Renal failure results from damage to the nephrons, causing them to not function properly. Renal failure can be divided into acute (a rapid onset) or chronic (slowly progressive over time). Acute renal failure is often the result of major damage to the kidneys, such as infection, decreased blood flow to the kidneys, drugs that can be harmful to the kidneys, toxins such as antifreeze, or blockage of urine outflow from the kidneys, such as stones lodged in the ureters or urethra. The pet usually becomes sick shortly after the kidney has been damaged. Depending on the cause and if acute renal failure is discovered early enough, it can sometimes be reversed and the kidneys can recover.
At the other extreme, irreversible damage can occur where the kidney is unable to be repaired and the disease can be fatal. Chronic renal failure is often more gradual in onset and by the time the pet is showing clinical signs it is difficult to pinpoint what initially caused the insult to the kidneys. As animals age, the number of functional nephrons begins to decrease. This is especially true in older cats and the decrease in functional nephrons often results in chronic renal failure. There is no way to cure chronic renal failure and it will continue to progress as more nephrons become non-functional. In many cases, pets with chronic renal failure can be treated with medications, fluid therapy and dietary changes to help them feel better and slow the progression of the disease for months or even years.
Blood work and urinalysis are helpful tests for looking at kidney function. With blood work, substances that are eliminated from the blood by the kidney can be evaluated. The major substances evaluated are BUN (blood urea nitrogen) and creatinine. These become elevated with renal failure because the kidneys aren’t working well enough to clear them from the blood. There are many other toxins that build up in the body with renal failure. Most of these lead to the clinical signs that we see, but they are difficult to evaluate with tests. Unfortunately, two thirds of the nephrons must be lost before the BUN and creatinine become elevated. Urinalysis can evaluate the ability of the kidneys to concentrate the urine as well as look for any evidence of infection, crystals or other substances such as protein that shouldn’t be in the urine under normal circumstances. Your veterinarian may also suggest other tests such as radiographs or ultrasound to get an idea of the size, shape and architecture of the kidneys.