How thin is she? Can you easily see/feel her ribs?
How much does she weigh?
Here is a good image to assess if she is too thin:

Here is a link that might be useful:

Kitten food has a higher calorie count...maybe some canned kitten food?

Have you had a full senior blood panel run on her in the last 6 mos, including a CBC, full chemistry, and Total T4? If not, you should have that done now. If you have had this bloodwork run, it would be very helpful if you would post the results here for us to take a look. If you don't have a copy of the lab report, go back to your vet and get one. You should ALWAYS keep copies of all of your cat's test results at home for your own records.

I can't tell you how many times vets have told me that bloodwork was fine, when, in fact, there were obvious red flags in the bloodwork when I saw the report myself. It's very important that you get the lab report and go over it yourself. There are a number of veterinary resources online to help you interpret the results and see if there really is anything in your cat's bloodwork that merits further investigation and discussion with your vet.

At 18 yrs old, it would be surprising, indeed, if your cat is as healthy as your vet has apparently implied. At the very least, your cat most likely has some degree of kidney disease that may benefit from home management. Proper management of any existing chronic diseases can help improve appetite and overall health. But you really need to know what, if anything, you're dealing with.

BTW, lactulose is not lactose-free milk. It is an osmotic laxative used in cats with constipation problems.

Laurie

Thanks for that chart! I would say Spirit is a 2, underweight, but no ribs showing. And not losing weight, but not gaining, either.

I will definitely try the kitten food.

Laurie, I meant she drinks Lactaid, not Lactulose. I give Lactulose to my other cat, who has mega-colon. I used the wrong word. And I will request those workup results.

Thanks, everybody

My 14-year-old cat had a kidney infection 2 years ago and we almost lost in in a matter of a couple of days (from completely normal to almost comatose). She was down to 5 pounds, but responded well to treatment. She was skin and bones, so I started giving her a tablespoon or two or half and half every morning, figuring the calories would help her to put and keep weight on. Plus (important!) she eats it readily.

It worked --- and she's now 16 and still going strong,,, vet was impressed with her at a recent visit. She's been stable at her normal weight of 8 pounds for 2 years.

Good luck with Spirit.

Doesn't milk/cream give cats diarrhea? (I never could spell that word!)

Many cats are lactose-intolerant and will develop diarrhea and other digestive upsets when given dairy products. Unless you know that your cat can tolerate dairy products, it's best to stay away from them or give only lactose-free dairy.

Have you gotten copies of her most recent lab reports yet?

Laurie

Update on Spirit - I took her to a new vet, who says he thinks she is closer to 19 or 20 years old.

Weight is up from 7.12 to 8 lbs. He extracted a badly infected tooth and ran blood tests.

Everything is within normal limits with the following exceptions:

BUN is 37 ,normal ranges from 14-36.

Cholesterol is 227, normal range is 75-220. Maybe that's from the Half and Half I was feeding her to fatten up?

HGB is 8.7, normal range is 9.3 - 15.9.
HCT is 27 Normal range is 29-48.
Lymphocytes are 1120, Normal range being 1200-8000

More results are pending lab results. He has not discussed what these results mean, so I have to try to get hold of him, now that the holidays are winding down.

He did give her an antibiotic for the infected tooth that was removed and Hi-Vite drops for her anemia.

Thanks very much, everybody, for your input and advice.