KIDNEY FAILURE AND RENAL INSUFFICIENCY:
WHAT IS CHRONIC RENAL FAILURE?
Whether the pet is brought in sick or whether the condition is discovered incidentally on routine lab work, the diagnosis of kidney failure can come as a shock. It does not help that the term "kidney failure" sounds dramatic and evokes images of on-going sickness, expensive hospitalization, and doom. In fact, the term "kidney failure" simply means that the kidneys are not able to do at least some of the tasks they are supposed to do as well as they are supposed to do them. Many clinicians prefer the term "renal insufficiency" or "insufficient kidneys" so as not to conjure up disturbing images when the patient is in a treatable state. Renal insufficiency is one of those conditions where early intervention can make a big difference and normal life quality can be maintained for months or even years. From a practical standpoint, the terms "chronic renal failure," "renal insufficiency," "chronic kidney failure," and "kidney insufficiency" all mean the same thing.
WHAT OUR KIDNEYS DO/WHAT INSUFFICIENT KIDNEYS CANNOT DO:
Before reviewing what failing kidneys can no longer do, it is important to know what normal kidneys do. Kidney function goes far beyond simply making urine. Kidneys are made of millions of processing units called "nephrons." These tiny little processing units are responsible for separating the chemicals you want to keep in your body from the chemicals you need to dispose of. The chemicals to remove are dissolved in water and make up the fluid we all know as urine. So here is what your kidneys do for you (and for your pet), what they become unable to do in renal insufficiency, and some of the parameters your veterinarian will want to track:
When we analyze a urine sample, one of the most important parameters is the “specific gravity.” This is a measure of how concentrated a urine sample is. Water has a specific gravity of 1.000. A dilute urine sample has a specific gravity less that 1.020 (often less than 1.010). A concentrated urine sample would have a specific gravity over 1.030 or 1.040. A failing kidney by definition cannot make a concentrated urine and the patient must drink excessively to get enough water to excrete the day’s toxic load.
The most important marker of uremia is called "creatinine." Creatinine is a by-product of muscle break-down and is always present in small amounts in the bloodstream. The kidney removes it continuously unless there is a kidney function problem. A newer parameter called "SDMA," ("symmetrical dimethylarginine") becomes abnormal much earlier than creatinine and is becoming more commonly tracked. We are able to stage a patient's kidney disease based on creatinine blood level and SDMA level (see the staging section below). Another marker is the "BUN" which stands for "blood urea nitrogen." This parameter is similar to creatinine but is influenced by dietary protein as well as kidney function. These three markers are central to determining the severity of a kidney problem.
Blood Pressure Regulation
Red Blood Cell Production
We are born with two kidneys and a huge excess of nephrons. As we live our lives, nephrons die off either as a consequence of disease or simply through wear and tear. When we are down to less than 1/3 of one kidney's worth of nephrons (in other words we have lost 5/6 of our total nephrons), test results become notably abnormal and it becomes important to make lifestyle changes. How big these changes are, depends on how far things have gone out of whack when the problem is discovered. Early detection of poor kidney function is one of the main reasons laboratory screening tests are recommended.
URINE SPECIFIC GRAVITY
One of the kidney’s most important jobs is the conservation of the body’s water. The kidney must excrete the toxic by-products created by the body’s metabolism but it will want to do so in the least amount of water possible. The healthy kidney is able to make a very concentrated urine. When we analyze a urine sample, one of the most important parameters is the “specific gravity.” This is a measure of how concentrated a urine sample is. Water has a specific gravity of 1.000. A dilute urine sample has a specific gravity less that 1.020 (often less than 1.010). A concentrated urine sample would have a specific gravity over 1.030 or 1.040. A failing kidney by definition cannot make a concentrated urine and the patient must drink excessively to get enough water to excrete the day’s toxic load.
BLOOD UREA NITROGEN (BUN)
This is a protein metabolite excreted by the kidney (it is one of the toxins we are concerned about, though it may be more of a marker for other toxins that are less easily measured). In a normal animal, the BUN is 25 mg/dl or so. Often at the time of diagnosis, BUN is well over 150, 200, or even 300. We’d like to keep the BUN no more than 60-80 mg/dl. BUN is influenced by dietary protein (including the patient’s own blood that has bled into the intestine), something which becomes important in certain situations.
