Ann Thorac Surg 2008;86:1-3. doi:10.1016/j.athoracsur.2008.05.007
© 2008 The Society of Thoracic Surgeons
The Statistician's Page
Estimated Glomerular Filtration Rate and Renal Function
Ruyun Jin, MDa,*,
Gary L. Grunkemeier, PhDa,
Jeremiah R. Brown, PhDb,
Anthony P. Furnary, MD
a Medical Data Research Center, Providence Health & Services, Portland, Oregon
b The Dartmouth Institute for Health Policy and Clinical Practice, Lebanon, New Hampshire
* Address correspondence to Dr Jin, Providence St. Vincent Hospital and Medical Center, 9205 SW Barnes R, LL#33, Portland, OR 97225 (Email: ruyun.jin{at}providence.org).
 |
Abstract
|
|---|
Glomerular filtration rate (GFR) is an important clinical indicator of kidney function. It can be used as an independent predictor of long-term survival after cardiac surgery. Definition, methods of measurement, and corrected value by body surface area are briefly introduced in this paper. Details of the calculation of estimated GFR (eGFR) by Levey's formula with serum creatinine, age, gender, and race are offered. The relationship between estimated GFR and the four factors is shown graphically and discussed. An eGFR/creatinine conversion table for individual patients derived from the eGFR equation with clinically significant cutpoints is given, which can be used by physicians as a pragmatic reference.
|
Introduction
|
|---|
In the article by Dr Brown and colleagues [1] published in this issue, postoperative estimated glomerular filtration rate (eGFR) is used as a risk factor to predict long-term survival after cardiac operations. As a technical supplement to that clinical paper, we will explain the details of the calculation of eGFR and attempt to increase its pragmatic value by using the eGFR equation to create a clinically significant eGFR/creatinine conversion table for individual patients.
|
Measured and Estimated Glomerular Filtration Rate
|
|---|
The GFR is "the volume of water filtered out of the plasma through glomerular capillary walls into Bowman's capsules per unit of time," and is defined as
The exact calculation of the GFR requires an injection of exogenous or endogenous markers and the subsequent measurement of its filtration by the kidneys. Inulin is normally used and is considered as the most accurate estimate of GFR [2]. However, this is seldom done in clinical practice because of the technical difficulty, inconvenience, and cost.
Measurement of endogenous creatinine clearance, which is another way to get the GFR, requires the concentration of creatinine in serum and urine, and 24-hour urine volume [3]. Again, this takes time and can be affected by muscle mass. It is seldom performed in postoperative cardiac surgical patients who are not in renal failure or acute renal insufficiency.
Because of these clinical difficulties, eGFR is often used in place of creatinine clearance or the actual GFR. The GFR can be estimated using one of several verified equations that use other easily definable factors, such as serum creatinine, age, gender, race, serum/urine urea nitrogen concentration, and serum albumin concentration [4–6].
|
Formula to Estimate GFR
|
|---|
The most popular, simplified equation to estimate the GFR was developed by Levey and colleagues [6], as a modification of a more complicated equation developed by the same author [5]:
GFR is related to body size: smaller persons should have a lower normal GFR and larger persons should have a higher normal GFR. To compare the GFR of differently sized people or to set its normal range, GFR is indexed by body surface area (BSA). The above formula really provides BSA-adjusted eGFR, calibrated to an average male's BSA of 1.73m2. This is called "corrected" GFR; thus, when using this formula, BSA is not needed.
|
Relationship Between eGFR and Age, Creatinine, Gender, and Race
|
|---|
There are four variables in the Levey formula: serum creatinine, age, gender, and race. Figures 1
and 2
show the relationship among these four variables and eGFR. The eGFR is inversely proportional to both age and creatinine: as either one increases, eGFR decreases. Both male gender and black race increase the eGFR compared with female gender and nonblack race, respectively.
View larger version (22K):
[in this window]
[in a new window]
|
Fig 1. The relationship between the estimated glomerular filtration rate (eGFR) and serum creatinine level, by gender (dashed line for females, solid line for males) and race (gray lines, nonblack patients; black lines, black patients) for a 60-year old patient based on Levey's equation [6]. Horizontal gridlines correspond to the National Kidney Foundation's thresholds of chronic kidney disease [7]. The areas within the boxes are expanded in Figure 2: dashed-line box is for female (left column) and the solid line box is for male (right column).
|
|
View larger version (44K):
[in this window]
[in a new window]
|
Fig 2. The relationship between estimated glomerular filtration rate (eGFR) and serum creatinine for three factors in the Levey equation: race, gender, and age. The thin lines are for a 90-year old; the medium lines are for a 60-year old; and the thick lines are for a 30-year old. The determinations of normal vs abnormal, based on eGFR and creatinine, agree with each other in the white areas (both normal or both abnormal), but they disagree with each other in the gray areas. A patient from the grey (disagreement) area in each panel is shown by an open circle, and described in the text.
|
|
The normal range of creatinine is 0.8 to 1.4 mg/dL for adult men and 0.6 to 1.1 mg/dL for adult women, with slight variations among different laboratories. If we consider an eGFR that exceeds 60 mL/min/1.73m2 as normal, then the determinations of normal/abnormal mostly agree between the two methods; that is, they lie within the white areas in Figure 2. This is not true in every situation, however, as can be seen from the portions of the lines that lie within the grey areas in Figure 2. For example, a creatinine of 1.05 in a 60-year-old white woman is considered to be in the normal range. However, this patient would have an eGFR of 57, which is considered below normal (see open circle in Fig 2A). Likewise, a creatinine of 1.3 mg/dL in a 90-year-old white man is in the normal range, yet converts to an abnormal eGFR of 55 (Fig 2B). A creatinine of 1.2 mg/dL in a 30-year-old black woman is not in the normal range, yet the eGFR is 68 and is thus considered "normal" (Fig 2C). Finally, a creatinine of 1.6 mg/dL in a 30-year-old black man is definitely elevated, but relates to an eGFR of 66, which is normal (Fig 2D).
