- Confirm the patient is 1 to 25 years old and the serum creatinine assay is IDMS-traceable (modern standard). Outside this age window, use the adult CKD-EPI 2021 eGFR instead.
- Enter age (years), sex, standing height (cm or in), and serum creatinine (mg/dL or µmol/L). Optional: serum cystatin C (mg/L) is offered for users who wish to cross-check (see disclaimer).
- The CKiD U25 creatinine eGFR uses an age- and sex-dependent coefficient k(age, sex): eGFRcr = k × (height_m ÷ SCr_mg/dL). The bedside Schwartz approximation (0.413 × height/SCr) is shown alongside as a sanity check.
- Use the eGFR with the KDIGO pediatric CKD G-stage below to track progression — but always interpret in clinical context (acute illness, hydration, muscle mass, growth, puberty).
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When to Use
Use the CKiD U25 equation whenever you need an age- and sex-aware eGFR for a patient 1 to 25 years old, especially across the pediatric-to-adult transition. It was developed and internally validated in the CKiD cohort against iohexol-measured GFR, and outperforms the legacy "bedside Schwartz" by removing systematic bias as children age and as biological sex begins to influence creatinine production around puberty.
Appropriate population
Children, adolescents, and young adults aged 1–25 years with known or suspected CKD (or a clinical need for a creatinine-based GFR estimate) and an IDMS-traceable serum creatinine. Particularly useful in pediatric nephrology clinics, in transplant follow-up, and during the pediatric-to-adult transition, because the same equation can be applied longitudinally as the patient grows into adulthood — avoiding the abrupt "Schwartz → CKD-EPI" step that often creates an artefactual GFR jump at age 18.
When NOT to rely on it
Do not use the CKiD U25 equation in infants under 1 year (no validated data), in acutely changing kidney function (AKI, dialysis transitions, rapidly changing fluid status, or non-steady-state creatinine), or in extremes of body habitus (severe wasting, amputation, marked muscle hypertrophy or hypotrophy from neuromuscular disease), where a creatinine-based eGFR is unreliable. In adults > 25 years, the adult CKD-EPI 2021 equation is preferred. Always interpret the number in clinical context: hydration, illness, growth velocity, and pubertal status.
Pearls & Pitfalls
Why U25 is "smoother" than Schwartz
The bedside Schwartz equation (eGFR = 0.413 × height/SCr) uses a single constant for everyone under 18. The CKiD U25 equation replaces that flat constant with an age- and sex-dependent k that rises through childhood, dips slightly at puberty, then plateaus into early adulthood — much closer to true biology. The result is far less age-related bias as a child grows up, and a continuous estimate that does not jump artefactually when the patient crosses an arbitrary age cut-off.
Bridging into adulthood
For the same young adult, the U25 equation typically gives a result that is concordant with the adult CKD-EPI 2021 eGFR around age 22–25, allowing a smoother transition handover. Switching abruptly from "bedside Schwartz" to CKD-EPI at age 18, by contrast, often creates a step change of 10–20 mL/min/1.73m² with no real change in kidney function — a common source of confusion in transition clinics. U25 is designed to be the same equation used continuously from age 1 to 25.
Pitfalls
(1) Requires an IDMS-traceable creatinine (modern enzymatic / standardised Jaffe). Non-standardised assays will systematically bias the result. (2) Acute kidney injury, dialysis, and rapidly changing creatinine all violate the steady-state assumption — eGFR is unreliable until creatinine has stabilised over 24–48 h. (3) Markedly low muscle mass (severe wasting, neuromuscular disease, post-amputation) inflates eGFR by lowering creatinine independent of GFR; in those patients consider a measured GFR or a cystatin-C-based estimate. (4) Cystatin C estimates are influenced by inflammation, thyroid status, and corticosteroids and should be interpreted with care. (5) Never confuse eGFR with a diagnosis of CKD. KDIGO requires ≥ 3 months of an abnormality of kidney structure or function to define CKD; an isolated low eGFR in an acutely ill child is not CKD.
Why Use It
Pediatric and adolescent eGFR has long been a difficult moving target. The 2009 bedside Schwartz equation (k = 0.413) was a major simplification but is well known to systematically over- or under-estimate GFR as children age, particularly through puberty. The CKiD U25 equation (Pierce 2021), derived from the same CKiD cohort with iohexol-measured GFR, addresses this by making the Schwartz "k" a continuous function of age and sex — producing a more accurate estimate from age 1 through 25 and, crucially, a smooth handover into the adult CKD-EPI 2021 equation. Using it consistently in pediatric and transition-age patients reduces apparent eGFR jumps that are equation artefacts rather than real changes in kidney function, and supports better KDIGO pediatric CKD staging and progression tracking.
CKiD U25 eGFR — Pediatric / Adolescent eGFR (Pierce 2021)
Enter age (1–25 yr), sex, height, and serum creatinine. The CKiD U25 creatinine-based eGFR uses an age- and sex-dependent k, with the bedside Schwartz approximation shown as a cross-check. Cystatin C is offered as an optional informational input (see disclaimer).
