Determining whether dogs with PU/PD but no azotemia have renal failure versus other causes of PU/PD (e.g. Cushing's disease, diabetes insipidus, primary polydipsia) can be challenging. Evaluation of the patient's clinical history and other laboratory data can provide important clues. Other diagnostic aids that can be helpful include:

• Check several urine specific gravity (USG) measurements to determine whether the dog is always isosthenuric.
• Evaluate changes in creatinine concentration, even if results are within the normal reference range.
• Look for size or architectural changes in the kidneys by radiography or ultrasonography.

Evaluation of GFR

• Iohexol clearance test
• Inulin clearance is an acceptable test for determining GFR, but disadvantages are that inulin is not readily available and the assay is not offered routinely.
• Technetium scans. Not considered to be reliable for determining GFR

Other options

• DDAVP (desmopressin) treatment trial to see if USG increases significantly
• Microalbuminuria (ERD® test kit, Heska Corp.) test. No data are available, but dogs with renal failure should test +ve. However, dogs with other causes of PU/PD ( e.g., Cushing's disease, chronic pyelonephritis, pyometra) could also test +ve. A negative test would make renal failure an unlikely cause of PU/PD.

• Phosphorus-restricted diets. These diets will also be protein restricted.
• Fish oil supplements also are appropriate, and are included in renal failure diets.
• Anti-oxidants are included in renal failure diets.
• As all renal failure diets are similar, any commercial or home-made equivalent product would be appropriate.
• Phosphate binders are used, whenever phosphorus restricted diets are ineffective in controlling serum phosphorus concentrations. Can use either Al- or Ca-based phosphate binders, although the Ca-containing salts (Ca acetate or Ca carbonate) tend to be more effective. This means using less drug, with less problem with palatability. Some cats may become hypercalcemic with use of Ca-containing phosphate binders. Mix it with food and warm up to increase palatability.
• A UK study showed that use of phosphorus restricted diets and phosphate binders was able to normalize PTH concentration in cats with CRF. It took ≡ 6 months for PTH to normalize.
• Protein restriction. Seems to be unimportant until BUN increases are pronounced.
• Clinical evaluation of dietary modification for treatment of spontaneous CRF in dogs (JAVMA 2002; 220:1163-1170). This study shows the importance of diet in renal failure management, but what makes the difference is unclear. The study diets were different with respect to protein, phosphorus, sodium and polyunsaturated fatty acid content.

[Excepted from conference with Dr. Scott Brown]