Thanks for your help! I decided not to give him any insulin and I'm going to check his blood again in 2 hours.
I am SO glad you decided not to give Simon any insulin tonight! 110 is way too low to shoot without more data on how he is handling the insulin. Also, 4 units is
awfully high. I would start over with just 1U or less, in Simon's case.
Vetsulin is a fast acting insulin that usually hits its peak around 3-5 hours after administration (what would be the nadir-lowest point in the cycle-for your cat). Blood glucose will then start to rise again, slowly, and will frequently drop a little bit again around +7 hours. This is because Vetsulin is actually comprised of two different components that have different actions. Vetsulin dosing is largely based on the pre-shot values. Since it is a fast-acting insulin it is best to wait 20-30 minutes after kitty eats to give the shot. This assures there is some food in the system when the insulin hits.
Blood glucose will almost always measure higher at the vet's office than at home because of stress. It is not unheard of for it to be 100-200 points higher at the vet's. This is one of the reasons home testing is so important. You also cannot compare your vet's reading, taken on a pet-specific meter, to yours, taken on a human meter. There is no comparison between them. Many of us here use human meters, and we just use a different scale than the pet meters do. Tests should be taken before every shot, and at least one other time during the cycle, ideally more than once, so you can determine your cat's nadir and see how he reacts to the insulin.
I see you are feeding Purina DM dry. This is very high in carbohydrates, well over 20%. We recommend diabetic kitties eat a canned/wet diet low in carbs, 10% or less. The Tiki Cat you mentioned is a very low/no carb food. Any food transition should be done slowly over a period of time to help avoid a hypo or any tummy issues, and you MUST be home testing for safety. I am surprised that Simon tested so low tonight when his AM shot should have been out of his system, and while he is eating hi-carb, dry food. Please do not give him any insulin in the morning without testing first and posting for help if BG is under 200. It looks to me like there may be a small chance that Simon's diabetes might be able to be controlled by diet alone. I don't want to get your hopes up in case it turns out he can't, but it is a possibility. I have to caution you against changing his diet, though, until you are testing full time and can monitor him closely for hypos. You will probably need to do a dose reduction when you transition him. In Simon's case, I would not switch foods at all until you work out his proper dose (I think 4 units is
much too high for him), I would be too afraid he would drop dangerously low.
It would be very helpful to you, and to us, if you could set up a spreadsheet for Simon. These links will help you:
http://www.felinediabetes.com/FDMB/threads/fdmb-spreadsheet-instructions.130337/
http://www.felinediabetes.com/FDMB/threads/understanding-the-spreadsheet-grid.156606/
If you need assistance setting it up just sing out, someone will help you!
I will be up for a little while longer, when you test Simon again please post the results, I'd like to see where his BG is now.

