eGFR

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.

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.

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:

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.