By Meredith Whitacre & Tejas Desai, MD 

Doctors often order laboratory tests to understand how a patient’s body is functioning. Often times, patients do not know what the tests are designed to measure or why they are necessary. An important first step toward understanding lab tests is a discussion with your physician regarding the indications for the tests. Physicians have various reasons to perform a test, and patients should discuss how a particular lab test applies to them. Such discussions will equip patients with the necessary knowledge to understand what their test results mean.

The patient-physician discussion is critical in early identification of kidney disease. Many common medical conditions and risk factors have the potential to cause kidney disease. These include: diabetes, hypertension, cardiovascular disease, a family history of kidney disease, age over 55, and African-American or Hispanic race. Your physician will determine your risk of kidney disease and use laboratory testing to monitor your kidney function.

Commonly used tests to identify kidney damage are: 1) blood creatinine, 2) eGFR, and 3) urine albumin-to-creatinine ratio. Understanding how each test is used will give you a better understanding of the health of your kidneys.

Creatinine is a waste product produced by muscle tissue. A healthy kidney will excrete creatinine in the urine. As the kidney looses function, creatinine levels in the blood increase. Generally, kidney damage is directly proportional to blood creatinine levels, which can be measured through a simple and quick standard blood test. While there is not a “normal” creatinine level, physicians start looking for kidney damage when the creatinine reaches over 1.0-1.2 mg/dl.

Because creatinine levels are influenced by race, gender, muscle mass and age, your physician may opt to use a more sophisticated method of determining your kidney function. The eGFR (estimated glomerular filtration rate) is a calculated value that adjusts for a patient’s race, gender, age, blood creatinine and albumin levels. One can consider the eGFR as the percentage of kidney function. eGFR’s of less than 90 ml/ min/1.73 m2 suggests less than 90 percent of the kidney is functioning normally and indicates early kidney damage (see Table 1 on page 11).

An equally important test is the urine albumin-to-creatinine ratio. Albumin is a blood protein that is found in small amounts in the urine of healthy patients. Kidney damage results in “protein leakage” or “spilling protein” which is detected by the presence of large amounts of albumin in the urine. A simple urine test can measure albumin and urine creatinine. Kidney damage is directly proportional to the urine albumin-to-creatinine ratio; the larger the ratio, the greater the kidney damage. Levels of < 30 mg albumin/gram of creatinine are considered normal; between 30-300 mg albumin/gram creatinine considered “microalbuminuria” and an early sign of kidney damage; greater than 300 mg albumin/gram creatinine considered “macroalbuminuria” and indicative of more severe kidney damage.

Patients who understand these tests can help their physicians identify early kidney damage. It is important for patients to ask their physicians what their most recent serum creatinine, eGFR and urine albumin-to-creatinine ratios are and the trend for each test over time. A proactive discussion with your physician about your kidney health will identify any damage early and help your physician institute protective measures to prevent further loss of kidney function.

Stage	eGFR (mL/min/1.73 m2)	Description
1	>90	Normal or increased GFR but other evidence of kidney damage
2	60-89	Slight decrease in GFR with other evidence of kidney damage
3a	45-59	Moderate decrease in GFR with or without other evidence of kidney damage
3b	30-44	Moderate decrease in GFR with or without other evidence of kidney damage
4	15-29	Severe decrease in GFR with or without other evidence of kidney damage
5	<15	Renal failure

References
de Jong PE, Gansevoort RT. Screening techniques for detecting chronic kidney disease. Curr Opin Nephrol Hypertens. 2005 Nov;14(6):567-72.

Nelson AW, Mackinnon B, Traynor J, Geddes CC. The relationship between serum creatinine and estimated glomerular filtration rate: implications for clinical practice. Scott Med J. 2006 Nov;51(4):5-9.

Brosnahan G, Fraer M. Chronic Kidney Disease: Whom to Screen and How to Treat, Part 1: Definition, Epidemiology, and Laboratory Testing. South Med J. 2010 Jan 8. [Epub ahead of print]

Hoerger TJ, Wittenborn JS, Segel JE, Burrows NR, Imai K, Eggers P, Pavkov ME, Jordan R, Hailpern SM, Schoolwerth AC, Williams DE. A Health Policy Model of CKD: 2. The Cost-Effectiveness of Microalbuminuria Screening: Centers for Disease Control and Prevention CKD Initiative. Am J Kidney Dis. 2010 Jan 28. [Epub ahead of print]

Levey AS, Coresh J, Balk E, Kausz AT, Levin A, Steffes MW, Hogg RJ, Perrone RD, Lau J, Eknoyan G. National Kidney Foundation practice guidelines for chronic kidney disease: evaluation, classification, and stratification. Ann Intern Med. 2003 Jul 15;139(2):137-47. Erratum in: Ann Intern Med. 2003 Oct 7;139(7):605.

Meredith Whitacre is a third year medical student at East Carolina University - Brody School of Medicine. She received her Bachelor of Science from Campbell University, with a major in Biology and minors in Chemistry and Spanish.

Tejas Desai, MD, is Assistant Professor of Medicine in the Division of Nephrology and Hypertension at East Carolina University - Brody School of Medicine. He is the Editor-in-Chief of Nephrology On-Demand (www.nephrologyondemand.org), an educational Internet site for patients and health care providers.

This article originally appeared in the May 2010 issue of aakpRENALIFE.

Posted 5/10/10.

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