What is GFR?

The Test

What is being tested?
Glomerular filtration rate (GFR) is a measure of the function of the kidneys. Glomeruli are tiny filters in the kidneys that allow waste products to be removed from the blood, while preventing the loss of important constituents, including proteins and blood cells. Every day, healthy kidneys filter about 200 quarts of blood and produce about 2 quarts of urine. The glomerular filtration rate refers to the amount of blood that is filtered by the glomeruli per minute. When a person's kidney function declines due to damage or disease, then the filtration rate decreases and waste products begin to accumulate in the blood.
Chronic kidney disease (CKD) is associated with a decrease in kidney function that is often progressive. CKD can be seen with a variety of conditions, including diabetes and high blood pressure. Early detection of kidney dysfunction can help to minimize the damage. This is important as symptoms of kidney disease may not be noticeable until as much as 30 to 40% of kidney function is lost.

A measured GFR is considered the most accurate way to detect changes in kidney status, but measuring the GFR directly is complicated, requires experienced personnel, and is typically performed in a research setting. Because of this, an estimate – the eGFR – is usually used.

The eGFR is a calculation based on a serum creatinine test. Creatinine is a muscle waste product that is filtered from the blood by the kidneys and excreted into the urine at a relatively steady rate. When kidney function decreases, less creatinine is excreted and concentrations increase in the blood. With the creatinine test, a reasonable estimate of the actual GFR can be determined.

How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm; depending on the formula used, your age, gender, race, height, and weight may also be needed.

How is it used?
The eGFR test is used to screen for and detect early kidney damage and to monitor kidney status. It is performed by ordering a creatinine test and calculating the eGFR. The creatinine test is ordered frequently as part of a routine Comprehensive Metabolic Panel (CMP) or Basic Metabolic Panel (BMP), or along with a Blood Urea Nitrogen (BUN) test whenever a doctor wants to evaluate the status of the kidneys. It is ordered to monitor those with known kidney disease and those with conditions such as diabetes and hypertension that may lead to kidney damage.

When is it ordered?
The eGFR can be determined, with no extra testing, at the same time that a blood sample is sent for a creatinine measurement. The National Kidney Foundation (NKF) has recommended that it be calculated automatically every time a creatinine test is done. A creatinine test and eGFR may be ordered any time that a doctor wants to evaluate a person's kidney function as part of a health checkup or if kidney disease is suspected. Warning signs of kidney disease may include:
Swelling or puffiness, particularly around the eyes or in the face, wrists, abdomen, thighs, or ankles
Urine that is foamy, bloody, or coffee-colored
A decrease in the amount of urine
Problems urinating, such as a burning feeling or abnormal discharge during urination, or a change in the frequency of urination, especially at night
Mid-back pain (flank), below the ribs, near where the kidneys are located
High blood pressure (hypertension)

As kidney disease worsens, symptoms may include:

Urinating more or less often
Feeling itchy
Tiredness, loss of concentration
Loss of appetite, nausea and/or vomiting
Swelling and/or numbness in hands and feet
Darkened skin
Muscle cramps
An eGFR may be ordered periodically when a person has a chronic kidney disease or a condition such as diabetes or hypertension that is associated with kidney damage.

What does the test result mean?

The eGFR test detects kidney disease in its early stages more reliably than the creatinine test alone. Because the calculation works best for estimating reduced renal function, the NKF suggests only reporting actual results once values are < 60 ml/min (normal values are 90-120 ml/min, according to the NKF). An eGFR below 60 ml/min suggests that some kidney damage has occurred. The NKF recommends that a person's eGFR result be interpreted in relation to his clinical history and presenting conditions.

The NKF has suggested that all persons "know their GFR number." They recommend interpreting eGFR results based on the following table:

KIDNEY DAMAGE STAGE    DESCRIPTION    GFR    OTHER FINDINGS
1     Normal or minimal kidney damage with normal GFR   90+    - Protein or albumin in urine are high, cells or casts seen in urine
2     Mild decrease in GFR   60-89
3     Moderate decrease in GFR   30-59
4     Severe decrease in GFR 15-29
5     Kidney failure <15

Is there anything else I should know?
Another method of evaluating renal function involves the measurement of the serum level of a molecule called cystatin C. There is increasing interest in the use of this test for this purpose.
The creatinine clearance test also provides an estimate of renal function and of the actual GFR. However, in addition to the serum creatinine, this test requires a timed urine collection (24 hours) for urine creatinine measurement in order to compare blood and urine creatinine concentrations and to calculate the clearance.

The actual amount of creatinine that a person produces and excretes is affected by their muscle mass and by the amount of protein in their diet. Men tend to have higher creatinine levels than women or children.

A person's GFR decreases with age and some illnesses and can increase during pregnancy.

The calculation for eGFR is intended to be used when kidney function, and creatinine production, is stable. If a creatinine level is measured when the kidney function is changing rapidly, such as with acute renal failure, then it will not give a useful estimate of the filtration rate. A slightly different equation should be used to calculate the eGFR for those under the age of 18. An eGFR test may not be as useful for those who differ from normal creatinine concentrations. This may include people who have significantly more muscle (such as a body builder) or less muscle (such as a muscle-wasting disease) than the norm, those who are extremely obese, malnourished, follow a strict vegetarian diet, ingest little protein, or who take creatine dietary supplements. Likewise, the eGFR equations are not valid for those who are 75 year of age or older because muscle mass normally decreases with age.

The eGFR test may also be affected by a variety of drugs, such as gentamicin, cisplatin, and cefoxitin that increase creatinine levels, and by any condition that decreases blood flow to the kidneys.

The most commonly used equation for calculating the eGFR, and the one recommended by the National Kidney Foundation for general use, is called the MDRD (Modification of Diet in Renal Disease study) equation. It requires a person's serum creatinine, age, and assigned values based upon gender and race.

http://www.labtestsonline.org/understanding/analytes/gfr/test.html