Evaluation of the creatinine-based chronic kidney disease in children (under 25 years) equation in healthy children and adolescents

Background: The Chronic Kidney Disease in children (CKiD) equation to estimate glomerular filtration rate eGFR = k × Ht / SCr (Ht = height in cm, SCr = serum creatinine in mg/dL), with fixed k = 0.413, has recently been optimized by introducing age/sex dependent k-values valid for young children up to young adults (the CKiD Under 25 years (CKiDU25) equation). Although the CKiD equation was designed for children with chronic kidney disease (CKD), this equation found common use both clinically and in research, and also in children without CKD. This report aims to evaluate properties of CKiD and CKiDU25 in healthy children and adolescents. Method: Sex-specific metadata (height and creatinine) for healthy children were obtained from national growth curves and creatinine versus age curves. These data were used to calculate average CKiD and CKiDU25 values for each year of age and compare them against age-independent measured GFR of 107 mL/min/1.73 m2. Results: The CKiD estimations show a steep decline with age (1.5 mL/min/1.73 m2/year in females and 2.0 mL/min/1.73 m2/year in males) over the entire age range (2–20 years) and large differences between adolescent males and females. Due to the age/sex specific k-values, the CKiDU25 equation solves this age-decline artifact of the CKiD equation. However, CKiDU25 still shows a systematic higher estimation of about 10% in healthy males compared to females over the entire age range. Conclusion: Although the CKiDU25 shows major improvements compared to the CKiD equation, as the unexpected age decline has been removed, a systematic difference is still observed between healthy males and females