Pediatric GFR Calculator
Enter a child's height, serum creatinine, age group, and sex to estimate glomerular filtration rate (eGFR) using both the original Schwartz (1976) formula and the revised bedside Schwartz (2009) formula calibrated for IDMS-traceable creatinine assays. Results include the KDIGO 2012 CKD stage, a worked calculation, and plain-language interpretation.
How pediatric GFR is estimated
Glomerular filtration rate (GFR) is the volume of fluid filtered by the kidneys per minute per 1.73 m² of body surface area. Direct measurement requires timed urine collections or plasma disappearance of exogenous markers (iohexol, inulin), which are impractical in routine care. Instead, clinicians use height-based creatinine equations validated in children. The Schwartz formula, originally published in 1976, exploits the observation that, in growing children, body muscle mass and therefore creatinine production scales closely with height. The equation is eGFR = K x height (cm) / serum creatinine (mg/dL), where K is a constant that varies by age and sex to account for differences in muscle mass. The 2009 CKiD bedside revision simplified the coefficient to a single value of 0.413 for all children aged 1 to 16 years, because creatinine assays shifted from Jaffe-based to isotope dilution mass spectrometry (IDMS)-traceable methods, which produce systematically lower creatinine values.
Schwartz 1976 vs. Schwartz 2009 - which to use?
The key practical distinction is the creatinine assay used by your laboratory. The original 1976 equation was validated against Jaffe assays, which tend to report higher creatinine values due to non-creatinine chromogens. Modern IDMS-calibrated assays, now standard in most clinical laboratories, produce creatinine values that are roughly 10-20 percent lower. Using the 1976 formula with an IDMS-traceable creatinine overestimates GFR by 20-40 percent. The 2009 revised bedside Schwartz (K = 0.413) was calibrated specifically against measured GFR (iohexol plasma disappearance) in the CKiD cohort using IDMS-traceable creatinine, making it the preferred formula when your lab uses a modern assay. If you are uncertain which assay your laboratory uses, contact the laboratory or review the assay method listed on the report. Both estimates are shown side by side here so you can apply the most appropriate one.
K coefficient values in the original Schwartz formula
The age and sex-specific K constants used in the 1976 Schwartz formula reflect differences in muscle mass at different stages of development. Preterm infants, who have very low muscle mass relative to body size, use K = 0.33. Term infants up to one year of age use K = 0.45. Children from one to thirteen years use K = 0.55 regardless of sex, because muscle mass differences between boys and girls are small before puberty. Adolescent males use K = 0.70, reflecting the disproportionate gain in muscle mass during male puberty; adolescent females remain at K = 0.55. Selecting the correct group is important: using K = 0.70 for a pre-adolescent child overestimates GFR by about 27 percent.
CKD staging and when to refer
The KDIGO 2012 Clinical Practice Guideline uses GFR categories G1 through G5 to describe the degree of kidney function loss. In children, GFR physiologically rises from birth (around 40 mL/min/1.73 m² in term newborns) to near-adult values (90-120 mL/min/1.73 m²) by age two. For this reason, GFR values that fall within the adult reference range are reassuring for children over two years of age, but mildly low values in infants must be interpreted against age-specific norms. CKD is defined by a GFR below 60 mL/min/1.73 m² persisting for more than 90 days, or by structural or functional kidney abnormalities even with a preserved GFR. A single eGFR estimate is therefore not sufficient to diagnose CKD; repeat measurement and assessment of albuminuria, blood pressure, and imaging findings are also needed. Nephrology referral is generally recommended at G3b or below, and is urgent at G4-G5.
KDIGO 2012 GFR Categories for CKD Staging
| Stage | eGFR (mL/min/1.73 m²) | Description | Action |
|---|---|---|---|
| G1 | >= 90 | Normal or high | Monitor if risk factors present |
| G2 | 60-89 | Mildly decreased | Monitor; diagnose CKD only with other markers |
| G3a | 45-59 | Mildly to moderately decreased | Evaluate and manage complications |
| G3b | 30-44 | Moderately to severely decreased | Nephrology referral recommended |
| G4 | 15-29 | Severely decreased | Nephrology referral; prepare for RRT |
| G5 | < 15 | Kidney failure | Renal replacement therapy if uraemic |
Applied to pediatric patients using eGFR in mL/min/1.73 m². The same GFR thresholds are used for children and adults.
Frequently asked questions
What is the Schwartz formula and why is it used in children?
The Schwartz formula estimates glomerular filtration rate from height and serum creatinine. It works in children because, during growth, a child's muscle mass and therefore creatinine production scales proportionally with height. Adult creatinine-based equations (MDRD, CKD-EPI) assume adult body composition and are not validated for pediatric patients. The Schwartz formula has been the standard pediatric GFR estimate since its publication in 1976.
Should I use the 1976 or the 2009 Schwartz formula?
Use the 2009 revised bedside formula (K = 0.413) if your laboratory uses an IDMS-calibrated creatinine assay, which is standard in most modern clinical laboratories. Use the original 1976 formula with the appropriate K coefficient if the creatinine was measured by an older Jaffe method. When in doubt, check with your laboratory. Using the 1976 formula with an IDMS-calibrated creatinine significantly overestimates GFR and can lead to underestimation of kidney disease severity.
What is a normal GFR for a child?
GFR in term newborns starts at around 40 mL/min/1.73 m² and rises rapidly to near-adult levels of 90-120 mL/min/1.73 m² by age two. From age two onwards, values above 90 mL/min/1.73 m² are considered normal. Values below 60 mL/min/1.73 m² on at least two measurements 90 days apart, along with markers of kidney damage, meet the KDIGO definition of chronic kidney disease.
Can this calculator be used for newborns?
The 2009 revised formula was validated in children aged 1 to 16 years and should not be applied to newborns. Neonatal GFR is highly variable and depends on gestational age, postnatal age, and kidney maturation. The preterm K value of 0.33 in the 1976 formula is used for premature infants, but neonatal GFR interpretation requires specialist guidance, as normal ranges are age-specific and differ substantially from older children.
What is the difference between eGFR and measured GFR?
Measured GFR (mGFR) uses exogenous filtration markers such as iohexol or inulin and is the gold standard, but it requires infusion, timed blood sampling, and specialized laboratory analysis. Estimated GFR (eGFR) uses serum creatinine and height to approximate mGFR using population-derived equations. eGFR is much more practical for routine monitoring but can be affected by creatinine assay calibration, muscle mass abnormalities (malnutrition, muscle disease), and rapid changes in kidney function. When precision is essential, for example before nephrotoxic chemotherapy or in living kidney donor evaluation, mGFR should be measured directly.
Does sex affect the eGFR result in children?
Sex only affects the result through the K coefficient in the original 1976 formula, and only for adolescents. Adolescent males are assigned K = 0.70 because testosterone-driven muscle development substantially increases creatinine production relative to height. Adolescent females and all younger children use K = 0.55. The 2009 revised formula uses a single K of 0.413 for all children aged 1 to 16, because the CKiD study did not find a statistically significant sex difference in the IDMS-calibrated cohort.