This inflammatory condition is very tricky to diagnose in
cats however a special test from Idexx (the
name of the
laboratory, they service all of north america) is available and it may shed some light on the subject.
Lastly, I am a little worried about cancer in this case. Your vet has done an amazing job assessing organ function and performing diagnostics like the ultrasound you mentioned, but with a cat of this many years and with normal kidney and liver enzymes, cancer is a real possibility.