Tests Used to Diagnose Kidney Disease
Several types of tests are used to diagnose and evaluate different types of kidney disease:
- Kidney function is determined using blood and urine tests.
- Imaging tests are a relatively painless and low-risk way of providing useful information about kidney structure.
- Kidney biopsy is a procedure for sampling a small portion of kidney tissue.
Determining Kidney Function with Blood and Urine Tests
The simplest way to monitor kidney function is to obtain blood tests for BUN (blood urea nitrogen) and creatinine. These two substances are normal metabolic waste products that are excreted by the kidneys. Urea is a byproduct of protein breakdown, and creatinine is a byproduct of normal muscle functioning. In kidney disease, these substances (as well as numerous others) are not excreted normally, so they accumulate in the body. This causes an increase in blood levels of urea and creatinine, which can be easily detected on blood tests.
One of the primary functions of the kidneys is to filter blood and remove waste products. Urine tests measure how well your kidney filters (see figure) are working. When functioning normally, glomerular filters are able to filter out wastes, but keep protein and red blood cells in the bloodstream. In many types of kidney disease, the tiny glomerular filters are damaged and become abnormally leaky. This allows proteins and red blood cells (that are normally kept in the bloodstream) to spill into the urine, where they can then be detected in several ways.
Urine protein determination – This test provides the actual number of milligrams or grams of protein in a sample. The test result is reported as a ratio of protein to creatinine. A urine protein/creatinine ratio greater than 100 mg protein per gram of creatinine is abnormal. A ratio greater than 3000 usually indicates serious damage to the glomerular filters of the kidneys. To learn more see the NetWellness original article Basic Information about the Kidneys.
Urine microalbumin determination – If only a very tiny amount of protein is present in the urine, it may not be detectable by standard tests (such as urine protein determination). In these cases, a “spot urine microalbumin/creatinine ratio” may be ordered. A ratio of greater than 30 micrograms of albumin per gram of creatinine indicates that a tiny amount of protein (albumin) is present. This can be the very earliest sign of kidney disease, particularly in diabetics.
Urine dipstick – A small flat plastic stick containing a row of several chemically-treated paper squares is dipped into a urine sample (see photo). The squares then turn different colors. By comparing the stick with a color chart, one can tell whether or not the urine contains various substances, such as protein, blood, glucose (or sugar, suggesting diabetes), and white blood cells (suggesting infection).
Urine sediment examination – Urine is poured into a test tube and spun in a centrifuge, and the sediment that goes to the bottom can then be examined under a microscope. The findings in the urine sediment can be very helpful in diagnosing kidney disease, and have even been referred to as the “poor man’s kidney biopsy.” Useful findings in the urine sediment may include: white blood cells (suggesting infection), red blood cells, bacteria, yeast, crystals (especially useful in people with kidney stones), and renal tubular cells (indicating damage to various parts of the kidney).
Measuring Kidney Function
The filtering function of the kidneys may be measured or estimated in several ways:
BUN and serum creatinine – As described above, a decrease in kidney function is reflected by an increase in the BUN and serum creatinine level. While there is a “normal range” for serum creatinine (approximately 0.7-1.2 mg/dL; 62-106 mM/L), it is important to note that an individual person’s “normal” serum creatinine depends on their personal muscle mass and their level of activity. Thus the normal serum creatinine for a young, very muscular man could be as high as 1.5 mg/dL (132 mM/L), while for an elderly, very small woman it might be as low as 0.5 mg/dL (44 mM/L). A more accurate measure of kidney function is the GFR, or glomerular filtration rate.
GFR (glomerular filtration rate) – GFR measures the rate at which the kidneys’ two million glomeruli filter plasma in order to process it and remove waste products from it. If the kidneys are injured by chronic kidney disease (CKD), the GFR gradually declines, and the amount of remaining kidney function can be estimated by measuring or calculating the GFR. The normal value for GFR in a normal-sized person is 100-150 ml/min. Currently the two most common methods for determining GFR are creatinine clearance and MDRD equation.
Creatinine clearance – Creatinine clearance requires collection of a 24-hour urine; the patient must save all the urine that they produce for a 24-hour period. A blood sample is drawn at some point during the 24-hour period, and creatinine clearance, which is an estimate of GFR, can then be calculated. A simpler method for estimating creatinine clearance is based upon a formula that includes a person’s age, gender, weight, and serum creatinine level, but does not require the collection of a 24-hour urine. This formula (the Cockcroft-Gault formula) can be found at several websites.
eGFR (estimated GFR) determined by the MDRD equation – This equation, developed as part of a study called the Modification of Diet in Renal Disease (MDRD) Study, can estimate GFR with surprising accuracy, based on a person’s age, gender, ethnicity, and blood serum creatinine level. The GFR (known as “eGFR” when determined by this equation) is the basis for classifying CKD into 5 stages as described below. The formula for the MDRD equation can be found at several web sites, including: The Nephron Information Center MDRD GFR Calculator – (with SI Units) and the National Institutes of Health Health GFR MDRD Calculators for Adults.
