What tests are required for the diagnosis of kidney disease?
The following screening tests are usually recommended when kidney disease is suspected in a pet: a complete blood count (CBC), a serum biochemical profile, and a urinalysis.
What might these tests indicate if my pet has kidney disease?
These screening tests may support the presence of kidney disease or a reduction in the functional capacity of the kidneys.
Complete Blood Count (CBC)
A complete blood count (CBC) involves the evaluation of the red blood cells, the white blood cells, and the platelet components of a blood sample. A hematology (blood) analyzer will provide your veterinarian with the total numbers of these cells, and evaluation of a blood smear will allow a look at the physical characteristics of these cells.
The kidneys produce a hormone called erythropoietin that is necessary for proper red blood cell production within the bone marrow. With kidney disease, the amount of erythropoietin produced may decline, causing a decrease in red blood cell production. This shows up in the CBC as anemia. Conversely, some tumors of the kidneys can create excessive erythropoietin, causing an increase in red blood cells known as polycythemia.
White blood cell numbers may be increased if the underlying cause of the kidney disease is due to an infection or inflammation. Rarely, platelet numbers may be reduced if a clotting problem has occurred because of the underlying kidney disease.
Serum Biochemistry Profile
The serum biochemistry profile requires a separate blood sample from which the serum (the liquid portion of blood) is separated from the cellular portion of blood. Serum contains many substances, including enzymes, proteins, lipids (fats), glucose (sugar), electrolytes, and metabolic waste products.
Of most importance in the diagnosis of kidney disease is the determination of blood urea nitrogen (BUN) and creatinine. BUN is a waste product of protein catabolism (breakdown) and creatinine is produced as a consequence of normal muscle metabolism. These two substances are normally cleared from the bloodstream by the kidneys and therefore are indicators of how well the kidneys are functioning.
"Increases in BUN and creatinine may be seen with kidney disease, as well as with dehydration and other causes of reduced blood flow to the kidneys."
Increases in BUN and creatinine may be seen with kidney disease, as well as with dehydration and other causes of reduced blood flow to the kidneys. The magnitude of increase in BUN and creatinine reflects the degree of kidney disease. Unfortunately, the serum concentration of these substances does not increase significantly until approximately 75% of kidney function is already lost.
An additional blood test for kidney function called symmetrical dimethylarginine (SDMA) is also available from many laboratories. This parameter becomes elevated in kidney disease before BUN and creatinine, and so can indicate kidney disease sooner.
The biochemistry profile will also measure other substances whose blood concentrations are normally regulated by the kidneys. These substances include electrolytes such as sodium, potassium, chloride, bicarbonate, phosphorus, and calcium. It is important to document the concentrations of these substances because marked increases or decreases due to kidney disease can lead to other complications. For example, severe increases in potassium may cause cardiac arrhythmias, and increased phosphorus can further worsen kidney function.
Urinalysis
A complete urinalysis is essential for proper interpretation of the urea, creatinine, and SDMA values in a serum biochemistry profile. Urinalysis may also provide important clues to the possible underlying cause of kidney disease.
In addition to its many other functions, the kidneys are responsible for regulation of body water by either concentrating or diluting urine. Normally, the degree of urine concentration depends upon the amount of fluid consumed by your pet and how well hydrated they are. For example, drinking a large amount of fluid will result in diluted urine, whereas drinking a limited volume will result in very concentrated urine.
"Healthy kidneys will respond to dehydration or reduced blood flow by producing concentrated urine to conserve body fluids."
The degree of urine concentration is determined by measuring the specific gravity of the urine. This is done using a special instrument called a refractometer. Specific gravity is important because, with kidney disease, the kidneys lose their ability to effectively concentrate or dilute urine. The specific gravity is vital in determining the significance of any elevations in the serum BUN and creatinine.
Dehydration or any other cause of decreased blood flow to the kidneys may also result in an increase in serum BUN, creatinine, and SDMA. Healthy kidneys respond to dehydration or reduced blood flow by producing concentrated urine to conserve body fluids. If kidney disease exists, the urine will not be concentrated, even in the face of dehydration and elevated serum BUN and creatinine.
Other components of urine (such as the pH and the amount of protein or blood present in a sample) can be measured by using urinary dipsticks. Urine dipsticks are small strips that have special pads attached to them. These pads contain different chemicals that react with different urinary components, measuring them through color changes.
The presence of blood or increased protein in a urine sample will alert your veterinarian to the possibility of urinary tract infection as a cause of kidney disease. Some kidney diseases are characterized by a large loss of protein in the urine. Such clues derived from the dipstick measurements may help your veterinarian to determine what factors are contributing to your pet's kidney disease.
The final component of the complete urinalysis is the evaluation of urine sediment. This involves spinning a urine sample in a special instrument called a centrifuge to separate the cellular or solid component from the liquid component. This sediment can then be transferred to a slide and examined under a microscope. The presence of certain elements in the sediment may provide clues as to the underlying cause of kidney disease. These may include red blood cells, white blood cells, bacteria, crystalline material, and cellular casts.
- Elevated numbers of red blood cells, white blood cells, and bacteria indicate that an infection may be partially or wholly responsible for the kidney disease.
- The presence of different types of crystals may suggest that urinary tract stones, or perhaps even the ingestion of toxins (for example, ethylene glycol, or antifreeze), has resulted in kidney disease.
- The presence of casts may indicate recent damage to the kidney cells. Casts are formed when damaged or destroyed cells are molded into the shape of the damaged internal kidney structures.
Will these screening tests determine the exact, underlying cause of the kidney disease?
Occasionally, specific changes in these screening tests will point to an infectious, inflammatory, or toxic cause of kidney disease. However, testing may not tell your veterinarian the exact reasons why the kidneys are malfunctioning, because there are many underlying causes of kidney disease and because the kidneys can compensate for damage so well.
A reduction in kidney function, characterized by either a decreased urinary concentrating ability or an increase in serum BUN and creatinine, is not measurable until two-thirds to three-quarters of kidney function is already lost. By this time, many kidney diseases have progressed to a chronic phase and the original cause can no longer be determined. This is one reason why the SDMA test described earlier is important, as it can detect subtle kidney disease much sooner.
Nevertheless, it is still important to evaluate your pet using these tests. They allow your veterinarian to confirm that the cause of your pet's clinical symptoms is kidney disease, and they also allow them to choose the most appropriate course of treatment and may indicate if further testing is required.
What other testing might be required?
Depending on your pet's clinical signs and the results of the initial screening tests, further testing for kidney disease may include any of the following:
- A urinary protein/creatinine ratio is recommended if the urine dipstick evaluation suggests that an excessive amount of protein is being lost in the urine. A urine sample is sent to a veterinary referral laboratory, and the ratio of protein to creatinine in that sample is calculated. An increased protein/creatinine ratio can indicate kidney damage.
- Urine culture may be performed if the urinalysis findings are suggestive of a bacterial infection as the cause of the kidney disease.
- Blood pressure assessment is often recommended, as many pets with kidney disease have high blood pressure due to hormonal changes associated with kidney disease.
- Testing for infectious diseases, including Leptospirosis and Lyme disease, may be recommended if either of these organisms are suspected based on geographical location.
- Imaging can be a valuable tool to further evaluate the kidneys. X-rays may be done to assess the size and shape of the kidneys, or to look for stones within the urinary tract. An ultrasound examination and/or kidney biopsy may also be recommended, especially if there is concern about cancer.
Determining the underlying cause and the stage (severity) of kidney disease is important. It allows your veterinarian to establish treatment plans and prognosis for your pet.
