Chronic kidney disease (CKD) is a common problem that affects an estimated 0.5 to 7% of dogs and 1.6 to 20% of cats. Nephron
damage associated with CKD is usually irreversible and can be progressive. Renal failure results when three-quarters or more
of the nephrons of both kidneys are not functioning. Whether the underlying CKD primarily affects glomeruli, tubules, interstitial
tissue, or renal vasculature, irreversible damage to any portion of the nephron renders the entire nephron nonfunctional.
Healing of irreversibly damaged nephrons occurs by replacement fibrosis and therefore a specific etiology is often not determined.
Chronic kidney disease occurs over a period of weeks, months, or years and is an important cause of death in dogs and cats.
It is often not possible to improve renal function in CKD and therefore treatment is aimed at stabilizing renal function.
Importantly, there is increasing evidence that dietary and anti-hypertensive/anti-proteinuric treatments (angiotensin-converting
enzyme inhibitors [ACEI]) can decrease the progressive nature of CKD.
Staging of Canine and Feline CKD
The following table was developed by International Renal Interest Society (IRIS) as guide to staging of canine and feline
CKD. Importantly, this staging system should only be used in patients with stable CKD.
Serum creatinine concentrations must always be interpreted in light of the patient's urine specific gravity and examination
findings in order to rule out pre- and post-renal causes of azotemia. The above stages are further classified by the presence
or absence of proteinuria and systemic hypertension as follows:
Clinical Signs and Diagnosis
Clinical signs of CKD may not be present in early stages and when present in later stages, are usually nonspecific (lethargy,
depression, anorexia, gastroenteritis, and dehydration). Occasionally uremic breath and/or oral ulcers may be observed. Unique
signs of CKD (vs. acute renal disease) include a history of weight loss and polydipsia-polyuria, poor body condition, nonregenerative
anemia, small and irregular kidneys, and renal secondary hyperparathyroidism. The classic diagnosis of renal failure based
on renal azotemia (persistent azotemia superimposed on the inability to concentrate urine) pertains to CKD stages II-IV. Stage
I CKD (non-azotemic CKD) could be diagnosed in cats and dogs with persistent proteinuria or renal origin, urine concentrating
deficits due to renal disease, increases in serum creatinine over time, even if the values remain in the normal range (e.g.,
serum creatinine that increases form 0.6 to 1.2 mg/dl could indicate a 50% reduction in GFR), or abnormal renal palpation
or renal ultrasound findings.
In general, the diagnostic approach to patient once CKD has been identified and staged is focused on three areas: 1) characterization
of the renal disease, 2) characterization of the stability of the renal disease and function, and 3) characterization of the
patient's problems associated with the decreased renal function. Further definition of the renal disease (beyond a standard
minimum data base) could include for example, quantitation of proteinuria, measurement of blood pressure, urine culture, kidney
imaging, and possibly kidney biopsy. The stability of the renal function would be assessed by serial monitoring of abnormalities
identified during the initial characterization of the renal disease. This monitoring should always include serum biochemistry
profiles, urinalyses, quantitation of proteinuria, and measurement of blood pressure but may also include follow-up urine
cultures and ultrasound examinations. Characterization of the renal disease and its stability is most important in the earlier
stages of CKD when appropriate treatment has the greatest potential to improve or stabilize renal function. Characterization
of patient problems becomes more important in the later stages of CKD when clinical signs tend to be more severe. In the later
stages of CKD, diagnostic (and subsequent therapeutic) efforts should directed at assessing anorexia, vomiting, acidosis,
potassium depletion, hypertension, anemia, etc.