© 2004 American Thoracic Society
Renal Function in Critically Ill, Morbidly Obese PatientsTo the Editor:We appreciate the Clinical Commentary by Dr. El Solh concerning critically ill, morbidly obese patients (1). The evaluation of renal function in such patients seems of importance and was not mentioned. Hypertension, diabetes mellitus, and other current complications of being overweight may severely compromise renal function (2). Glomerular hyperfiltration, hyperlipidemia, leptin, and adipocyte-derived hormone play critical roles in the development of focal and segmental glomerulosclerosis (FSG) (3). The long-term prognosis of obese patients with biopsy-proven FSG is poor, with almost one half ultimately experiencing development of advanced renal failure. The management of such patients, with frequently impaired renal function, may require dose adjustment especially for low molecular weight heparins, iodine contrast media, and drugs excreted by the kidneys. The glomerular filtration rate (GFR) is traditionally considered the best overall index of renal function. Most clinicians estimate the GFR from the serum creatinine concentration (SCr). However, the accuracy of this estimation is limited because the SCr is affected by factors other than GFR. No formula is more widely used than that proposed by Cockcroft and Gault (4). More recently, the Modification Diet in Renal Disease (MDRD) Study Group proposed a new formula, including SCr, sex, age, and ethnicity (black or others): 214 x [SCr, mg/dL]–0.113 x [age, years]–0.174 x [0.762 if patient is female] x [1.180 if patient is black] (5). The GFR is expressed in ml/minute per 1.73 m2. This model has been validated in 1,628 patients whose main weight was 79.6 ± 16.8 kg. In opposition to the Cockcroft and Gault formula, it does not include the weight and may be less reliable in obese patients. Indeed, after correction for surface body area, using the Dubois and Dubois formula, it seems more accurate for determining obesity. Data obtained in an intensive care unit of a morbidly obese patient are summarized in Table 1 , using I51Cr-ethylenediamine tetraacetic acid clearance as the gold standard. In Figure 1 , we evaluate an example for different values of creatinine, using the two formulas. The Cockcroft and Gault formula doesn't seem of value in obese patients, overestimating the GFR. We thus believe that MRDR has to be used in the critically ill morbidly obese patient.
Hôpital Tenon Paris, France FOOTNOTES Conflict of Interest Statement: F.V. does not have a financial relationship with a commercial entity that has an interest in the subject of this letter; N.E-K. does not have a financial relationship with a commercial entity that has an interest in the subject of this letter; E.R. does not have a financial relationship with a commercial entity that has an interest in the subject of this letter. REFERENCES
From the Author: I thank Dr. Vincent and his colleagues for their insightful comments regarding my recent Clinical Commentary (1). Creatinine clearance is considered the most practical measure of glomerular filtration rate (GFR) in hospitalized patients. Although inulin clearance represents the gold standard for measuring GFR, it is both costly and labor intensive making it impractical in clinical settings. Numerous formulas and nomograms have been devised on the basis of age, sex, body surface area, and serum creatinine concentration to estimate creatinine clearance (2–4); however, none of these derived equations has been proven accurate in critically ill, morbidly obese patients when compared with measured creatinine clearance obtained by 24-hour urine collection (5). The new prediction formula derived from the Modification Diet in Renal Disease (MDRD) Study Group (6) has the advantage of predicting GFR rather than creatinine clearance, can be readily calculated from available demographic variables, and seems more accurate than the other equations. Nonetheless, the performance of the MDRD study equation has not been validated in critically ill, morbidly obese patients across the spectrum of renal function. The equation may vary with conditions that interfere with creatinine secretion (e.g., cimetidine) or creatinine assay (e.g., diabetic ketoacidosis) and may be inaccurate in patients not in steady state of creatinine balance. The data provided by Dr. Vincent and coworkers are promising, but pending a larger comparative prospective study, measurement of creatinine clearance by 24-hour urine collection should not yet be abandoned.
University at Buffalo Buffalo, New York FOOTNOTES Conflict of Interest Statement: A.E-S. does not have a financial relationship with a commercial entity that has an interest in the subject of this letter. REFERENCES
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