Revised Schwartz Equation for Glomerular Filtration Rate (GFR) (2009)

Revised Schwartz Equation (2009)

Height

Serum Creatinine

mg/dL

Estimates GFR in pediatric patients.

Why Use

Determines stage and severity of kidney dysfunction in children. Helps in adjusting medication doses according to renal function. Can provide prognostic information for children with CKD and identify potential candidates for renal replacement therapy (generally, eGFR <15). The newer CKiD equation , which requires BUN and cystatin-C in addition to serum creatinine, may overestimate GFR, as cystatin-C is not as well standardized as serum creatinine. Cystatin-C is also less widely available than serum creatinine.

When to Use

Use in infants, children, and adolescents with or without chronic kidney disease (CKD).

Formula

Revised Schwartz equation: Estimated GFR, mL/min/1.73 m 2 = k × height, cm / serum creatinine, mg/dL where k = 0.413 Note: the original version of the Schwartz equation used different values for k based on age and sex. For historical interest, they are listed here: Patient type k Preterm and small for gestational age (SGA) infants <1 year 0.33 Term infant age ≤1 year 0.45 Child (female or male) 0.55 Adolescent female 0.55 Adolescent male 0.7

Pearls / Pitfalls

Although the original formula was designed using data from a cohort of children without CKD, the revised equation (used here) has been shown to apply to populations of children with CKD with relatively high accuracy. This formula is inaccurate in acute kidney injury (AKI), as GFR in AKI has not reached a steady state. Only in children with normal renal function or established CKD does GFR reach a steady state that can be estimated by using this formula. Neonates have higher normal values of serum creatinine than do older infants and children, as maternal creatinine may circulate in the neonate for several days after birth. Children have lower normal levels of creatinine compared to adolescents and adults because of their smaller muscle mass. Use caution in interpreting serum creatinine levels in children, as levels considered normal in adults and adolescents may be elevated and indicative of impaired renal function in younger children. A k value of 0.413 is more accurate for labs that use enzymatic methods (as opposed to the Jaffe method) to determine serum creatinine levels.

Management

Urine output is an important consideration in interpreting estimates of GFR. Anuric patients are more immediately concerning and may require renal replacement therapy (RRT) to avoid severe electrolyte disturbances, arrhythmias, and seizures. Consider renal ultrasound in children with newly-diagnosed CKD to evaluate for congenital renal malformation or other genetic issues such as polycystic kidney disease. GFR <15 indicates end-stage renal disease and will likely require RRT.

Critical Actions

Remember that serum creatinine levels may be elevated initially after birth as neonates’ kidneys work to clear out maternal creatinine. Infants’ and children’s creatinine levels are much lower than those in adults. A creatinine value of 1.0 mg/dL, considered normal in an adult male, may indicate renal impairment in a male child.

Advice

Nephrotoxic medications may require renal dosing. Different dose adjustments may be required based upon the degree of renal functional impairment.

More Information

Interpretation: GFR, mL/min/1.73 m² CKD stage Severity ≥90 1 Normal 60-89 2 Mild 45-59 3a Mild-moderate 30-44 3b Moderate-severe 15-29 4 Severe <15 5 End-stage For reference, age-specific normal ranges for GFR are provided here. Standard CKD staging (e.g. for medication dose adjustments) is as above. Age GFR normal range, mL/min/1.73 m² 4-28 days 26-68 1-6 months 39-114 6-12 months 49-157 12-30 months 67-231 30 months-5 years 92-292 5-10 years 79-251 10-15 years 73-220 15-25 years 86-206 From Heilbron et al 1991 and Du 2009 .