Estimation of Creatinine Clearance in Morbidly Obese Patients

Jasmina A. Demirovic; Amy Barton Pai; Manjunath P. Pai

Am J Health Syst Pharm. 2009;66(7):642-648.

Abstract and Introduction

Abstract

Purpose: Estimates of creatinine clearance (CL_cr) based on equations and various body-size descriptors were compared with 24-hour measured CL_cr values in morbidly obese patients.
Methods: Patients age 18-75 years with a body mass index (BMI) of ≥40 kg/m² with stable serum creatinine values were enrolled. Covariates known to contribute to alteration in CL_cr were used to exclude patients. Twenty-four-hour urine collection was performed to measure CL_cr. Bioelectric impedance analysis was used to estimate fat-free weight (FFW). Glomerular filtration rate was estimated using the four-variable Modification of Diet in Renal Disease (MDRD4) equation. CL_cr was estimated using the Cockcroft-Gault and Salazar-Corcoran methods using total body weight (TBW). Body-size descriptors, such as ideal body weight (IBW), adjusted body weight (ABW), and lean body weight (LBW), and FFW were substituted in the Cockcroft-Gault equation to generate additional estimates of CL_cr.
Results: Fifty-four patients (mean ± S.D. age, 48.4 ± 12.9 years; TBW, 142.3 ± 41.7 kg; BMI, 50.5 ± 12.6 kg/m²) completed the study. All three equations were biased in their estimation of CL_cr. Use of MDRD4 and IBW in the Cockcroft-Gault equation underestimated CL_cr, while the Salazar-Corcoran equation and use of TBW or ABW in the Cockcroft-Gault equation overestimated this value. Substitution of fat-free weight or LBW in the Cockcroft-Gault equation provided unbiased estimates of CL_cr.
Conclusion: An LBW estimate, based on TBW and BMI, incorporated into the Cockcroft-Gault equation provided an unbiased, relatively precise, accurate, and clinically practical estimate of 24-hour measured CL_cr in morbidly obese patients.

Introduction

Accurate estimation of renal function is particularly problematic in morbidly obese patients. We compared estimates of creatinine clearance (CL_cr) that used various body-size descriptors to determine which method most closely approximated the actual measured 24-hour CL_cr in morbidly obese patients.

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Abstract and Introduction
Background
Methods
Results
Discussion
Conclusion
References
Appendix: Equations and Body Weights Used in Study

Table 1. Patient Characteristics
Variable	Women (n = 34)	Men (n = 20)	Total (n = 54)
Age (yr)
Mean ± S.D.	51.3 ± 12.5	43.6 ± 12.4	48.4 ± 12.9
Median (range)	52.0 (29.0-74.0)	47.5 (21.0-65.0)	48.5 (21.0-74.0)
Race/ethnicity (no. [%])
Non-Hispanic white	18 (53)	9 (45)	27 (50)
Hispanic	14 (41)	9 (45)	23 (43)
Native American	1 (3)	2 (10)	3 (6)
Black	1 (3)	0	1 (2)
Serum creatinine concentration (mg/dL)
Mean ± S.D.	0.8 ± 0.3	0.9 ± 0.2	0.8 ± 0.3
Median (range)	0.7 (0.5-1.8)	0.8 (0.6-1.3)	0.8 (0.5-1.8)
Total body weight (kg)
Mean ± S.D.	132.7 ± 35.0	158.7 ± 47.6	142.3 ± 41.7
Median (range)	123.8 (99.0-256.0)	147.0 (112.0-320.0)	128.2 (99.0-320.0)
Body mass index (kg/m²)
Mean ± S.D.	50.6 ± 12.0	50.3 ± 13.9	50.5 ± 12.6
Median (range)	45.6 (40.1-88.6)	45.2 (40.2-95.6)	45.6 (40.1-95.6)

Table 2. Performance of Various Methods of Estimating Creatinine Clearance (CL _cr) or Glomerular Filtration Rate (GFR) ^a
Method	Mean ± S.D. or GFR CL_cr (mL/min)	Mean Bias (mL/min) (95% CI)	Precision		Accuracy (% Patients)
Method	Mean ± S.D. or GFR CL_cr (mL/min)	Mean Bias (mL/min) (95% CI)	r ²	RMSE	≤30% of Measured CL_cr	≤50% of Measured CL_cr
Measured CL_cr	109.5 ± 44.4
Cockcroft-Gault_TBW	216.9 ± 113.1	-107.4 (-133.2, -81.5)	0.272	37.83	12.9	29.6
Cockcroft-Gault_IBW	85.3 ± 29.4	24.3 (14.6, 33.9)	0.326	36.41	48.1	88.9
Cockcroft-Gault ABW_0.3	129.4 ± 55.3	-19.8 (-32.1, -7.6)	0.339	36.07	53.7	75.9
Cockcroft-Gault ABW_0.4	141.9 ± 63.1	-32.3 (-46.2, -18.5)	0.326	36.40	51.8	66.7
Cockcroft-Gault_FFW	102.7 ± 48.1	6.8 (-4.5, 18.2)	0.324	36.46	61.1	83.3
Cockcroft-Gault_LBW	101.5 ± 43.3	8.1 (-2.6, 18.7)	0.352	35.68	55.6	87.0
Salazar-corcoran	155.2 ± 65.1	-45.7 (-59.9, -31.5)	0.326	36.40	46.2	55.6
MDRD4	96.3 ± 29.4	13.3 (2.3, 24.3)	0.172	40.36	51.9	87.0

^aCI = confidence interval; RMSE = root mean-squared error; TBW = total body weight; IBW = ideal body weight; ABW_0.3 = adjusted body weight (ABW) using 0.3 to represent gain in lean body weight (LBW) between TBW and IBW; ABW_0.4 = ABW using 0.4 to represent gain in LBW between TBW and IBW; FFW = fat-free weight; MDRD4 = four-variable Modification of Diet in Renal Disease equation.

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