Thanks for the additional information and clarification.
This is really quite a challenging and frustrating situation, isn't it:(
It's interesting you should mention FIP because as I was pondering her situation after I logged off last night, this possibility did occur to me.
You may already know this from your research but there's a wet form (where fluid builds up in the abdominal or thoracic cavity) and the dry form (which causes pyrogranulomas to develop and is harder to diagnose).
Signs can be vague and can depend on the location of pyogranulomas in the dry form. Mild intermittent fever, weight loss, lethargy, and decreased appetite may be seen. Irregular kidneys or irregular abdominal organs can be detected. The reason I thought of it was because of the ultrasound results of her kidneys and her gastrointestinal issues (diarrhea could be seen if the pyogranulomas affected her intestines).
The problem with previous antibody testing is that it will only tell us that she's been exposed to the corona virus, not that she necessarily is ill from the disease...which I'm sure you already know. But, there's a new test which is more reliable (LINK
I don't see a lot of FIP where I practice so I don't have a lot of familiarity with this test and haven't had to use it.
The only reason I mentioned a biopsy is because of the change in her stool consistency but I'm not entirely certain that this is the next test to consider since it's fairly invasive. Of course, if she had this done, then the entire abdominal contents could be evaluated and biopsies taken of any suspicious tissue, including her kidneys.
I've seen some young cats develop lymphoma which can affect their kidneys and intestines. A kidney biopsy would be needed to specifically diagnose it, though.
I guess I'm a little surprised that they aren't pursing possible causes of the irregular kidney since clearly this is not considered normal in a cat this age.
I'm afraid that I don't have a lot to contribute and I'm really sorry for that. A consult with the surgeon might be worthwhile. X--rays are certainly non-invasive and might help shed some light on the situation.But, it saddens me to say that I'm getting a bad feeling about this given her age and continued symptoms.
I'm really so sorry that you, MIchelle and Tonk have having to go through this. I truly thought her problems were transient ones and it's more than distressing to hear that they aren't.
Please keep me posted about her. I'll continue to think about her situation and if I have any epiphanies, I'll let you know. Deb