Kidneys Part 2: Chronic Kidney Disease Basics
July 25, 2019
After our crash course in how kidneys are supposed to work, let’s dive into one common kidney disease: Chronic Kidney Disease (ckd).
Chronic kidney disease is an irreversible, progressive loss of normal kidney function. Once diagnosed, it often stays stable for a period of time ranging from months to years. Occasionally, we can identify a specific cause for ckd such as a toxin, trauma, or infection. More often, we do not know a specific cause; in these cases ckd is likely the result of a lifetime of “wear and tear” or accumulated small insults. Whatever the cause, nephrons are lost and cannot be replaced.
The remaining nephrons respond by increasing their filtration rate, which can compensate for their decreased numbers for a period of time. This increased workload, however, triggers events that can damage additional nephrons. This cycle continues until there are too few nephrons left to compensate and the disease progresses to end-stage kidney failure.
Because the kidneys are so important to maintaining balance in the body, symptoms are often nonspecific and may include changes in appetite, lethargy, decreased grooming, and weight loss or loss of muscle mass. A more specific sign is increased thirst and urination, although these also overlap with several other diseases. Therefore, laboratory tests are needed to diagnose ckd.
We use a combination of blood and urine tests to diagnose ckd. The three main kidney values on blood work are called SDMA (symmetric dimethylarginine), creatinine, and BUN (blood urea nitrogen). All three are wastes produced during the metabolism of proteins and are normally excreted in the urine. These are an indirect measure of filtration rate and if they are elevated (which we call azotemia), it can mean the kidneys are not excreting wastes normally. (Directly measuring the filtration rate is a costly and cumbersome test in our pets.)
However, there is a complication: Some non-kidney factors can cause azotemia. Two common causes are dehydration or a recent meal. To help us decide whether azotemia is due to ckd or not, we also look at a value called urine specific gravity or Usg. This is a measurement of how concentrated urine is. If a pet is azotemic due to dehydration, their urine should be very concentrated (as their kidneys work to conserve water). Azotemia plus dilute urine lead us to a diagnosis of ckd.
There are several complications of ckd that may also be apparent on laboratory results: Abnormal levels of electrolytes (especially potassium) or phosphorus, high blood pressure, or anemia can occur with ckd.
Once diagnosed, we assign a stage of ckd of 1-4 based on recommendations of IRIS, the International Renal Insufficiency Society. Identifying the stage helps guide our treatment options.
Next time we will delve into the specifics of treatment, but one thing to understand is that in cases of ckd we cannot “fix” the kidneys. They will never be normal again and this disease will progress. However, we have many tools to manage pets to keep them feeling well with a good quality of life. One way to summarize treatment goals is to use the acronym NEPHRONS to identify important considerations: Nutrition, Electrolytes, Proteinuria (protein in the urine), Hydration, Retention of wastes (azotemia), Other renal insults to be avoided, Neuroendocrine changes (such as anemia resulting from the kidneys not telling the bone marrow to make more red blood cells), Serial monitoring (of laboratory work).