Chronic Kidney Disease in Children (CKiD) U25 eGFR Calculator

Introduction

The CKiD U25 equations estimate kidney function (glomerular filtration rate, or GFR) in children, teenagers, and young adults aged 1 to 25. They were developed by researchers from the Chronic Kidney Disease in Children (CKiD) study, led by Christopher Pierce and colleagues, and published in 2021. They update the widely used 2009 “bedside Schwartz” equation. According to the study team, the data used to develop the bedside equation came mainly from children aged 8 to 15, with too few young children under five and young adults 18 and over. The U25 equations fix this by allowing the calculation to change with age and sex. They are supported for clinical use by the current 2024 KDIGO guidelines.

How it’s calculated

There are three equations, depending on which labs are available. Each uses a factor, K, that changes with the patient’s age and sex.

• The creatinine-based estimate requires sex, age, height and serum creatinine

• The cystatin C-based estimate requires sex, age, and cystatin C

• The combined estimate requires sex, age, height, serum creatinine and cystatin C.

Results appear based on selection of the equation desired.

Results

1. CKiD U25 Creatinine: eGFR = k × (height, m / sCr, mg/dL)

   1. For males, k is calculated as:

      1. Age 1 to <12 years old: 39.0 × 1.008(age-12)

      2. Age 12 to <18 years old: 39.0 × 1.045(age-12)

      3. Age 18 to 25 years old: 50.8

   2. For females, k is calculated as:

      1. Age 1 to <12 years old: 36.1 × 1.008(age-12)

      2. Age 12 to <18 years old: 36.1 × 1.023(age-12)

      3. Age 18 to 25 years old: 41.4

2. CKiD U25 Cystatin C: eGFR = k × (1 / cysC, mg/L)

   1. For males, k is calculated as:

      1. Age 1 to <15 years old: 87.2 × 1.011(age-15)

      2. Age 15 to <18 years old: 87.2 × 0.96(age-15)

      3. Age 18 to 25 years old: 77.1

   2. For females, k is calculated as:

      1. Age 1 to <12 years old: 79.9 × 1.004(age-12)

      2. Age 12 to <18 years old: 79.9 × 0.974(age-12)

      3. Age 18 to 25 years old: 68.3

3. CKiD U25 Creatinine-Cystatin C: eGFR = (sCr-based eGFR + cycC-based eGFR) / 2

Management Considerations

• Using both creatinine and cystatin C may provide a more accurate GFR estimate as it averages the results from both equations.

• Serum creatinine levels can be influenced by malnutrition, decreased muscle mass, and certain medications. Cystatin C may be preferable in such cases.

• May underestimate GFR in patients with normal renal function, particularily for cystatin C equations.

• Refer to the KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease (CKD) for management of CKD.

Pierce CB, Muñoz A, Ng DK, Warady BA, Furth SL, Schwartz GJ. Age- and sex-dependent clinical equations to estimate glomerular filtration rates in children and young adults with chronic kidney disease. Kidney Int. 2021 Apr;99(4):948-956.