Determining Renal Function: What’s the Best Way to Evaluate?

Our experts talk you through which tests are most helpful in determining whether a patient has, or is at risk for, chronic kidney disease.

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Q: One of my patients is a 72-year-old woman who weighs 59 kg. Her creatinine clearance by Cockcroft-Gault (CG) came back low (49 mL/min). Is this due to her age, gender, and weight loss during the past five months (subsequent to a femur fracture), or does she have underlying kidney disease? Would a 24-hour urine creatinine test be the best way to determine her level of kidney function—and would it be appropriate for someone her age? Is there a better way to evaluate her kidney function?

Accurate measurement of renal function is vital for any patient suspected of having chronic kidney disease (CKD). More than 20 million adults in the United States, or more than 10% of the adult population, have CKD.1 The 2012 US Renal Data System (USRDS) Annual Data Report states that the prevalence of chronic kidney disease in the Medicare population alone rose more than three-fold between 2000 and 2010, from 2.7% to 9.2%.2

CKD consumes a large proportion of Medicare dollars: more than $23,000 per person per year (PPPY) annually. For end-stage renal disease (ESRD) patients on hemodialysis, the cost is an astounding $88,000 PPPY.2 The cost of treating 871,000 ESRD patients was more than $40 billion in both public and private funds in 2009.3

Risk factors for CKD include but are not limited to: advancing age, male sex, race, hypertension, diabetes mellitus, smoking, family history of kidney disease, proteinuria, exposure to nephrotoxins, and atherosclerosis.4

In the US, the most common methods used to estimate renal function are the CG (Cockcroft-Gault) equation, Modification of Diet in Renal Disease (MDRD) study equations, and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. It is often difficult to determine which test is best suited for a patient, because there are pros and cons to each formula and no one test is perfectly suited for every clinical application.4

Since we know this patient’s renal function is low via CG (49 mL/min), the next important question to ask is, “Is it progressive?” I would recommend obtaining a urinalysis to look for hematuria and albuminuria. Proteinuria is an all-encompassing term. Albumin is only one type of protein and is the single most predictive risk factor for kidney disease progression. Persistent albuminuria alone is diagnostic of renal disease.5 The recommended test is a random urine albumin-to-creatinine ratio (ACR; see Table 1).6

You asked if a 24-hour urine creatinine clearance might evaluate her renal function better. Creatinine clearance can be determined by a 24-hour urine test and a serum blood sample in a steady state. However, this test should be interpreted with caution due to both collection errors and the fact that creatinine clearance overestimates true glomerular filtration rate (GFR) due to tubular secretion of creatinine.7,8 Thus, this test is no longer routinely recommended to determine kidney function.8
Catherine B. York, MSN, APRN-BC
Springfield Nephrology 
Associates, Springfield, MO

1. CDC. National chronic kidney disease fact sheet: general information and national estimates on chronic kidney disease in the United States, 2010. Atlanta, GA: US Department of Health and Human Services, CDC; 2010.

2. US Renal Data System. USRDS 2012 annual data report: atlas of end-stage renal disease in the United States. Bethesda, MD: National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases; 2012.

3. US Renal Data System. USRDS 2012 annual data report: atlas of end-stage renal disease in the United States. Bethesda, MD: National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases; 2011.

4. Clarkson MR, Brenner BM. Clinical assessment of the patient with kidney disease. In: Clarkson MR, Brenner BM. Pocket Companion to Brenner & Rector’s The Kidney. 7th ed. Philadelphia, PA: Elsevier Saunders; 2005: 3-19.5.

5. Hsu C. Clinical evaluation of kidney function. In: Greenberg A, Cheung A, Coffman T, et al, eds. Primer on Kidney Diseases, 5th ed. Philadelphia, PA; Saunders Elsevier; 2009:19-237.

6. Kidney Disease Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl. 2013;3:1-150.

7. National Kidney Foundation. Guideline 5: assessment of proteinuria. K/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification; 2000.

8. Stevens LA, Coresh J, Greene T, et al. Assessing kidney function-measured and estimated glomerular filtration rate. N Engl J Med. 2006;354:2473-2483.

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