Comparison of different equations to assess glomerular filtration in critically ill patients.

PURPOSE: To evaluate equations for estimation of glomerular filtration rate (GFR) and measured urinary creatinine clearance, compared to measured GFR in critically ill patients.
METHODS: GFR was measured using inulin clearance. Multiple blood samples were collected per patient for determination of serum creatinine, cystatin C and inulin. GFR was estimated by the use of the following estimation equations (eGFR): four commonly used creatinine-based equations [Cockcroft-Gault, Modification of Diet in Renal Disease (both the short and long formula) and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI)], five cystatin C based estimation equations (Hoek, Larsson, Filler, Le Bricon, CKD-EPIcys) and one equation combining cystatin C and serum creatinine (CKD-EPIcr-cys). In addition we measured urinary creatinine clearance. Bias, precision and accuracy of all estimates were compared to those of the inulin clearance.
RESULTS: Data were collected from 83 patients, of whom 68 were considered evaluable. The median age was 58 years [interquartile range (IQR) 39-68]. The median inulin clearance was 80 mL/min/1.73 m(2) (IQR 31-114). Equations based on creatinine had much bias and poor precision and accuracy. Measured urinary creatinine clearances overestimated GFR. Equations based on cystatin C were free of bias, but also had limited precision and accuracy.
CONCLUSIONS: In this cohort of patients, estimates of GFR had low accuracy and precision. Cystatin C based formulas, especially CKD-EPIcr-cys, showed limited bias; however, the accuracy and precision of these estimates were still insufficient. Measured urinary creatinine clearance overestimates GFR, but may provide a cheap alternative, when this is taken into account.