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Technical Innovation |
1 Both authors: London Health Sciences Centre, University of Western Ontario, 375 South St., London, ON N6A 4G5, Canada.
Received December 16, 2002;
accepted after revision February 11, 2003.
Address correspondence to S. F. Millward
(steven.millward{at}lhsc.on.ca).
One of the keys to preventing contrast agent–induced nephropathy is identifying the patients who are at an increased risk [1]. Thus, the identification of patients with preexisting renal insufficiency is of great importance. Most practicing radiologists use the serum creatinine level for this purpose, even though the serum creatinine level does not increase until the glomerular filtration rate is reduced by approximately 50% [4]. Furthermore, because serum creatinine levels depend on lean body weight, age, and sex, the estimation of serum creatinine alone cannot be used to diagnose renal insufficiency in approximately one third of patients 40–49 years old and approximately 90% of patients 70 years old or older [5]. It is important that these patients be identified at least 24 hr before administration of the contrast agent so that preventive measures, such as IV fluids, N-acetylcysteine, or possibly fenoldopam can be initiated at that time [1–3, 6].
Consequently, measuring glomerular filtration rate is preferable to relying on serum creatinine levels. Accurate measurements of glomerular filtration rate are best obtained using inulin clearance, isotope clearance, or creatinine clearance [4]. However, each of these tests is labor-intensive and can be of limited practical value.
Three equations have been described to estimate glomerular filtration rate:
the Cockroft and Gault equation, the Modification of Diet in Renal Disease
equation, and the Bjorssen equation. All of these equations have similar
predictive estimates, and none is necessarily superior to the others
[4]. The Modification of Diet
in Renal Disease equation and the Bjorssen equation are useful, but they use
several variables that make them laborious and impractical. The Cockroft and
Gault formula can be used to estimate creatinine clearance (and hence
glomerular filtration rate) using age, lean body weight, and serum creatinine
levels [1,
5]. A recent study involving
2543 serial patients over the age of 16 years with normal serum creatinine
levels (
1.5 mg/dL, or 130 µmol/L) showed that serum creatinine
measurements missed 387 patients with a glomerular filtration rate of 50
mL/min or less when the glomerular filtration rate was subsequently estimated
by the Cockroft and Gault formula
[7].
The many variations of the Cockroft and Gault formula [5] include these commonly used equations, shown in SI units (equation 1) and English units (equation 2):
Lean body weight is calculated for men as follows [1]: A lean body weight of 50 kg (110 lb) is assigned to a man 5 feet (60 inches) tall; another 2.3 kg (5.06 lb) of lean body weight is assigned for each inch of height above 60 inches. For women, 45.5 kg (100.1 lb) is assigned as the lean body weight for 60 inches of height, and 2.3 kg (5.06 lb) is added per inch above 60 inches. Consequently, it is possible to estimate the heights for patients of various lean body weights.
Consider a 75-year-old, 5 feet 10 inch tall man. The lean body weight is 50 kg plus 2.3 kg per inch over 5 feet. Therefore, the lean body weight for a 5 feet 10 inch tall man is 73 kg, or 160.6 lb. Thus, with the weight of 73 kg, or 160.6 lb, as an equivalent to 5 feet 10 inches tall, we can determine the serum creatinine at a glomerular filtration rate of 50 mL/min by plugging the appropriate values into the Cockroft and Gault equation and solving for serum creatinine (equation 3):
According to the recommendations of Waybill and Waybill [1], patients with diabetes mellitus or other risk factors for contrast agent–induced nephropathy who are scheduled for IV contrast injection and all patients scheduled for angiography should have their serum creatinine measured. Afterward, creatinine clearance should be estimated using the Cockroft and Gault formula. In the authors' department, however, this strategy proved impractical for all patients, and therefore a simpler method of identifying patients with preexisting renal insufficiency was sought.