As is evident in the Figures, as the serum creatinine value gets smaller, especially at values of less than 1.0 mg/dL, the slope of the eGFR curve gets much steeper. This means that a small rise in creatinine could markedly reduce the eGFR. For example, a 60-year-old white woman whose creatinine is 0.7 mg/dL has an eGFR of 91; but if her creatinine is 0.8 mg/dL, her eGFR would decrease to 78; and at 0.9 mg/dL, it decreases to 68.
|
Important Clinical Thresholds of eGFR
|
|---|
The GFR assesses the excretory function of the kidneys and is considered the gold standard used to evaluate renal function. On the basis of the National Kidney Foundation's guidelines for chronic kidney disease, a GFR exceeding 90 mL/min/1.73m2 is considered normal; GFR of 60 to 89 is mildly decreased; a GFR of 30 to 59 is moderately decreased and may signify "renal insufficiency"; a GFR of 15 to 29 mL/min/1.73m2 is considered severely decreased; and a GFR of less than 15 is considered kidney failure [7].
Because a postoperative eGFR of less than 60 mL/min/1.73m2 predicts poor long-term survival, Brown and colleagues recommend "physicians should monitor patients with eGFR < 60 following procedure closely until renal function recovers" [1].
Because eGFRs are not always consistent with creatinine, we recommend the eGFR should be used for diagnosis or as a risk factor in a risk model. Also, eGFR accounts for the variability due to age, gender, and race. And it is more sensitive than creatinine, especially in the normal ranges. Thus, to provide the practicing cardiac surgeon with easily referable creatinine related to clinically significant eGFR thresholds described by the National Kidney Foundation, we have produced Table 1. Note that any creatinine level above 3.1 mg/dL should be considered overt kidney failure, because all values in the columns of eGFR that equal 15 in Table 1 are greater than 3.1. Also, note that even "normal" serum creatinine values may be considered renal insufficiency (eg, 0.9 mg/dL in a 90-year-old nonblack woman).
View this table:
[in this window]
[in a new window]
|
Table 1 Serum Creatinine Values (mg/dL) by Gender, Race, and Age for the National Kidney Foundation's Estimated Glomerular Filtration Rate Thresholds of Chronic Kidney Disease [7]
|
|
|
References
|
|---|
- Brown JR, Cochran RP, MacKenzie TA, et al. Long-term survival after cardiac surgery is predicted by estimated glomerular filtration rate Ann Thorac Surg 2008;86:4-12.[Abstract/Free Full Text]
- Lamb EJ, Tomson CR, Roderick PJ, Clinical Sciences Reviews Committee of the Association for Clinical Biochemistry Estimating kidney function in adults using formulae Ann Clin Biochem 2005;42:321-345.[Abstract/Free Full Text]
- Popper H, Mandel E. Filtrations- und Reabsorptionsleistung in der Nierenpathologie Erg Inn Med Kinder 1937;53:685-695.
- Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine Nephron 1976;16:31-41.[Medline]
- 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;130:461-470.[Abstract/Free Full Text]
- Levey AS, Greene T, Kusek JW, Beck GL,, MDRD Study Group A simplified equation to predict glomerular filtration rate from serum creatinine(abstract) J Am Soc Nephrol 2000;11(suppl):155A.
- National Kidney Foundation Clinical practice guidelines for chronic kidney disease: evaluation classi.cation and strati.cation Am J Kidney Dis 2002;39(suppl 1):S1-S266.[Medline]
Related Article
-
Long-Term Survival After Cardiac Surgery is Predicted by Estimated Glomerular Filtration Rate
- Jeremiah R. Brown, Richard P. Cochran, Todd A. MacKenzie, Anthony P. Furnary, Karyn S. Kunzelman, Cathy S. Ross, Craig W. Langner, David C. Charlesworth, Bruce J. Leavitt, Lawrence J. Dacey, Robert E. Helm, John H. Braxton, Robert A. Clough, Robert F. Dunton, Gerald T. O'Connor Northern New England Cardiovascular Disease Study Group
Ann. Thorac. Surg. 2008 86: 4-11.
[Abstract]
[Full Text]
[PDF]
This article has been cited by other articles:
|
|
|
|
 
C. Diez, P. Mohr, D. Koch, R.-E. Silber, C. Schmid, and H.-S. Hofmann
Age- and gender-specific values of estimated glomerular filtration rate among 6232 patients undergoing cardiac surgery
Interactive CardioVascular and Thoracic Surgery,
October 1, 2009;
9(4):
593 - 597.
[Abstract]
[Full Text]
[PDF]
|
|
|
Top
Abstract
Introduction
