The Performance of Equations That Estimate Glomerular Filtration Rate against Measured Urinary Creatinine Clearance in Critically Ill Patients

Abstract

The performance of glomerular filtration rate- (GFR-) estimating equations was studied against creatinine clearance measured by 24-hour urine collection (CrCl_24h-urine) in critically ill patients. Methods. In this substudy of the PermiT trial (https://clinicaltrials.gov/ct2/show/ISRCTN68144998), patients from King Abdulaziz Medical City-Riyadh who had CrCl_24h-urine were included. We estimated GFR using Cockroft-Gault (CG), modification of diet in renal disease study (MDRD), chronic kidney disease epidemiology collaboration (CKD-EPI), and Jelliffe equations. For the CG equation, we entered the actual weight in one calculation (CG_actual-wt), and if BMI ≥30 kg/m², we entered the ideal body weight (CG_ideal-wt) and the adjusted body weight (CG_adjusted-wt) in two calculations. We calculated the MDRD equation based on 4 (MDRD-4) and 6 variables (MDRD-6). The performance of these equations was assessed by different ways including Spearman correlation, bias (difference between estimated GFR and CrCl_24h-urine), precision (standard deviation of bias), and Bland-Altman plot analysis. Results. The cohort consisted of 237 patients (age 45 ± 20 years, males 75%, mechanically ventilated 99% with serum creatinine 101 ± 94 µmol/L and CrCl_24h-urine 108 ± 69 ml/min/1.73 m²). The correlations between the different equations and CrCl_24h-urine were modest (r: 0.62 to 0.79; p < 0.0001). Bias was statistically significant for CG_actual-wt (21 ml/min), CG_adjusted-wt (12 ml/min), and MDRD-6 (-10 ml/min) equations. Precision ranged from 46 to 54 ml/min. The sensitivity of equations to correctly classify CrCl_24h-urine 30-59.9 ml/min/1.73 m² was 17.2% for CG_actual-wt, 30.0% for CG_ideal-wt, 31.0% for CG_adjusted-wt, 31.0% for MDRD-4, 39.1% for MDRD-6, 13.8% for CKD-EPI, and 34.5% for Jelliffe equation. Conclusions. Commonly used GFR-estimating equations had limited ability to properly estimate CrCl_24h-urine and to correctly classify GFR into clinically relevant ranges that usually determine dosing of medications.

Figure 1

Mean values of the measured creatinine clearance by 24-hour urine collection (CrCl_24h-urine) and estimated glomerular filtration rate by different equations. The first 237 24-hour urine samples were used in this analysis. Error bars represent 95% confidence interval. The difference between the different methods was significant (p < 0.001) by ANOVA test. CG: Cockroft–Gault equation; MDRD-4: 4-variable modification of diet in renal disease equation; MDRD-6: 6-variable modification of diet in renal disease equation, and CKD-EPI: chronic kidney disease epidemiology collaboration equation.

Figure 2

Bland–Altman plot of the creatinine clearance measured by 24-hour urine collection (CrCl_24h-urine) versus the equations estimating glomerular filtration rate. (a) Cockroft–Gault equation using actual body weight; (b) Cockroft–Gault equation using ideal body weight; (c) Cockroft–Gault equation using adjusted body weight; (d) 4-variable modification of diet in renal disease equation; (e) 6-variable modification of diet in renal disease equation; (f) chronic kidney disease epidemiology collaboration equation; and (g) Jelliffe equation. The X-axis represents the difference between CrCl_24h-urine and the equation estimating glomerular filtration rate. The Y-axis represents the mean of CrCl_24h-urine and the equation estimating glomerular filtration rate. The solid line represents the bias (mean difference obtained across the range of values), whereas the dashed lines are the limits of agreement (±1.96 × standard deviation).