Protein in the urine can be caused
by a variety of different conditions; if you are looking for non or minimal invasive procedures then
ultrasound isn't invasive and can tell your Veterinarian a lot about the internal structure of the kidney's and any abnormalities, also a regular blood test (just taken with a needle from the fore leg) with biochemistry will give a good indication about protein levels in the blood more importantly the albumin to globulin
ratio as well as creatinine and urea which will help to determine Sebastian's internal health.