Average creatinine-urea clearance: Revival of an old analytical technique?

Background Creatinine-based equations such as Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) are recommended for estimating GFR (eGFR) in clinical practice but have reduced performance in advanced stages of CKD. However, rare studies have evaluated the performance of eGFR by measuring the average of the urinary clearances of creatinine and urea (mClUN-cr) compared to the eGFR equations. Methods This cross-sectional study evaluated the usefulness of mClUN-cr in a population of 855 participants who performed a GFR measurement by urinary inulin clearance. The performance of mClUN-cr was compared with those of CKD-EPI 2009, CKD-EPI 2021, considering three criteria: bias, precision, and accuracy. Results In the whole sample, the mClUN-cr performed similarly to CKD-EPI equations (2009 and 2021) (precision: 11.5 [95% CI: 10.5; 12.5] vs. 19.0 [95% CI: 17.2; 20.1] and 19.1 [95% CI:17.4; 20.4]), and accuracy P30: 97.0 [95% CI: 95.8; 98.0] vs. 82.0 [95% CI: 79.2; 84.4] and 77.2 [95% CI: 74.5; 80.0]). The CKD-EPI equations (2009 and 2021) had the best performance when mGFR > 60 mL/min/1.73m2. In contrast, the mClUN-cr performed better than others, with lowest mGFR values, more noticeable when mGFR < 60 mL/min/1.73m2. Conclusions The study described the best performance of mClUN-cr at GFR levels below 60 mL/min/1.73m2 and a satisfactory result in the overall cohort. The findings point to a role of this tool, especially for estimating GFR in Chronic Kidney Disease patients in developing countries, when reference measurement of GFR is not available.