This is another protein metabolite (though this one is less dependent on dietary protein intake than is BUN). A normal creatinine is less than 1.4 mg/dl, certainly less than 2.0. Patients begin to feel sick when values meet or exceed 5.0 so we try to keep the value at 4.5 or less. BUN and creatinine will be tracked (as will several other parameters) over time and in response to different treatments.
The calcium/phosphorus balance becomes deranged in kidney failure due to hormone changes as well as the inability of the failing kidney to excrete phosphorus. If calcium and phosphorus levels become too high, the soft tissues of the animal's body will develop mineralized deposits which are inflammatory, uncomfortable, and often cause intestinal bleeding. The bones will weaken as well, in some cases actually becoming rubbery. Keeping phosphorus levels in the low normal range has been correlated with improved survival.
The failing kidney is unable to conserve potassium efficiently and supplementation may be needed. Signs of "hypokalemia" (the scientific name for low blood potassium) include weakness, especially drooping of the head and neck.
PACKED CELL VOLUME / HEMATOCRIT
This is a measure of red blood cell amount. More literally it represents the percentage of the blood made up by red blood cells. The hormone which stimulates the production of red blood cells is made by the kidney. The failing kidney does not make this hormone in normal amounts leading to a reduction in red blood cells, in turn leading to weakness, poor appetite, and overall poor life quality.
Blood pressure is not something measured off a laboratory result sheet but it is important to monitor this parameter in kidney patients as there is a tendency for hypertension to develop in kidney failure. Special medications may be needed to manage this problem should it arise.
One of the functions of the kidney is to prevent loss of the body’s proteins, in particular the blood proteins. The kidney’s filtering mechanism that enables it to remove toxins is designed to leave larger molecules (such as proteins) inside the body where they belong. But if holes develop in the filter, protein can be lost. If this complication cannot be controlled, survival time is dramatically shortened.
If you are a “hands-on” kind of pet owner, it is a good idea to request copies of monitoring lab work so you can make a chart of these parameters. In this way you can see how the disease is progressing or improving and what the problem areas are.
The International Renal Interest Society (IRIS) has posed the following staging criteria for pets with Stage I being pets with evidence of kidney disease but no evidence of a change in actual kidney function all the way up to Stage IV where pets actually begin to feel sick from their disease. These stages are based on blood creatinine levels measured in mg/dl. It is worth noting that MANY patients are well past IRIS Stage IV at the time of diagnosis with creatinine of 8, 9, 12 mg/dl or even higher. IRIS does not have criteria beyond the relatively low creatinine value of 5.0 so it is best to think of the IRIS system as a way to intervene in the earlier stages of renal insufficiency. For patients in more dire initial states, the goal is to drive their toxin levels down to where the IRIS system becomes relevant.
Values above reflect creatinine blood levels measured in mg/dl and SDMA levels measured in mcg/dl.
Sub-staging of kidney disease involves screening for urine protein loss and measuring blood pressure. A urine protein:creatinine ratio is performed on a urine sample and the protein amount is classified as either "non-proteinuric," "borderline proteinuric," or "proteinuric." Blood pressure is checked and the patient is classified as "normotensive" (normal), "borderline hypertensive," "hypertensive," or "severely hypertensive." The ultimate classification for the patient will reflect all of these things ("Stage 3, non-proteinuria, normotensive" would be an example). Further testing and monitoring is determined based on what the actual parameters are. For more details on these substages, see the articles on glomerular disease, and hypertension.
A SPECIAL NOTE ON PYELONEPHRITIS
"Pyelonephritis" is another name for kidney inflammation and usually means kidney infection. Patients get kidney infections when a bladder infection goes unnoticed or incompletely treated long enough and the bacteria ascend up the ureters into the kidney where they set up shop and cause damage and pain. This form of kidney failure actually has the most potential for reversal or partial reversal so it is important to culture the urine early for its presence. Urinalysis is frequently unable to detect infection in this situation as the patient drinks so much water that the visible markers of infection are diluted out and cannot be found. The infection also will produce a significant amount of urinary protein so the patient will be sub-staged incorrectly if infection is not ruled out.
The bottom line is that there is a wide range of what is involved when a patient has kidney failure or insufficient kidneys. Each patient will have a different stage and a different constellation of issues to contend with. Some will need only a diet change and some will need on-going fluid therapy at home or hospitalization. Prognosis depends on all the factors involved. Your veterinarian will let you know what your particular pet needs and will be watching many different lab tests to keep control of your pet's condition.
Other Pages in the Chronic Renal Failure Center:
Page posted: 7/25/2007