Patients at risk for contrast agent–induced nephropathy are those with moderate or severe preexisting renal insufficiency [1, 5]. There is controversy regarding what constitutes moderate and severe renal insufficiency. However, a cutoff level of 50 mL/min for creatinine clearance (for a 50-year-old, 5 feet 9 inch tall man with a lean body mass of 70 kg, this equates to a serum creatinine of 156 µmol/L, or 1.75 mg/dL) is reasonable [1, 5]. Also, this glomerular filtration rate was used in studies that dictate the use of preventive therapies [2]. Creatinine clearance levels of less than 50 mL/min indicate the presence of moderate renal insufficiency, whereas a creatinine clearance of 25 mL/min can be used as a cutoff below which severe renal insufficiency is present [1, 5].
Figures 1, 2, 3, 4 were created so that patients with a creatinine clearance of 50 mL/min or less could be readily identified. Figure 1 is a graph of serum creatinine (in µmol/L) and age for men of various heights with a creatinine clearance of 50 mL/min. Figure 2 is a similar graph for women. Figures 3 and 4 are graphs with serum creatinine measured in milligrams per deciliter. The lines on the graphs were derived using the Cockroft and Gault formula. A creatinine clearance level of 50 mL/min was entered into the formula, as were different heights (corresponding to lean body weights) and ages. The equation was then solved for serum creatinine to determine the appropriate serum creatinine for a creatinine clearance of 50 mL/min. Because most patients in North America refer to their height in English units, the graphs are based on feet and inches.
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To use the graphs, determine the point on the graph corresponding to the patient's serum creatinine and age and correlate that to the appropriate height line. If the point is on or above the height line, the creatinine clearance level may be close to or less than 50 mL/min. In this situation, it is suggested that the Cockroft and Gault formula be used to more accurately estimate creatinine clearance.
These calculations carry limitations. For the elderly, for example, no estimating formula can effectively predict glomerular filtration rate. It must be noted, however, that the Cockroft and Gault formula is less accurate at a glomerular filtration rate greater than 60 mL/min, and thus this limitation may not influence treatment or prediction of renal insufficiency [8]. The Cockroft and Gault equation often underestimates the glomerular filtration rate for patients older than 70 years and thus overestimates renal insufficiency. Nevertheless, the estimate it provides has been used in several studies and is still a better estimate of glomerular filtration rate and renal function than serum creatinine alone. The Cockroft and Gault formula is not applicable for children.
Several randomized trials have been published recently, some of which used
estimation of glomerular filtration rate with the Cockroft and Gault equation
for some patients, whereas others used serum creatinine levels alone to
determine if renal insufficiency existed
[2,
3,
6]. Most of these studies used
a serum creatinine level ranging from 1.2 mg/dL or greater (
106
µmol/L) to 1.4 mg/dL or greater ( 124 µmol/L) as the cutoff for
inclusion or exclusion of preexisting renal insufficiency
[2,
3,
6]. Use of a cutoff level of
1.4 mg/dL (124 µmol/L) has potential limitations as the examples will
demonstrate.
To illustrate how to use the graphs, we give two examples. With a man 60 years old who is 6 feet tall and has a serum creatinine of 1.4 mg/dL, we use the English units graph (Fig. 3). The serum creatinine is on the y-axis and the age on the x-axis. The coordinates determined lie below the appropriate height line, and hence the patient does not have moderate to severe renal insufficiency. For a woman 70 years old who is 5 feet tall and has a serum creatinine of 100 µmol/L, we use the International System of Units graph (Fig. 2). When the values are coordinated, the point lies far above the appropriate height line, and hence she has moderate renal insufficiency despite a "normal" creatinine.
The authors hope that practicing radiologists find these graphs to be a useful tool for identifying patients with preexisting renal insufficiency and a better guide than serum creatinine levels alone.
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  B. R. Herts, E. Schneider, E. D. Poggio, N. A. Obuchowski, and M. E. Baker Identifying Outpatients with Renal Insufficiency before Contrast-enhanced CT by Using Estimated Glomerular Filtration Rates versus Serum Creatinine Levels Radiology, July 1, 2008; 248(1): 106 - 113. [Abstract] [Full Text] [PDF]
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= 5 feet 3 inches, 
= 5 feet 6 inches, 
=
5 feet 9 inches, x = 6 feet, * = 6 feet 3 inches, • = 6 feet 6
inches, + = 6 feet 9 inches.
