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    Patent Pending

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

    Estimates GFR in pediatric patients.
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    When to Use
    Pearls/Pitfalls
    Why Use

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

    • 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.
    • 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.
    in
    mg/dL

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    Next Steps
    Evidence
    Creator Insights

    Advice

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

    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.

    Formula

    Revised Schwartz equation:

    Estimated GFR, mL/min/1.73 m2 = 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

    Facts & Figures

    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 Pediatrics in Review and Du 2009.


    Evidence Appraisal

    The original Schwartz formula was developed using a cohort of children (n = 586) with normal renal function. In that study, different values of the k coefficient according to age and sex were proposed to more accurately estimate GFR. A recent re-examination by the original author (Schwartz 2009) found that a single k coefficient of 0.413 better estimates GFR in all children with CKD. A value of 0.413 for k was validated in children without CKD in a later study by Staples et al (2010). Note that the updated (2009) Schwartz formula may not apply equally to all ethnic groups (Zheng 2017).

    Literature

    Dr. George J. Schwartz

    From the Creator

    Why did you develop the Schwartz Equation for GFR? Was there a particular clinical experience or patient encounter that inspired you to create this tool for clinicians?

    Estimating GFR takes much less time, and measurement of GFR is very laborious. We used to calculate all the GFRs as fellows at Albert Einstein in the Bronx, New York. We hypothesized that serum creatinine by itself might not be a great marker because growth occurs in children, and with growth is the accretion of muscle mass, which raises serum creatinine. Thus, we tested formulas with a body habitus component as well as serum creatinine, and found that height divided by serum creatinine (Ht/Scr) worked the best.

    What pearls, pitfalls and/or tips do you have for users of the Schwartz Equation for GFR? Do you know of cases when it has been applied, interpreted, or used inappropriately?

    The equation must be applied to a stable creatinine value, and is inaccurate when serum creatinine is rapidly changing.

    The equation is not useful in children with reduced muscle mass, amputations, cachexia, or those taking creatine supplements.

    What recommendations do you have for doctors once they have applied the Schwartz Equation for GFR? Are there any adjustments or updates you would make to the score based on new data or practice changes?

    The updated eGFR from 2009 works well, especially with enzymatic serum creatinine. One has to average this with the CKD-EPI eGFR for adolescents and young adults between 18 and 26.

    How do you use the Schwartz Equation for GFR in your own clinical practice? Can you give an example of a scenario in which you use it?

    I use this regularly to stage my patients. If I am unclear as to whether or not this estimate makes sense, I send out a cystatin C. GFR can be estimated by 70.69 x CysC-0.931. If the two univariate estimates agree within 10%, then that is the eGFR. If there is disagreement, try to see which estimate is likely off. If this is not obvious, you may have to measure GFR in such a patient.

    Any other research in the pipeline that you’re particularly excited about?

    The Chronic Kidney Disease in Children Study (CKiD) hopes to have a more globally applied eGFR after recruitment of the third cohort. Work will probably begin on this eGFR in a year or so. Meanwhile, the full CKiD eGFR is even more accurate than is the Ht/Scr (published in Kidney Int 82:445-53, 2012).

    About the Creator

    George J. Schwartz, MD, is chief of the nephrology division in the department of pediatrics at the University of Rochester Medical Center. He is also an elected member of the American Society for Clinical Investigation. Dr. Schwartz's research interests include acidosis and renal tubular disorders.

    To view Dr. George J. Schwartz's publications, visit PubMed

    Content Contributors
    Reviewed By
    • Juan Kupferman, MD
    About the Creator
    Dr. George J. Schwartz
    Content Contributors
    Reviewed By
    • Juan Kupferman, MD