How is the glomerular filtration rate (GFR) calculated in the workup for chronic kidney disease (CKD) in children?

Updated: Jul 21, 2020
  • Author: Sanjeev Gulati, MD, MBBS, DNB(Peds), DM, DNB(Neph), FIPN(Australia), FICN, FRCPC(Canada); Chief Editor: Craig B Langman, MD  more...
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Answer

The glomerular filtration rate (GFR) is equal to the sum of the filtration rates in all of the functioning nephrons; thus, estimation of the GFR gives a rough measure of the number of functioning nephrons. A reduction in GFR implies progression of the underlying disease.

The Kidney Disease Outcomes Quality Initiative (KDOQI) guidelines state that estimates of GFR are the best overall indices of the level of kidney function. [4] The reference range of GFR in young adults is 120-130 mL/min per 1.73 m2. However, the reference range of estimated GFR (eGFR) is much lower in early infancy, even when corrected for body surface area, and subsequently increases in relationship to body size for as long as 2 years. Hence, the eGFR ranges that are used to define the 5 CKD stages apply only to children aged 2 years and older (see Staging). The eGFR can be estimated from the constant k, plasma creatinine concentration (PCr) (in mg/dL), and body length (L) (in cm) according to the Schwartz formula, as follows:

  • GFR = (k X L) / PCr

The value of k is different at different ages:

  • k = 0.4 for preterm infants),

  • k = 0.45 for full-term infants

  • k = 0.55 for those aged 2-12 years in children and adolescent girls

  • k = 0.7 years in adolescent boys

Therefore, all children with chronic kidney disease should have an eGFR calculated. This should be calculated from the Schwartz (or Counahan-Barratt prediction) equation in children, because it is convenient, reasonably precise, and practical. The constants used in the equations differ slightly, likely related to the different assays to measure creatinine.

Creatinine clearance estimates are difficult and imprecise, because they require 24-hour urine collections, which may be incomplete for various reasons. It is a known fact that estimation of GFR or creatinine clearance from serum creatinine critically depends on calibration of the serum creatinine assay, specific to the expected lower levels found in children without chronic kidney disease.

For young adults, the Chronic Kidney Disease in Children study (CKiD) formula underestimates eGFR, whereas the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula overestimates eGFR. Averaging results from the 2 formulas provided an eGFR similar to an iohexol GFR. Cystatin C–based equations do not outperform the creatinine-based formulas in the general population and are therefore not recommended for routine assessment of kidney function but may be considered for special clinical situations in which patients have reduced muscle mass. [18]


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