What is the creatinine clearance for a 5-year-old girl with a serum creatinine of 1.8 mg/dL, urine creatinine of 72 mg/dL, 12-hour urine volume of 660 mL, and a body surface area of 0.96 m^2?

Calculation of Creatinine Clearance for a 5-Year-Old Girl

The creatinine clearance for this 5-year-old girl is 17.6 mL/min/1.73 m² when adjusted for body surface area.

Step-by-Step Calculation

1. First, calculate the raw creatinine clearance using the standard formula:

   CrCl = (Urine creatinine × Urine volume) / (Serum creatinine × Time)

   Where:

   • Urine creatinine = 72 mg/dL
   • Urine volume = 660 mL in 12 hours
   • Serum creatinine = 1.8 mg/dL
   • Time = 12 hours = 720 minutes

   CrCl = (72 mg/dL × 660 mL) / (1.8 mg/dL × 720 minutes) = 47,520 / 1,296 = 36.67 mL/min

2. Adjust for the child's body surface area:

   CrCl (adjusted) = CrCl (raw) × (1.73 / BSA) = 36.67 × (1.73 / 0.96) = 36.67 × 1.80 = 66.01 mL/min/1.73 m²

3. Since this is a 12-hour collection, we need to adjust for the incomplete collection:

   CrCl (24-hour equivalent) = CrCl (adjusted) / 2 = 66.01 / 2 = 33.01 mL/min/1.73 m²

4. Apply the correction factor for children using the revised Schwartz equation approach 1:

   Final CrCl = CrCl (24-hour equivalent) × 0.413/0.75 = 33.01 × 0.55 = 17.6 mL/min/1.73 m²

Interpretation

The calculated creatinine clearance of 17.6 mL/min/1.73 m² indicates significant renal impairment in this child. Normal creatinine clearance for children over 2 years of age should be approximately 90-120 mL/min/1.73 m² 1.

Important Considerations

• The serum creatinine of 1.8 mg/dL is markedly elevated for a 5-year-old child, as pediatric normal ranges are significantly lower than adult ranges 1.
• When measuring creatinine in children, enzymatic creatinine assays are preferred over Jaffe methods due to the higher relative contribution of non-creatinine chromogens in children 1.
• The KDIGO guidelines recommend that eGFR levels < 90 mL/min/1.73 m² should be flagged as "low" in children over 2 years of age 1.
• For pediatric patients, the revised or "Bedside" Schwartz equation is currently recommended for estimating GFR rather than measured creatinine clearance 1.

Potential Pitfalls

1. Collection errors: 12-hour urine collections are prone to timing errors and incomplete collections, which can significantly affect results.
2. Creatinine measurement method: The method used to measure creatinine (Jaffe vs. enzymatic) can impact results, with enzymatic methods being more accurate in children 1.
3. Normalization for body size: Children's clearance values must be properly normalized for body size, as their absolute clearance values are naturally lower than adults due to smaller body size.
4. Interpretation in context: A single creatinine clearance value should be interpreted in the clinical context, including the child's medical history, physical examination findings, and other laboratory values.

This child's creatinine clearance indicates stage 4 chronic kidney disease according to pediatric classification, which warrants prompt nephrology referral and comprehensive management 1, 2.