By Cat Troiano
According to the National Kidney Foundation, one-third of the American population is at risk of developing kidney disease, and the degenerative condition is the ninth leading cause of death in the United States. Over time, kidney disease leads to end-stage renal failure. Performing renal function screenings is crucial for early diagnosis and monitoring so that the disease can be slowed in its progression.
Kidney Form and Function
The two bean-shaped kidneys are located beneath the ribcage toward the back of the abdominal cavity, with one kidney situated on each side of the vertebral column. The kidneys are responsible for several life-preserving functions. Each day, 180 liters of blood pass through millions of filters, called nephrons, within the kidneys to remove metabolic wastes. Glomeruli are additional filters within the nephrons. Glomeruli pass the filtered toxins and excess water along to be eliminated from the body through urination, and they direct the filtered blood to continue circulating through the body. When renal function is compromised, the kidneys are unable to carry out the filtration process as efficiently, and toxins build up in the bloodstream.
The kidneys also play roles in other critical functions:
• Kidneys produce renin, an enzyme that aids in blood pressure regulation.
• Kidneys produce erythropoietin, a hormone that aids in red blood cell production.
• Kidneys maintain the body’s water, acid and electrolyte balances.
• Kidneys produce an activated form of vitamin D that aids in maintaining bone strength.
Kidney disease hinders all of these functions, and that causes a number of complications.
Causes of Kidney Disease
Chronic kidney disease occurs when damage to the kidneys and the deterioration of their function is gradually sustained. Chronic kidney disease is diagnosed when kidney function is decreased for more than three months. Acute kidney failure is a sudden onset of kidney damage that poses an immediate threat of kidney failure.
Some examples of chronic kidney disease include:
• Polycystic kidney disease, which is a genetic condition in which fluid-filled cysts form and overtake the kidneys
• Diabetic kidney disease, which is caused by uncontrolled type 1 or type 2 diabetes as high blood glucose levels inflict damage to the kidneys
• Hypertensive nephrosclerosis, which is caused by uncontrolled hypertension as the high blood pressure inflicts damage to the kidneys’ blood vessels
• Analgesic nephropathy, which results from extensive use of nonsteroidal anti-inflammatory drugs, such as ibuprofen, naproxen and celecoxib
Diabetics and patients with hypertension are at high risk for developing chronic kidney disease. Other conditions can cause chronic kidney disease as well, including autoimmune disorders, HIV/AIDS, hepatitis B and hepatitis C.
Some conditions that can result in acute kidney failure include:
• Prostate enlargement, which can cause a urinary obstruction and allow a backflow of urine to the kidneys
• Kidney stones
• Pyelonephritis, which is the development of scar tissue in the kidneys caused by repeated bouts of urinary tract infections
• Glomerulonephritis, which is inflammation of the glomeruli
Regularly monitoring blood pressure and checking blood glucose levels on all patients are important first steps toward the preservation of renal health. Ordering renal function laboratory tests enables physicians to assess and monitor how efficiently their patients’ kidneys are working. Renal function tests are performed on blood serum and urine samples.
Blood Serum Kidney Values
A comprehensive metabolic panel reveals two specific values that are key indicators of renal performance.
Blood urea nitrogen (BUN)
• BUN is the waste that is produced when the liver breaks down dietary protein.
• The normal reference range for BUN is between 7 and 20.
• A high BUN result is indicative of a decrease in blood flow through the kidneys. As kidney disease advances, the BUN value escalates.
• Creatinine is the metabolic waste from muscle activity.
• The general normal reference range for creatinine is 0.84 mg/dL to 1.21 mg/dL, with some variance when age and gender are considered.
• A high creatinine result is indicative of kidney disease. Like the BUN value, the creatinine value increases as kidney disease worsens.
If the BUN or creatinine levels are higher than the normal ranges, other comprehensive metabolic panel values can provide additional clues for assessing renal function, including:
The normal reference range for phosphorus in adults is 2.5 to 4.5 mg/dL, with a slight variation based on a patient’s age.
