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Thank you for your question. My name is***** I have been a small animal veterinarian for 14 years and I'd like to help with your question.
So, vomiting in cats is a really nonspecific symptom and can be caused by a number of problems..... in general we separate these into two categories, intestinal and non intestinal causes. Blood work is good at identifying non intestinal causes of vomiting, but when the problem is in the intestine itself the bloodwork will often be normal. The problem is there are just not great, easy tests to tell us what I going on in the intestine. At her age cancer is a concern. Intestinal lymphoma is an often slowly progressive cancer that causes vomiting (or/and diarrhea) and weight loss. It can be impossible to diagnose without an intestinal biopsy. Other potential problems include different inflammatory bowel diseases, dietary intolerance, foreign material (like hair balls or string) in the intestine. You have to decide how far you want to go with tests because it gets both expensive and invasive. And abdominal ultrasound is probably worth doing. Sometime you can see evidence of intestinal cancer on these and if there is a foreign body it is sometime identified. But the gold standard for knowing for sure what is going on is an intestinal biopsy obtained at surgery.
Without doing surgery it is a matter of doing some diet trial with hypoallergenic foods and seeing if those help and maybe a trial with prednisone. Prednisone decreased inflammation in the intestine and can really improve their symptoms and quality of life, but doesn't solve the underlying problem. I frequently put older vomiting cats on prednisone if they have normal bloodwork and their owner's don't want to pursue surgical biopsies, just to reduce their vomiting with the understanding that this isn't a cure.
I hope that helps, but if you have more question please let me know. Also, when our conversation is over, please rate my answers.