Hi JAB and welcome to FDMB.
I've been away from the board for a short time, but am from UK.
Don't have any experience of the particular insulin you are using, but have a feeling it may be what my friend used a while back so will check with her.
5U is a very high dose and once a day just isn't going to cut it.
When you said 30, would I be right in thinking mmol? In USA it's mgdl.You would times that by 18-that's a seriously high diabetic number.
Can you explain how the vet worked you up to that dose?
You do have to be careful with cutting doses, but at the same time it's rare for cats to be on doses above 2 or 3u (most here are are under 2). We do have what we call high dose cats, they are on very hih doses.
I would suggest you navigate your way round this site, do lots of reading (hope you have time).
Also wikia has lots of sections on felinediabetes, alot of it written by Steve and Jock who are also UK based (Steve and Jock were the pioneers of using Levemir, found to be one of the most successful insulins at giving you a chance of getting your cat into remission)
The Hills stuff is next to rubbish.
I fed Lucky Felix 'as good as it looks'. This was just over 3% carbs. Hills is much higher, but don't want you to swap till understood where your 5u dose came from.(I can also give you some other UK brands that are better than the Hills)
The reason for this is, the higher the carb content of food, the higer dose of insulin cat needs. Carbs raise glucose number.
If your cat is on to high a dose, the higher carb food may be evening that out a bit.
Although fructosamine test gives you an average, that's exactly what it is-an average BG over the last 1-3 weeks. If it comes out at say 300, it could stillmean at some time in the day, kitty has been at 100.
This is why the board advocates strongl for home testing. It takes dedication, but once your into a routine it takes minutes out of your day.
I have no doubt in my mind it gave me an extra 10 months with Lucky, and the only reason I lost her was because of something that the vets did (she died in their care)
If your interested in learning about hometesting there's plenty of beans (us) here who will help you.
I used to get test strips of ebay and a couple of the monitors cheap. Sometimes the strips were costly, others cheap (supply and demand).
Shame you weren't on here (and me) couple of weeks ago, I have 2 tubs that now will go to waste as they are accucheck aviva and run out 31st March (microchip so can't use past sell by date like you can other brands).
Your right about kitty getting stressed at vets,can raise BG by over 100 points-Lucky always did. It's a nice little money earner too.
I sawsome mention of crystals-can you give more info, also if any other health concerns?
How long had kitty been peeing/eating excessively before being diagnosed?
Don't want to overload you with info (and late here-I'm in Germany at present)and if you want to just stick with vets no point in bombarding you either.
Ask lots of questions if you like, will get lots of replies as this is a 24 hr board from all over the world.
Good luck
ps-look at somygi rebound. If cat being given too high a dose, you get a roller coaster ride of numbers and vets who don't know better just keepo increasing dose.
Also reason twice a day shots is cos cats utilise insulin differently to dogs (twice as fast basically)This should be basic knowledge!!! (sos just my little dig at whitecoats)