Interstitial nephritis is an uncommon cause of chronic kidney disease (CKD). A kidney biopsy is the definitive way to make a kidney diagnosis, so you have had the definitive test for kidney disease. There are many causes of interstitial nephritis, mostly related to allergic disease or chronic drug toxicity. Many chemotherapy agents used in cancer patients are known to cause interstitial nephritis. Exposure to lead, gout, and exposure to other toxic heavy metals are known to be associated with interstitial nephritis.
I know of no other testing that could be done to definitively point to a cause of interstitial nephritis. As far as treatment, I would follow the advice of your nephrologist, lead a healthy lifestyle, maintain a healthy weight and follow a low salt diet.