How is Glomerular Filtration Rate (GFR) estimated in infants with suspected Impaired renal function?

Estimating Glomerular Filtration Rate (GFR) in Infants with Suspected Impaired Renal Function

In infants with suspected impaired renal function, GFR should be estimated using the revised Schwartz Formula, which utilizes serum creatinine, patient height, and a constant to derive estimated creatinine clearance: GFR (mL/min/1.73 m²) = 0.413 × [height (cm) / serum creatinine (mg/dL)]. 1, 2

Primary Methods for GFR Estimation in Infants

• The revised Schwartz Formula is the most practical bedside method for estimating GFR in infants with suspected renal impairment 1, 2
• For infants in the first year of life, a modified constant of 0.45 (instead of 0.413) may be used in the Schwartz Formula 3, 4
• Serum creatinine alone should not be used to assess renal function in infants, as it is affected by muscle mass, age, gender, and renal tubular function 1
• Cystatin C is a valuable alternative marker for GFR estimation in infants, as it is not affected by muscle mass, age, or gender 1, 5

Advantages of Cystatin C for Infant GFR Estimation

• Cystatin C is almost completely filtered by the glomerulus and is not excreted or absorbed by renal tubules 1
• Normal values for cystatin C are high in infants but approach normal adult levels (0.51-0.98 mg/L) by 1 year of age 1
• A cystatin C level of 1.06 mg/L predicts a GFR <80 mL/min/1.73 m² with a sensitivity of 91% and specificity of 81% in pediatric patients 1
• For infants with low muscle mass, cystatin C-based GFR measurements are preferred as creatinine-based equations may overestimate kidney function 5

Combined Approaches for Improved Accuracy

• Combined creatinine and cystatin C-based equations provide superior accuracy for GFR estimation in infants 5
• For acute renal injury assessment in infants, the pediatric modified RIFLE criteria uses a combination of estimated creatinine clearance by the Schwartz method and urine output 1, 6
• The most accurate GFR estimation formula for infants with chronic kidney disease incorporates height, serum creatinine, cystatin C, blood urea nitrogen, and gender 2

Laboratory Considerations for Infant GFR Assessment

• Enzymatic creatinine assays should be used in infants rather than Jaffe assays due to the higher relative contribution of non-creatinine chromogens in infant samples 5, 6
• Laboratories measuring creatinine in infants must ensure their quality control process includes the lowest end of the expected range of values 5
• Small changes in serum creatinine may represent relatively large changes in actual GFR in infants 6

Direct GFR Measurement Methods

• Direct measurement of GFR using exogenous filtration markers (e.g., iohexol plasma clearance) is considered the gold standard but is impractical in routine clinical settings for infants 1, 7
• Endogenous filtration markers such as creatinine clearance are hampered by the difficulties of timed urinary collections in infants 1
• Iohexol plasma disappearance offers the best combination of safety, accuracy, and precision for direct GFR measurement when needed in infants 7

Special Considerations for Infants

• 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 infant's height 6
• The trend of GFR assessments over time is more relevant than a single measurement, especially in growing infants 5
• The pRIFLE definition for acute kidney injury has not been fully validated for neonates (under 1 month of age) 6

Common Pitfalls in Infant GFR Estimation

• Standard eGFR formulae may overestimate GFR in infants with low muscle mass 5
• Accurate height measurement is necessary to calculate eGFR using the Schwartz formula, which can be challenging in sick, ventilated infants 6
• The Schwartz formula may overestimate GFR, especially at lower GFR levels 1