Hi Catrina,
Well done getting that test !

It is very normal for a newly diagnosed diabetic cat to have BGs all over the place. His body is still getting used to the insulin, which is a hormone and not a medication like an antibiotic that goes to work right away to fix the problem.
The dose of 0.5 U is taking him down nicely to a lovely nadir and for a newly diagnosed kitty it is great he is responding well to the dose. Some cats take a lot longer.
The reason he is up high again at the end of the cycle is because the dose has worn off.
The normal cycle of Lantus is...onset at around +2 and then gradually goes down to Nadir around +4 to +7 or 8 ( every cat is different). The the dose starts to wear off then and the BG starts rising again.
As you get closer to the best dose, the preshots will start to come down. This can take time.
You have only been giving the dose for 4 days and the depot will take up to 7 days to fill and the full effect to be felt.
Feline diabetes is a journey not a race and the main think
I had to learn in my journey was patience.
Another reason the BG can go high is
bouncing which is very normal in newly diagnosed cat. This is from The Basics on the Lantus page
- Bouncing - Bouncing is simply a natural reaction to what the cat's system perceives as a BG value that is "too low". "Too low" is relative. If a cat is used to BGs in the 200's, 300's, or higher for a long time, then even a BG that drops to 150 can trigger a "bounce". Bouncing can also be triggered if the blood glucose drops too low and/or too fast.The pancreas, then the liver, release glucogon, glycogen and counter-regulatory hormones. The end result is a dumping of "sugar" into the bloodstream to save the cat from going hypoglycemic from a perceived low. The action is often referred to as "liver panic" or "panicky liver". *Usually*, a bounce will clear kitty's system within 3 days (6 cycles).
You may also like to read
THE BASICS from the lantus page.
Are you feeding snacks during both the cycles?
I would also try and get more tests in during the PM cycles as that gives valuable information. Always try and get a before bed test. If that test is below the preshot BG that is a bit of a heads up to you that the BG might drop lower, so I would se the alarm and get up and test a bit later.