Accuracy of estimating equations for the assessment of glomerular filtration rate in critically ill patients versus outpatients.

Background: Estimating equations for the assessment of glomerular filtration rate (GFR) have been poorly investigated in the critical care setting. We evaluated the agreement between the GFR measured with ⁵¹CrEDTA/iohexol (mGFR) and four estimating equations based on serum concentrations of creatine and/or cystatin C (eGFR) in two cohorts: critically ill patients and outpatients with normal-to-moderately reduced GFR.

Methods: Forty-three patients in the critical care group and 48 patients in the outpatient group were included. GFR was measured (mGFR) by plasma infusion clearance of ⁵¹Cr-EDTA/iohexol (critical care group) and the single injection, one-sample plasma ⁵¹Cr-EDTA clearance technique (outpatients). The following estimating equations (eGFR) were used: the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation for creatinine (CKD-EPI_Cr), cystatin C (CKD-EPI_{Cys C}), creatinine+cystatin C (CKD-EPI_{Cr + Cys C}) and the Lund-Malmö creatinine+cystatin C equation (LM_{Cr + Cys C}). Agreement between mGFR and eGFR was assessed by the Bland-Altman method and accuracy by calculating P30 and P10.

Results: In the critically ill group, the bias between the estimating equations and mGFR was -3.6 to 2.8 mL/min/1.73 m², while the error was 121%-127% and the accuracy (P30) 33%-40%. In the outpatients, the bias between the estimating equations and mGFR was -13.0 to 7.6 mL/min/1.73 m², while the error was 31%-41% and the accuracy (P30), 67%-96%.

Conclusions: All four equations performed poorly in assessing GFR in the critically ill cohort with an unacceptably high error and low accuracy in contrast to the outpatient group. To accurately assess GFR in critically ill patients, GFR must be measured not estimated.

Editorial comment: For the assessment of glomerular filtration rate (GFR), it can be measured directly, but is frequently estimated using a point measure of serum creatinine concentration. In this study, ICU case GFR estimations, by different adjusted equations, done also for a cohort of outpatients, showed that these serum creatinine-based estimations for ICU cases are not highly precise or reliable.