Answer. The glomerular filtration rate (GFR) is the test that doctors use to show how well kidneys function. They enable a standard and accepted measure to evaluate the extent of the disease, place it in one of five classifications and track its progress over time.

Each kidney is comprised of a million filters known as glomeruli. A filter separates substances based upon their unique characteristics. Each filter depends upon pressure (gravity is the pressure used in filtration of coffee), the selectivity characteristics of the membrane and the size or surface area. When optimal, the rate of filtration is high, but with disease the ability to clear substances across a filter decreases.

When measuring GFR, a marker substance such as creatinine is used. Creatinine is a metabolite from muscle. Each day our muscles go through “maintenance.” As new muscle builds, old muscle cells are torn down and release this metabolite into the blood where it is eliminated by the kidneys and excreted into the urine. In the past, we used the creatinine clearance formula that required an accurate and timed 24-hour collection of urine creatinine. This test was awkward and it was hard to get perfectly accurate collections. Prediction equations, such as the Cockcroft-Gault formula, were often used as a substitute.

In 1994, the Modification of Diet in Renal Disease Study Group (MDRD) published a study that looked at the effects of dietary protein restriction and blood pressure control on kidney disease progression.¹ A total of 1,628 patients enrolled and very accurate methods of measuring GFR were required. Using these results made it possible to create a formula that predicted the GFR using demographic characteristics such as age, race, gender and the serum creatinine level. Originally, the blood urea nitrogen (BUN) and the serum albumin were also included, but it was found that the serum creatinine formula was equally suitable. The MDRD formula was compared with the earlier Cockcroft-Gault prediction formula and found to be more accurate.²

The National Kidney Foundation (NKF) published the Kidney Disease Outcomes Quality Initiative (K/DOQI) clinical practice guidelines for chronic kidney disease (CKD) and used the MDRD formula to classify kidney disease into five stages ranging from a GFR greater than 90 (Stage 1), GFR 60-89 (Stage 2), GFR 30-59 (Stage 3), GFR 15-29 (Stage 4) and less than 15 cc/min (Stage 5). This classification enables one to determine what level of CKD is present for each individual patient, to define a specific action plan and to easily reference the widely-accepted guidelines of the NKF. A GFR below 60 cc/min may indicate that CKD is present if the level persists for more than three months.³

The MDRD GFR formula is complicated to compute even when using a hand calculator, but the program is readily available on hand held devices (PDAs) at www.kidney.org, and on the web at nephron.com, nkdep.nih.gov or www.kidney.org.

Answer provided by Stephen Z. Fadem, MD, FACP, who serves as a member of AAKP’s Medical Advisory Board and Vice President of the AAKP Board of Directors. Dr. Fadem is a practicing nephrologist in Houston, Texas.

References:

1. Klahr S, Levey AS, Beck GJ, et al. The effects of dietary protein restriction and blood-pressure control on the progression of chronic renal disease. Modification of Diet in Renal Disease Study Group. N Engl J Med 1994; 330:877-84.

2. Levey AS, Bosch JP, Lewis JB, Greene T, Rogers N, Roth D. A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group. Ann Intern Med 1999; 130:461-70.

3. K/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification. Am J Kidney Dis 2002; 39:S1-266.

The American Association of Kidney Patients presents Ask the Doctor, an opportunity for readers to submit kidney related health questions to healthcare professionals who specialize in an area of concern. The answers are not to be construed as a diagnosis and therefore, alterations in current healthcare should not occur until the patient’s physician is consulted.

This article originally appeared in the September/October 2004 issue of Kidney Beginnings: The Magazine, Vol. 3, No. 3.

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