# 008 Laboratory Evaluation of Kidney Function in Geriatric Patients
Authors: Christopher J. Anderson, Edmund H. Duthie MD, Bambi Wessel
Objectives:
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To discuss limitations of Creatinine and GFR lab values in the Geriatric population.
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To review formulae for estimating Glomerular Filtration Rate (eGFR).
ACGME Competencies: Patient Care, Medical Knowledge, Practice-Based Learning & Improvement
Context: Geriatric patient with abnormal creatinine or eGFR.
Action: React to clinical laboratory values in a geriatric patient.
Incidence/Prevalence: Currently, GFR is the best index available to assess kidney function in an individual. In a young, healthy adult, normal GFR ranges from 120 to 130 ml/min per 1.73 m2. After age 40, GFR has been shown to decrease by 0.8-0.9 ml/min per 1.73 m2 per year1.
Underlying Science:
Serum Creatinine (SCr)
Serum Creatinine is often the initial laboratory value ordered to “assess” renal function. In general, an increase in SCr by >15% likely correlates with a significant decrease in GFR not attributable to biologic variables such as age, gender, muscle mass, diet and medications1. Estimation of GFR using SCr alone has been shown to be unreliable. With aging, the concomitant decline in lean body mass results in a decline in creatinine production, resulting in a non clinically detectable rise in the serum creatinine with the age associated drop in GFR .
Creatinine Clearance (CCr)
The 24 hour Creatinine clearance can be difficult to collect in any population. With the higher prevalence of cognitive and functional decline, this may be even more difficult among the geriatric patient population. Because of tubular secretion of Creatinine, CCr systematically overestimates GFR and therefore estimation of GFR using the CCr alone has been shown to be unreliable.
Cystatin C
One novel method for measurement of GFR is the endogenous substance cystatin C. This substance is like creatinine but is constitutively produced by all nucleated cells, freely filtered, reabsorbed, and catabolized by the kidney. Early studies have shown that serum cystatin C levels correlate better with GFR than does serum creatinine alone, especially at higher levels of GFR1.
Limited testing availability for cystatin C and lack of larger supporting data make cystatin C a promising lab value for more accurate measurement of GFR in the future.
Assessment: As discussed above, all currently available methods of estimation of GFR have several limitations in the geriatric population. In addition to these limitations, the Baltimore Longitudinal Study on Aging showed that age-related decline in GFR may not always occur. This information can make differentiating age-related decline from chronic kidney disease difficult.
The MDRD method is currently the most widely reported laboratory method of GFR estimation in the elderly person1. Serum creatinine and GFR estimation equations can be applied only when creatinine is stable. One geriatric based study showed that the MDRD formula most accurately predicts GFR in the geriatric population2. The Cockcroft Gault equation provides an estimate of Creatinine clearance.
Formulae
• The abbreviated version or four variable version of the Modification of Diet in Renal Disease (MDRD) equation (ml/min per 1.73 m2)
Estimated Glomerular Filtration Rate (eGFR) = 186 x (SCr) -1.154 x (Age) -0.203
(multiply by 0.742 if patient is female; multiply by 1.212 if patient is black)
• The Cockcroft-Gault (C & G)equation:
Est. Creatinine Clearance = [[140 - age(yr)] x weight(kg)]/[72 x SCr (mg/dL)]
(multiply by 0.85 for women)
• Estimation of GFR using Cystatin C
eGFR =177.6 X SCr-0.65 X CysC-0.57 X Age -0.20
(multiply by 0.82 if female; multiply by 1.11 if black)
Conclusion: Clinicians cannot use the serum creatinine alone as a measure of renal function in the geriatric patient. No matter what measure is used to estimate the age related physiologic decline in renal function, clinicians must recall that these are estimates. Individual variability exists and patients need clinical monitoring of pharmacotherapy to insure adequate drug dosing and effect, avoidance of toxicities, and optimal patient outcomes. In life threatening situations and for drugs that have significant toxicity, pharmacokinetic profiling with monitoring of drug levels may be needed as an adjunct to estimates of renal function so that precise dosing can occur.
References:
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http://www.asn-online.org/education_and_meetings/geriatrics/OnlineGeriatricsCurriculum.pdf
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Aras, S; Varli, M; Uzun, B; Atli, B; Keven, K; and Turgay, M. Comparison of Different Glomerular Filtration Methods in the Elderly: Which Formula Provides Better Estimates? Renal Failure, 1/24/12 Epub.
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Earley A, Miskulin D, Lamb EJ, Levey AS and Uhlig K. Estimating equations for glomerular filtration rate in the era of creatinine standardization - a systematic review. Ann Intern Med published online February 6, 2012.
Author Affiliation: Medical College of Wisconsin, Milwaukee, Wisconsin.
Geriatric Fast Facts are edited by the faculty of the Geriatric Education Teams (GETS) program funded by the Reynolds Foundation, Kathryn Denson, MD, Steven Denson, MD, & Edmund Duthie, MD from the Division of Geriatrics, Medical College of Wisconsin, and are published by the Geriatric Fast Facts Website at the Medical College of Wisconsin. For more information write Kathryn Denson, MD. More information, as well as the complete set of Fast Facts, available at www.mcw.edu/Geriatric-Fast-Facts.htm
Copyright/Referencing Information: Users are free to download and distribute Geriatric Fast Facts for educational purposes only. Citation: Christopher J. Anderson, Edmund H. Duthie MD, Bambi Wessel, Fast Fact #8: Laboratory Evaluation of Kidney Function in Geriatric Patients: February, 2012. Geriatric Fast Facts, available at www.mcw.edu/Geriatric-Fast-Facts.htm
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