Limitations of U25 CKiD and CKD-EPI eGFR formulae in patients 2-20 years of age with measured GFR > 60 mL/min/1.73 m-a cross-sectional study.

Background: When applying Pierce U25 formula for estimating glomerular filtration rate (eGFR), we observed a higher proportion of eGFR < 90 mL/min/1.73 m (chronic kidney disease (CKD) stage 2). We compared agreement and accuracy of the Pierce U25 (ages 2-25), Pottel (ages 2-100), and CKD-EPI (ages 18-100) formulae to GFR measurements.

Methods: Post hoc analysis of the three eGFRs compared to 367 m technetium-diethylene-triamine penta-acetic acid (Tc DTPA) GFR measurements (240 patients) using 3 sampling points and Brockner/Mørtensen correction (body surface area calculation based on ideal weight) on simultaneous serum creatinine and cystatin C measurements.

Results: Overall, the U25 formula performed well with a Spearman r of 0.8102 (95% confidence interval 0.7706 to 0.8435, p < 0.0001) while diagnostic accuracy was low in patients with normal mGFR. The U25 formula reclassified 29.5% of patients with normal mGFR as CKD stage 2; whereas the average of the modified Schwartz formula based on serum creatinine and the Filler formula based on cystatin C, only over-diagnosed CKD stage 2 in 8.5%, 24.5% within 10% and 62.7% within 30%. We therefore combined both. The average Schwartz/Filler eGFR had 36.5% of results within 10%, 84.7% within 30%, and normal mGFR accuracy was 26.8%, 63.9% for 10% and 30%, respectively, outperforming the CKD-EPI and Pottel formulae.

Conclusions: The Pierce U25 formula results correlated well with mGFR < 75 mL/min/1.73 m. Over the entire GFR range, accuracy was better for patients with a higher mGFR, when averaging the combined Schwartz/Filler formulae. More work is needed to prospectively confirm our findings in other centers.