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Renal Function Estimation in Pediatric Patients

Evaluation of a patient's renal function often includes the use of equations to estimate glomerular filtration rate (GFR) (eg, estimated GFR [eGFR] creatinine clearance [CrCl]) using an endogenous filtration marker (eg, serum creatinine) and other patient variables. For example, the Schwartz equation estimates renal function by calculating eGFR and is typically used to steer medication dosing or categorize chronic kidney disease (CKD) staging and monitor progression. The rate of creatinine clearance does not always accurately represent GFR; creatinine may be cleared by other renal mechanisms in addition to glomerular filtration and serum creatinine concentrations may be affected by nonrenal factors (eg, age, gender, race, body habitus, illness, diet). In addition, these equations were developed based on studies in limited populations and may either over- or underestimate the renal function of a specific patient.
Nevertheless, most clinicians use an eGFR or CrCl as an indicator of renal function in pediatric patients for the purposes of adjusting medication doses. These equations should be used in the clinical context of patient-specific factors noted during the physical exam/work-up. Decisions regarding drug therapy and doses must be made on clinical judgment.

RENAL FUNCTION ESTIMATION EQUATIONS

Commonly used equations to estimate renal function utilizing the endogenous filtration marker serum creatinine include the Schwartz and Traub-Johnson equations. Both equations were originally developed using a serum creatinine assay measured by the alkaline picrate-based (Jaffe) method. Many substances, including proteins, can interfere with the accuracy of this assay and overestimate serum creatinine concentration. The National Kidney Foundation and The National Kidney Disease Education Program advocated for a universal creatinine assay, in order to ensure an accurate estimate of GFR in patients. As a result, a more specific enzymatic assay with an isotope dilution mass spectrometry (IDMS)-traceable international standard was developed. Compared to the older methods, IDMS-traceable assays may report lower serum creatinine values and may, therefore, overestimate renal function when used in the original equations. An updated Schwartz equation (eg, Bedside Schwartz) based on serum creatinine measured by the IDMS-traceable method has been proposed for pediatrics (Schwartz 2009); the Traub-Johnson equation has not been re-expressed. The original Schwartz and Traub-Johnson equations may overestimate renal function when used with a serum creatinine measured by the IDMS-traceable method. However, at this point, all laboratories should be using creatinine methods calibrated to be IDMS traceable.
The following factors may contribute to an inaccurate estimation of renal function (Stevens 2006):
  • Increased creatinine generation (may underestimate renal function):
    • Black patients
    • Muscular body habitus
    • Ingestion of cooked meats
  • Decreased creatinine generation (may overestimate renal function):
    • Increased age
    • Female patients
    • Asian patients
    • Amputees
    • Malnutrition, inflammation, or deconditioning (eg, cancer, severe cardiovascular disease, hospitalized patients)
    • Neuromuscular disease
    • Vegetarian diet
  • Rapidly changing serum creatinine (either up or down):
    • In patients with rapidly rising serum creatinines (ie, increasing by >0.5 to 0.7 mg/dL/day), it is best to assume that the patient's renal function is severely impaired
Use extreme caution when estimating renal function in the following patient populations:
  • Low body weight (actual body weight < ideal body weight)
  • Liver transplant
  • Prematurity (especially very low birth weight)
  • Dehydration
  • Recent kidney transplantation (serum creatinine values may decrease rapidly and can lead to renal function underestimation; conversely, delayed graft function may be present)
IDMS-traceable method: Bedside Schwartza
Note: This equation is for use in ages 1 to 16 years.
eGFR = (0.413 X Height) / Creatinine
where:
eGFR = estimated GFR; calculated in mL/minute per 1.73 m2
Height (length) is input in cm
Creatinine = SrCr input in mg/dL
Alkaline picrate-based (Jaffe) methods
Note: These equations have not been updated for use with serum creatinine methods traceable to IDMS. Use with IDMS-traceable serum creatinine methods may overestimate renal function; use with caution.
Method 1: Schwartz equation
Note: This equation may not provide an accurate estimation of creatinine clearance for infants <6 months of age or for patients with severe starvation or muscle wasting.
eGFR = (k X Height) / Creatinine
where:
eGFR = estimated GFR; calculated in mL/minute per 1.73 m2
Height (length) is input in cm
k = constant of proportionality that is age-specific
<1 year preterm: 0.33
<1 year full-term: 0.45
1 to 12 years: 0.55
>12 years female: 0.55
>12 years male: 0.7
Creatinine is input in mg/dL
Method 2: Traub-Johnson equation
Note: This equation is for use in ages 1 to 18 years.
CrCl = (0.48 X Height) / Creatinine
where:
CrCl = estimated creatinine clearance; calculated in mL/minute per 1.73 m2
Height (length) is input in cm
Creatinine = SrCr input in mg/dL
FOOTNOTES
aNational Kidney Disease Education Program preferred equation
REFERENCES
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National Kidney Disease Education Program. GFR calculators. http://www.nkdep.nih.gov/professionals/gfr_calculators. Accessed April 24, 2013.
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