How this instrument works
Estimated glomerular filtration rate approximates how many millilitres of blood a child's kidneys clear each minute, standardized to a body surface area of 1.73 square metres. In children this is estimated far more simply than in adults: height in centimetres, divided by serum creatinine in mg/dL, multiplied by a single constant. That constant is 0.413, and this instrument is nothing more than that one division and one multiplication.
The formula in current use is the 2009 'bedside Schwartz' equation, published by Schwartz, Muñoz, Schneider and colleagues in the Journal of the American Society of Nephrology. It replaced an earlier 1976 version by Schwartz, Haycock, Edelmann and Spitzer, which used different, age- and sex-dependent constants — a k of about 0.55 for most children, higher for adolescent boys. That original formula grew systematically less accurate through the 2000s not because the underlying kidney physiology changed, but because clinical labs shifted from the older Jaffe colorimetric method for measuring creatinine to modern enzymatic assays, which read somewhat lower for the same blood sample. The 2009 recalibration re-derived a single constant against creatinine measured the modern way, against filtration measured directly by iohexol clearance in a cohort of children with chronic kidney disease.
This is the current standard, not a timeless one. Height and creatinine alone cannot capture muscle mass, acute illness, or hydration, all of which shift creatinine without necessarily reflecting a real change in filtration, and the formula was derived in children who already had reduced kidney function, so its accuracy in a child with entirely normal kidneys is less firmly established. Whichever assay a given lab reports creatinine from also matters, since the 0.413 constant was fitted to the enzymatic method — a result computed from an old-style Jaffe value can run off in the same direction the 1976 formula eventually did.
- Enter Height in centimetres.
- Enter Serum creatinine in mg/dL from a recent blood draw, ideally from an enzymatic assay.
- Read eGFR in mL/min/1.73m², and compare it against normal ranges for the child's age.
- Recheck the figure whenever a new creatinine value or an updated height measurement is available.
Worked example — 100 cm, creatinine 0.4 mg/dL
A child measuring 100 cm has a serum creatinine of 0.4 mg/dL. Multiply height by the constant: 0.413 × 100 = 41.3. Divide by creatinine: 41.3 ÷ 0.4 = 103.3 mL/min/1.73m², comfortably within the normal range for a child that size.
A taller 130 cm child with a slightly higher creatinine of 0.6 mg/dL works out to 0.413 × 130 = 53.69, divided by 0.6, for about 89.5 mL/min/1.73m² — still a normal reading. Contrast that against an 80 cm child with a creatinine of 0.9 mg/dL: 0.413 × 80 = 33.04, divided by 0.9, gives only about 36.7 mL/min/1.73m², a figure low enough to flag reduced kidney function for a child of that height and prompt further evaluation rather than a routine follow-up.
Questions
Why did the bedside Schwartz formula change from its 1976 version?
The original 1976 formula used age- and sex-dependent constants fitted to creatinine measured by the older Jaffe colorimetric assay. When clinical laboratories broadly shifted to modern enzymatic creatinine assays through the 2000s, which read somewhat differently for the same sample, the 1976 constants stopped matching real filtration rates. The 2009 bedside Schwartz equation re-derived a single constant, 0.413, against creatinine measured the modern way and filtration measured directly by iohexol clearance, restoring accuracy for current lab methods.
Is this the same equation used for adults?
No. Adult kidney function on this site is estimated with the 2021 CKD-EPI equation, a more complex formula using age, sex, and a piecewise exponent on creatinine, fitted to adult physiology. Height-based bedside Schwartz is specific to children and adolescents, whose creatinine production scales more directly with height than with the age and sex terms an adult formula relies on.
What counts as a normal eGFR for a child?
Roughly 90 mL/min/1.73m² and above is considered normal for most children past early infancy, with values below that warranting closer review of trend and clinical context. Very young infants normally run lower than this, since kidney filtration is still maturing over the first two years of life, so age-appropriate reference ranges matter more than a single adult-style cutoff.
Does creatinine assay type actually change the result meaningfully?
Yes, potentially by a clinically relevant margin. The 0.413 constant was fitted specifically to enzymatic creatinine assays; a value drawn from an older Jaffe-method assay tends to read higher for the same true filtration rate, which would push the calculated eGFR down. Knowing which assay a lab uses matters when interpreting a borderline result.
How accurate is bedside Schwartz for a specific child?
In its original validation against directly measured iohexol clearance, the formula performed well for children with chronic kidney disease, which is the population it was built and tested in. Height and creatinine alone cannot see muscle mass, hydration, or acute illness, so results in a child with normal kidneys, or during an acute illness, carry more uncertainty than the validation cohort suggests.
What should happen after a low eGFR result?
A single low reading calls for clinical correlation, not an immediate diagnosis — repeat testing, a check of hydration and recent illness, and often a urine study are typical next steps before chronic kidney disease is confirmed. Sustained reduced filtration over time, alongside other markers, is what actually establishes a diagnosis, not one calculated number.
References
- Schwartz GJ, Muñoz A, Schneider MF, et al., 2009, J Am Soc Nephrol — bedside Schwartz equation (PubMed)
- Schwartz GJ, Haycock GB, Edelmann CM Jr, Spitzer A, 1976, Pediatrics — original Schwartz formula (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.