For a discussion of the stages of chronic kidney disease (CKD) and how they are defined by level of GFR, see: Kidney Basics: What is chronic kidney disease, and how common is it?
Other Kidney Tests
Various imaging tests can provide useful information about the kidneys with little or no discomfort and minimal risk to the patient.
- CAT scan (computed tomography)
- MRI (magnetic resonance imaging)
Ultrasound – Ultrasound uses sound waves to bounce off structures in the body and give images, in the same way as sonar is used by submarines to map the ocean floor, or as sound waves are used by flying bats. This procedure is good for determining the size of the kidneys and for detecting cysts (round fluid-filled pockets that are common in older people and usually of no significance), solid masses that may be benign or malignant tumors, and kidney stones. Ultrasound can also help to estimate the amount of scarring in a kidney, and can detect whether there is a blockage to urine flow anywhere in the kidney, the ureters, or the bladder.
IVP (intravenous pyelogram) – IVP uses traditional x-rays to produce pictures of the kidneys, ureters, and bladder. A substance called “contrast dye” is injected into a vein, circulates through the bloodstream, and is processed and excreted by the kidneys. This technique produces two-dimensional black-and-white images and can provide fairly-detailed information about the size and shape of the kidneys, as well as the presence of kidney stones and sometimes cysts or tumors. However, if kidney function is reduced, IVPs are not done, for two reasons: 1) the kidneys will not process the dye as well and will not show up well in the pictures; and 2) there is some risk of the dye’s causing renal failure (temporary) in people who already have reduced kidney function to begin with. The contrast dye may also produce an allergic reaction in some people.
CAT (Computed Axial Tomography) scan – CAT scan uses x-rays to produce pictures in crosswise slices (as though the body was sliced like a loaf of bread). Like an ultrasound exam, a CAT scan can detect kidney stones, blockage, cysts, and solid masses. Generally, a CAT scan involves the entire abdomen, so that each organ – not just the kidneys – can be seen; even spinal vertebrae and layers of the abdominal wall can be examined using this technique. CAT scans are sometimes done using contrast dye, which (as with IVPs) carries the risk of inducing an allergic reaction and/or causing renal failure, especially in people who already have reduced kidney function.
MRI (Magnetic Resonance Imaging) scan – MRI scan exposes the body to a strong magnetic field and creates images based on the molecular composition of different organs and tissues. The amount of fine detail in the pictures is greater than with either the ultrasound or CAT scan. However, the procedure, since it involves exposing the patient to a strong magnetic field, cannot be used in those who have metal devices in their bodies (such as pacemakers or defibrillators). The procedure may also be somewhat difficult for patients to undergo, since the patient must lie still in a dark, enclosed tunnel and must not be upset by banging noises made by the machinery. MRI scans are sometimes done with a special type of dye (administered by IV) called gadolinium: this dye is not used in patients with moderately- to severely-reduced renal function because of a risk of adverse effects particularly in a condition called “nephrogenic systemic fibrosis”.
The procedure of obtaining and examining a small tissue sample from the kidney is called a kidney, or renal, biopsy.
A biopsy is generally performed using ultrasound or CAT scan to localize the kidney. The patient lies prone (face down), since the kidneys are located close to the surface of the back. Either the right or the left kidney is biopsied; it is not necessary to biopsy both. Once the spot for the biopsy is located using ultrasound or CAT scan, the area is anesthetized using xylocaine (the same medication that dentists use to numb teeth). A small incision (approximately ¼” long) is made, and a special needle is inserted into the kidney. The needle is spring-loaded so that when a button is pushed, the device automatically samples a small piece of kidney, about the width of a No. 2 pencil lead, and approximately ½” long. Generally 2 or 3 pieces of tissue are obtained, each time using the same procedure. Following the biopsy, the patient is asked to lie still for 4-6 hours and is closely observed for any signs of kidney bleeding.
The biopsy specimens are frozen, sliced, stained, and studied by a variety of microscopic methods. It usually takes one to two weeks for the processing to be completed and for a diagnosis of the kidney disease to be made.
Learn more about the kidneys:
- Basic Information About the Kidneys
- Kidney Pain
- Kidney Stones
- Signs and Symptoms of Kidney Disease
- Treatment & Management of Chronic Kidney Disease (CKD)
Kidney failure information:
- Kidney Failure – Understanding End Stage Renal Disease (ESRD)
- Peritoneal Dialysis
- Kidney Transplantation
- Organ Donation and Transplantation
For more information:
Go to the Kidney Diseases health topic.