Kidney function tests, also known as renal function tests, are important for monitoring the progression of the disease and your response to therapy. They are also vital to staging the disease and can help differentiate CKD from an acute kidney injury (AKI).
Labs and Tests
Chronic kidney disease develops when the kidneys, for whatever reason, are unable to filter waste and regulate the balance of water and acids in the body. The disease can be diagnosed by measuring certain substances, known as markers, that either rise or fall when the kidneys are impaired.
The screening process begins with a battery of blood and urine tests that evaluate standard diagnostic markers such as serum creatinine (SCr), glomerular filtration rate (GFR), urinary albumin, and blood urea nitrogen (BUN).
Serum Creatinine
Serum creatinine (SCr) tests measure the amount of a substance called creatinine in your blood. Creatinine is a by-product of muscle metabolism that is excreted in urine.
The normal ranges of SCr are:
0. 5 to 1. 0 milligrams (mg) per deciliter (dL) in women0. 7 to 1. 2 mg/dL in men
Glomerular Filtration Rate
Glomerular filtration rate (GFR) is an estimation of how well your kidneys are functioning. Specifically, it estimates how much blood passes through the tiny filters of your kidneys, called glomeruli, each minute.
To calculate your GFR, the lab will use an algorithm that factors in your SCr value, age, ethnicity, gender, height, and weight.
The GFR reference range is as follows:
90 to 120 milliliters (mL) per minute: Normal Below 60 mL/min: CKD Below 15 ml/min: Kidney failure, also known as end-stage renal disease (ESRD)
GFR values can vary slightly from one lab to the next, as some may use a different formula to calculate the GFR. To this end, it helps to have your tests performed at the same lab to ensure consistency.
Creatinine Clearance
Another means of estimating GFR is a test called creatinine clearance (CrCl), which compares your serum creatinine with the amount of creatinine excreted in urine over 24 hours. The algorithm would also factor in your age, ethnicity, height, and weight.
The normal range of CrCl values are:
88 to 128 mL/min for women97 to 137 mL/min for men
Urinary Albumin
Glomeruli are components of a filtration unit called a nephron. The role of nephrons is to filter out larger cells like red blood cells and proteins so that they are not excreted. When the nephrons are damaged, protein and blood can escape and exit the body in urine.
One such protein, called albumin, can be used by the lab to detect proteinuria, the excessive accumulation of protein characteristic of kidney disease. To diagnose proteinuria, the healthcare provider may either perform a 24-hour urine test or, alternatively, calculate the ratio of urinary albumin to serum creatinine.
The normal ranges of urinary albumin are:
0 to 8 mg/dL for the 24-hour urine test0 to 30 for a urinary albumin/creatinine ratio
Blood Urea Nitrogen
Blood urea nitrogen (BUN) measures the amount of a waste product in your blood, called urea nitrogen. Urea nitrogen is created when the liver breaks down protein and, like serum creatinine, is produced and excreted at a fairly consistent rate.
The normal range of BUN values are:
6 to 21 mg/dL for women8 to 24 mg/dL for men
High BUN value can also suggest the underlying cause of a kidney failure.
Urine Volume
Urine volume is the amount of fluid you urinate over a given period of time. It is primarily used to diagnose AKI and is measured in milliliters (mL) per kilograms of your body weight (kg) per hour.
Oliguria, the production of abnormally small volumes of urine, is characteristic of an acute kidney injury and is defined as anything less than 0.5 mL/kg/h. Oliguria is less common with CKD.
Imaging
In addition to blood and urine tests, imaging tests can be used to identify and evaluate any damage that the kidneys may have sustained. Among them:
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Ultrasound is the preferred method of imaging. It can be used to measure the size and appearance of the kidney and help locate tumors, lesions, and blockages. A newer technique called a Color Doppler can be used to identify clots, strictures (narrowing), or ruptures in the blood vessels of the kidneys. X-rays are mainly used to assess the size of kidney stones or to measure the size and shape of the kidney. Computed tomography (CT) is a type of X-ray that can produce cross-sectional images of the kidneys. CT scans can be useful in detecting cancer, lesions, abscesses, obstructions, kidney stones, and the accumulation of fluid around the kidneys. They can be used for obese people for whom an ultrasound may not provide a clear picture. Magnetic resonance imaging (MRI) uses magnetic waves to produce high-contrast images without radiation. While it can be just as useful as a CT scan, an MRI often requires a gadolinium contrast dye, which can cause a potentially fatal skin disease called nephrogenic systemic fibrosis (NSF) in people with poor kidney function.
Kidney Biopsy
A kidney biopsy involves taking a tissue sample of your kidney to examine under the microscope. The biopsy may either be percutaneous (in which a needle is inserted into the kidney through your back) or open (where tissue is obtained through a one- to two-inch laparoscopic surgical incision).
A kidney biopsy may be indicated in certain situations, including:
Kidney disease with no clear cause Hematuria (blood in urine) Severe proteinuria accompanied by other signs of renal impairment (nephrotic syndrome) Kidney trauma Renal tumors Kidney failure
Serious complications of a kidney biopsy are uncommon. In rare cases, bleeding can cause the formation of a renal clot and the inability to urinate. Infection is also possible but less likely if wound care is adhered to.
Staging
The purpose of kidney disease staging is two-fold: to establish how functional the kidney is, and to determine the most appropriate course of treatment. For CKD, the staging is based solely on GFR results, which correspond to appropriate treatment plans.
There are a number of diagnostic clues that can help distinguish between the two conditions.
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