Q.During the March 20 AAKP HealthLine call titled, “Risk Factors for Kidney Disease: Understanding Diabetes and Hypertension,” Dr. Eli Friedman stressed the importance of creatinine level. Is creatinine level more important than GFR level?

A.Your question is excellent as it penetrates a key issue in monitoring patients at risk of progression of chronic kidney disease (CKD). Ideally, your doctor would like to look inside your kidneys to judge ongoing damage in terms of injured tissue, unwanted cells or infection. Because that would require either penetrating the kidney with some sort of scope device or sticking a biopsy needle through your skin and muscle to get a piece of kidney to look at under the microscope (biopsy), we rely on measuring blood chemicals that rise or fall as waste products are passed into urine.

Wastes are removed from blood by being filtered through parts of the kidneys tubes called glomeruli, while the blood cells and big proteins like albumin stay in the blood. Think of it as what happens when coffee is made by filtration but the coffee grounds stay behind on the filter. This process is called glomerular filtration and the rate at which it occurs is the glomerular filtration rate or GFR. To measure GFR requires collection of urine over time – hours to a full day, meaning you have to carry urine in a bottle at work or over the day. Once it was noted that a chemical test, measuring creatinine in blood, gives a good estimate of GFR, many kidney doctors switched from the urine collection to getting a blood creatinine test and using what is called the estimated GFR or eGFR.

Put simply, as kidney function decreases, eGFR falls while creatinine increases. The exact level at which the kidney “problem” is judged seriously depends on age and gender and is still being studied. For adults, a GFR below 60 or a creatinine above 2.0 generally calls for evaluation by a kidney specialist.

Eli Friedman, MD, is Distinguished Teaching Professor at SUNY Health Science Center Brooklyn, NY. Dr. Friedman is also the Chairperson of the AAKP Medical Advisory Board.

To listen to the AAKP HealthLine call titled, “Risk Factors for Kidney Disease: Understanding Diabetes and Hypertension,” visit www.aakp.org/events/HealthLine/archived/Mar08. 

This article originally appeared in the December 2008 issue of Kidney Beginnings: The Magazine.

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