⚕ CKiD U25 creatinine equation, Pierce CB et al., Kidney Int. 2021;99(4):948–956. The creatinine-based eGFR shown above uses the published age- and sex-dependent k(age, sex) for ages 1–25 with height in metres and creatinine in mg/dL (eGFRcr = k × height_m / SCr_mg/dL). The bedside Schwartz value (eGFR = 0.413 × height_cm / SCr_mg/dL) is provided as a sanity check only. Cystatin C eGFR is not computed here because the full piecewise cystatin C coefficients were not independently re-verified by us at build time — for an authoritative cystatin-C or combined CKiD U25 (cr-cys) estimate, please use the official tool at NIDDK pediatric eGFR calculators or the MDCalc CKiD U25 calculator. eGFR is unreliable in AKI, dialysis, and non-steady-state creatinine. For licensed clinicians; not a substitute for individualised pediatric assessment.
Next Steps
Use the eGFR with the KDIGO pediatric CKD staging table to plan management and follow-up.
- eGFR ≥ 90 (G1) or 60–89 (G2): low-stage. Confirm chronicity (≥ 3 months of an abnormality of kidney structure or function) before labelling as CKD. Check first-morning urine albumin-to-creatinine ratio, blood pressure, and a renal ultrasound; treat underlying causes (CAKUT, glomerular disease, reflux, obstruction).
- eGFR 45–59 (G3a) or 30–44 (G3b): moderately reduced. Pediatric nephrology referral is essential. Optimise blood-pressure control (ACEi/ARB if proteinuric), monitor growth and bone-mineral metabolism (Ca, PO₄, PTH, 25-OH-D), screen for anaemia, and address acidosis and hyperkalaemia.
- eGFR 15–29 (G4): severely reduced. Coordinate transplant-first planning, vaccination catch-up, and dialysis preparation if indicated. Pay attention to growth, nutrition, and the psychosocial transition to adult care.
- eGFR < 15 (G5): kidney failure. Initiate kidney-replacement therapy planning (pre-emptive transplant strongly preferred in children) and ensure structured pediatric-to-adult transition before age 21–25.
- Track progression by re-using the same equation longitudinally — switching equations mid-follow-up creates artefactual jumps. Pair with the bedside Schwartz and pediatric eGFR comparison tools when reviewing historical labs.
Evidence & References
Formula (Pierce 2021)
| Quantity | Formula |
|---|---|
| CKiD U25 eGFRcr (mL/min/1.73 m²) | k(age, sex) × (height_metres ÷ SCr_mg/dL) |
| Bedside Schwartz cross-check | 0.413 × height_cm ÷ SCr_mg/dL |
| Creatinine unit conversion | SCr_mg/dL = SCr_µmol/L ÷ 88.4 |
| Height unit conversion | height_cm = height_in × 2.54; height_m = height_cm ÷ 100 |
k(age, sex) — creatinine equation (Pierce 2021, Table 2)
| Sex | Age range (yr) | k |
|---|---|---|
| Male | 1 to < 12 | 39.0 × 1.008(age − 12) |
| Male | 12 to < 18 | 39.0 × 1.045(age − 12) |
| Male | 18 to 25 | 50.8 (constant) |
| Female | 1 to < 12 | 36.1 × 1.008(age − 12) |
| Female | 12 to < 18 | 36.1 × 1.023(age − 12) |
| Female | 18 to 25 | 41.4 (constant) |
KDIGO pediatric CKD G-stage (by eGFR)
| Stage | eGFR (mL/min/1.73 m²) | Description |
|---|---|---|
| G1 | ≥ 90 | Normal or high |
| G2 | 60 – 89 | Mildly decreased |
| G3a | 45 – 59 | Mildly to moderately decreased |
| G3b | 30 – 44 | Moderately to severely decreased |
| G4 | 15 – 29 | Severely decreased |
| G5 | < 15 | Kidney failure |
Pierce and colleagues used the longitudinal Chronic Kidney Disease in Children (CKiD) cohort with iohexol-measured GFR to derive a continuous, age- and sex-dependent k for use from age 1 to 25. In internal validation it outperformed bedside Schwartz and several adult equations across the pediatric-to-adult transition. The cystatin C and combined creatinine-cystatin C variants are described in the same paper but are not computed by this tool — see the official NIDDK or MDCalc calculators for those.
References
- 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;99(4):948–956. doi:10.1016/j.kint.2020.10.047.
- Schwartz GJ, Muñoz A, Schneider MF, et al. New equations to estimate GFR in children with CKD. J Am Soc Nephrol. 2009;20(3):629–637. doi:10.1681/ASN.2008030287.
- Kidney Disease: Improving Global Outcomes (KDIGO). 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2024;105(4S):S117–S314.
