Schwartz Formula for Estimating GFR in Children
Recommended Formula
The revised "Bedside Schwartz equation" is the recommended formula for estimating GFR in children aged 1-16 years: GFR (mL/min per 1.73 m²) = 0.413 × [height (cm) / creatinine (mg/dL)] or GFR (mL/min per 1.73 m²) = 36.5 × [height (cm) / creatinine (μmol/L)]. 1, 2, 3
This revised formula requires enzymatic creatinine assays that are traceable to isotope-dilution mass spectrometry (IDMS) reference standards, not the older Jaffe-based methods. 1
Age-Specific Constants for the Original Schwartz Formula
If using the older Schwartz formula with Jaffe creatinine methods, different constants (k values) apply based on age and sex: 4
- Preterm infants: k = 0.33 4
- Full-term infants (first year): k = 0.45 5, 4
- Children and adolescent girls: k = 0.55 4
- Adolescent boys: k = 0.70 4
The original formula is: GFR (mL/min per 1.73 m²) = k × [height (cm) / creatinine (mg/dL)] 1
Critical Implementation Requirements
Enzymatic creatinine assays must be used in pediatric populations rather than Jaffe methods because non-creatinine chromogens contribute disproportionately at the low creatinine values seen in children, leading to significant measurement errors. 1, 2, 6
Accurate height measurement is essential for calculation, which can be challenging in critically ill or ventilated patients. 3, 6
Important Limitations and Caveats
The Schwartz formula overestimates GFR, particularly at lower GFR levels and in children with less renal impairment. 1, 2, 7 Research demonstrates the revised Schwartz formula is accurate until a height/creatinine ratio of 251 (corresponding to GFR ~103 mL/min per 1.73 m²), but becomes significantly unreliable above this threshold. 7
Serum creatinine alone should never be used to assess renal function in children because it is affected by muscle mass, age, gender, and tubular function. 1, 2
Alternative and Complementary Approaches
For children with low muscle mass, cystatin C-based GFR measurements are preferred as creatinine-based equations may overestimate kidney function in this population. 1, 2, 6
Cystatin C levels of 1.06 mg/L predict GFR <80 mL/min/1.73 m² with 91% sensitivity and 81% specificity in pediatric patients. 1, 2
Combined creatinine and cystatin C-based equations provide superior accuracy compared to either marker alone. 2, 6
Special Population Considerations
The pRIFLE criteria for acute kidney injury uses the Schwartz method combined with urine output to assess severity of renal injury in children over 1 month of age. 1, 3
For neonates under 1 month, the pRIFLE definition has not been validated and requires further research. 3
When no baseline creatinine is available, kidney function can be imputed by assuming a normal GFR of 100 mL/min/1.73 m² and using the child's height to back-calculate an expected creatinine. 2, 3
Clinical Application Principles
The trend of GFR assessments over time is more clinically relevant than single measurements, especially in growing children where monthly assessment is recommended to guide increases in dialyzer size, blood flow rates, or treatment time as patients grow. 1, 2, 6
Laboratories measuring pediatric creatinine must ensure quality control processes include the lowest expected range of values to maintain accuracy. 2, 6