A high phosphorus level, or hyperphosphatemia, can be the result of kidney disease.
The normal reference range for total calcium is 8.6 to 10.5 mg/dL, with slight variations based on a patient’s gender and age.
A low calcium level, or hypocalcemia, can be indicative of kidney disease.
Since the kidneys help to maintain normal electrolyte balances, imbalances can reveal inadequate renal function.
Urine Kidney Values
A urinalysis also provides valuable insight into renal function. Some particular kidney values that are evaluated in a urinalysis include:
Urine specific gravity
The normal reference range for urine specific gravity is between 1.005 and 1.025.
If the urine specific gravity result is lower than the normal range, kidney disease is suspected.
The normal reference range for urinary pH is 4.5 to 8.
If the urinary pH exceeds the normal range, this may indicate kidney disease.
Creatinine Clearance Test
If any of the aforementioned kidney function values do not fall within normal parameters, an additional urine test that evaluates kidney filtration may be ordered. The creatinine clearance test
is run on a sample of all urine that has been collected over a 24-hour period. When the level of creatinine in this sample is compared to that of the blood serum sample, a calculation can then determine how much blood per minute the kidneys are able to filter creatinine.
The normal creatinine clearance for women is approximately 95 milliliters per minute, and the normal creatinine clearance for men is approximately 120 milliliters per minute. A lower creatinine clearance result may be indicative of kidney disease.
Glomerular Filtration Rate and Disease Staging
The glomerular filtration rate (GFR) is calculated by a physician from the results of the patient’s serum creatinine value and other factors, including age, gender and weight. The normal reference range for GFR in healthy adults is greater than 90. The GFR indicates how much renal functioning capacity a patient has and enables physicians to determine the stage of a patient’s chronic kidney disease. The stages for GFR are designated as follows:
Stage 1 – there is evidence of kidney damage, but the GFR is a normal value of 90 or greater.
Stage 2 – there is evidence of kidney damage and the GFR has dropped to between 60 and 89.
Stage 3 – the GFR is between 30 and 59.
Stage 4 – the GFR is between 15 and 29.
Stage 5 – the GFR is less than 15.
Kidney disease is diagnosed when the GFR is at least 60. A GFR of 15 prompts immediate treatment for kidney failure. At this stage, the options of dialysis, a kidney transplant and palliative care must be considered.
If any renal function tests yield abnormal results, additional tests may be ordered. Such tests may include diagnostic imaging, such as an ultrasound or CT scan of the kidneys, or a renal biopsy.
Who Should Be Tested?
Comprehensive metabolic screenings should be ordered for all patients as part of their routine physicals. Abbreviated tests that screen specifically for renal function may be ordered more frequently for patients who carry higher risks for kidney disease. High risk factors include:
• Cardiovascular disease
• Family history of kidney failure
Hispanic Americans, African Americans, Pacific Islanders, Asians and American Indians all have an increased risk for kidney disease due to the higher prevalence of diabetes and hypertension in these groups.
Symptoms of chronic kidney disease do not emerge until the illness has advanced to its later stages. Renal function tests should be ordered for any patient who presents with the following complaints:
• Nausea or vomiting
• Decreased appetite
• Urinating more frequently than normal, including at night
• Swelling around the ankles
• Puffiness around the eyes
• Unintentional weight loss
• Shortness of breath
• Foamy appearance to urine
• Muscle cramping
• Dry or itchy skin
In patients who have been diagnosed with chronic kidney disease, periodic orders for renal function screenings will serve to monitor the advancement of the disease so that adjustments in treatment protocols can be made.
Kidney damage and diminishing function cannot be reversed. The goal of treatment for chronic kidney disease is to slow down the rate of degeneration and maintain an adequate level of kidney function to stave off kidney failure as long as possible. Treatment includes a therapeutic diet, management of diabetes or hypertension, medications to reduce complications and dialysis. The best chance for a longer survival time is to make renal function testing part of preventative patient care.
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