetsulin.
When a dose is given, some of it is used in the current cycle and some goes towards filling up/replenishing the 'tank'. This means that it takes several cycles before you can see the full effect of a given dose. Lily will be starting on 0.25IU of Lantus. The starting dose is held for a week (unless a dose reduction is earned - see below), and a curve is run. The daily readings and curve results are then evaluated and the dose adjusted if required. Here's an excerpt from the SLGS dosing guidelines:
Starting Dose:
- 1u BID if kitty is not on a wet/canned low carb diet
- 0.5u BID if kitty has been switched to a wet/canned low carb diet
- If the cat was previously on another insulin, the starting dose should be increased or decreased by taking prior data into consideration
- Generally, shots are to be given 12 hours apart.
Hold the dose for at least a week:
- Unless your cat won’t eat or you suspect hypoglycemia
- Unless your kitty falls below 90 mg/dL (5 mmol/L). If kitty falls below 90 mg/dL (5 mmol/L) decrease the dose by 0.25 unit immediately.
After 1 week at a given dose perform a 12 hour curve, testing every 2 hours OR perform an 18 hour curve, testing every 3 hours. Note: Random spot checks are often helpful to "fill in the blanks" on kitty's spreadsheet. The goal is to learn how low the current dose is dropping kitty prior to making dose adjustments.
- If nadirs are more than 150 mg/dl (8.3 mmol/L), increase the dose by 0.25 unit
- If nadirs are between 90 (5 mmol/L) and 149 mg/dl (8.2 mmol/L), maintain the same dose
- If nadirs are below 90 mg/dl (5mmol/L), decrease the dose by 0.25 unit
With Lantus, dose adjustments are based primarily on the nadir BG level.
Hoping that Lily will get on great with her Lantus!
(((Lily)))
Mogs
* Note: There may be exceptions, e.g. cats on a relatively high dose of the first insulin might start Lantus treatment at a somewhat lower dose.
.