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

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/m(2), 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 m(2)). 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 m(2) 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.