The Schwartz Formula: Purpose and Application in Pediatric GFR Estimation
The Schwartz formula is used to estimate glomerular filtration rate (GFR) in children aged 1-16 years, not for calculating medication doses. This bedside calculation provides a practical method to assess kidney function when direct GFR measurement is unavailable 1, 2, 3.
Primary Formula and Calculation Method
The revised "Bedside Schwartz equation" is the recommended approach for estimating GFR in pediatric patients 1, 3:
- GFR (mL/min per 1.73 m²) = 0.413 × [height (cm) / serum creatinine (mg/dL)] 1, 3
- Alternative units: GFR (mL/min per 1.73 m²) = 36.5 × [height (cm) / creatinine (μmol/L)] 1, 3
This formula was developed using isotope-dilution mass spectrometry traceable creatinine assays and has been validated in both chronic kidney disease (CKD) and non-CKD pediatric populations 1.
Critical Laboratory Requirements
Enzymatic creatinine assays must be used instead of Jaffe-based methods because non-creatinine chromogens contribute disproportionately at the low creatinine values encountered in children, leading to significant measurement errors 1, 3. The lack of specificity in Jaffe methods is particularly problematic in pediatric populations 1.
Clinical Applications
The Schwartz formula serves multiple purposes in pediatric nephrology:
- Assessment of acute kidney injury (AKI) using the pRIFLE criteria, which combines estimated creatinine clearance with urine output 1, 3
- Monitoring chronic kidney disease progression in children with reduced GFR 1
- Guiding dialysis adjustments as children grow, informing increases in dialyzer size, blood flow rates, or treatment time 3
Important Limitations and Pitfalls
The Schwartz formula overestimates GFR, particularly at lower GFR levels and in children with less severe renal impairment 3, 4. The formula performs best in children with CKD (reduced GFR) but is not optimal for those with GFR >75 mL/min/1.73 m² 4.
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 2, 3.
Accurate height measurement is essential for the calculation, which can be challenging in sick or ventilated infants 2.
Alternative and Enhanced 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 3.
Combined creatinine and cystatin C-based equations provide superior accuracy for GFR estimation, with approximately 87.7% of estimates falling within 30% of measured iohexol-GFR and 45.6% within 10% 5. The enhanced formula incorporating both markers is: GFR = 39.1[height/Scr]^0.516 × [1.8/cystatin C]^0.294 × [30/BUN]^0.169 × [1.099]^male × [height/1.4]^0.188 5.
Clinical Interpretation
The trend of GFR assessments over time is more clinically relevant than single measurements, especially in growing children 2, 3. If no prior creatinine is available, baseline kidney function can be imputed by assuming a normal GFR of 100 mL/min/1.73 m² and using the child's height 2.
Direct measurement of GFR using exogenous filtration markers (e.g., iohexol plasma clearance) remains the gold standard but is impractical in routine clinical settings 2.