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ketones and DKA.
There are a number of things that you can do to keep Gibson safe. I'm hoping the vet told you that you acted promptly and likely saved your cat's life. DKA can be a life threatening condition. Many of the cats here, mine included, have survived DKA. In simple terms, DKA results from an infection or inflammation + not enough insulin + not enough calories. This combination of factors throws off your cat's metabolism and he became acidotic (i.e., his electrolytes weren't where they should be). Gibson wasn't metabolizing food. He was metabolizing stored fat and this is what causes ketone bodies to be produced. Typically, treatment involves IV fluids because of dehydration and it's a medium for adding back electrolytes (such as potassium), using a short acting insulin to get his blood glucose (BG) numbers down to a better level rapidly along with a longer acting insulin such as Prozinc, making sure he is eating even if it means adding a medication for nausea and an appetite stimulant, and treatment of the underlying infection.
The first two things I would encourage you to do is to get a glucometer. You will want to test Gibson's blood if you're not already doing so in order to keep track of his BG levels. Home testing is the best way you can make sure Gibson's levels are not going too low. It can also serve as a heads up if you see his numbers become unusually high. I'd also suggest getting Ketostix or a blood ketone monitor. The Ketostix are a way to test for urinary ketones whereas the meter allows you to test blood. The Ketostix are less expensive and fine to use providing Gibson doesn't mind that you will need to stalk him to the litter box. Ketones that are trace or lower are manageable at home. If they are above trace levels, it can be an emergency. Any human glucometer is fine. Many people here use the Walmart Relion brand meter because the strips are inexpensive. You should be able to get Ketostix at Walmart. Any pharmacy will stock glucometers and Ketostix. Adding water to Gibson's food can also help. Additional fluid helps to dilute the ketones.
Talk to the vet about Gibson's eating. He will need to eat more calories than he typically does. We suggest 1.5 times the usual amount of what he consumes. If he's still not feeling great, an appetite stimulant (e.g., mirtazepine) can help. If he's nauseated, that will need to get controlled otherwise, he's likely to not want to eat. (Ondansatron and/or Cerenia are often used to manage nausea. Ondansatron is a human medication and available at any pharmacy; Cerenia is a veterinary medication your vet should stock.) While we encourage having your cat on a low carb diet, if he's not feeling great and we're working with a cat that's recently overcome DKA, personally, if all your cat wants to eat is ice cream or the salmon you cooked yourself for dinner, let him eat!!! The calories are important.
The other factor is his insulin. If you weren't home testing, you don't have a means of assessing if Gibson was getting enough insulin. With a cat that is immediately out of the hospital, we encourage keeping an eye on insulin needs (and BG levels) since enough insulin is a part of the equation that keeps ketones in check. While likely not what you want to hear, some cats are more prone to another episode of DKA once they have been there before. That said, my cat had one episode at the time she was diagnosed and never had ketones appear again.
We tend to be pretty aggressive about supporting you and and your cat when it comes to ketones. It is completely unnerving for the caretaker to have a cat that is critically ill. It's also dreadfully expensive. We want to be able to help you set Gibson up for success. Please let us know how we can help. The members here are very generous with their time and knowledge so please ask questions.