What labs are used to evaluate kidney function in a 9-year-old?

Laboratory Evaluation of Kidney Function in a 9-Year-Old Child

The primary laboratory tests for evaluating kidney function in a 9-year-old child include serum creatinine with estimated glomerular filtration rate (eGFR) and urinalysis with urine albumin-to-creatinine ratio (ACR). These tests provide essential information about both glomerular and tubular function.

First-Line Laboratory Tests

Glomerular Function Assessment

• Serum Creatinine with eGFR Calculation

  • Must use pediatric-specific equations (Schwartz formula) 1
  • Enzymatic creatinine assay preferred over Jaffe method in children 1
  • An eGFRcr < 90 mL/min/1.73m² should be flagged as "low" in children over 2 years 1
  • Normal creatinine values vary significantly by age; a value of 1.0 mg/dL represents normal function in an adolescent but >50% loss of function in a 5-year-old 2

• Cystatin C (when more accurate GFR assessment needed)

  • Less affected by muscle mass than creatinine
  • Consider eGFRcr-cys (combined creatinine and cystatin C) when eGFRcr is less accurate and clinical decisions depend on precise GFR 1

Urinalysis and Protein Assessment

• Urinalysis (first morning void preferred) 1

  • Microscopic examination for RBCs, WBCs, casts
  • Dipstick for protein, blood, glucose, nitrites, leukocyte esterase

• Urine Albumin-to-Creatinine Ratio (ACR) 1

  • First morning void sample preferred
  • Confirm positive dipstick findings with quantitative ACR
  • ACR ≥ 30 mg/g requires confirmation with a subsequent first morning void sample

Second-Line Laboratory Tests

Additional Blood Tests

• Blood Urea Nitrogen (BUN) 1

  • Assess along with creatinine (BUN/creatinine ratio)
  • Helps distinguish prerenal from intrarenal causes

• Electrolytes 1

  • Sodium, potassium, chloride, bicarbonate
  • Calcium, phosphorus, magnesium

• Complete Blood Count 1

  • Anemia may indicate chronic kidney disease

Additional Urine Tests

• Urine Protein-to-Creatinine Ratio (PCR) 1, 3

  • Alternative to ACR when total protein assessment is needed
  • Particularly useful when non-albumin proteinuria is suspected

• Urine Culture 1, 3

  • Indicated when infection is suspected
  • Essential when pyuria or nitrites are present on urinalysis

Special Considerations in Children

1. Laboratory Quality Control

   • Labs measuring creatinine in children must ensure quality control processes include the lowest end of expected values 1
   • Higher contribution of non-creatinine chromogens to measured creatinine in children when using Jaffe assay 1

2. Interpretation Challenges

   • Normal values change with age and growth 2
   • Muscle mass affects creatinine levels more significantly in growing children
   • Concentrating ability reaches adult values by 6-12 months of age 2

3. When to Consider More Advanced Testing

   • Persistent proteinuria (≥2+ by dipstick) 1
   • Hematuria with proteinuria (suggests glomerular disease) 1, 3
   • Abnormal kidney function with unclear etiology
   • Monitoring known kidney disease

Indications for Nephrology Referral

• Persistent significant proteinuria (protein-to-creatinine ratio >0.2 for 3 specimens) 1
• Persistent microscopic hematuria 1
• Gross hematuria in the absence of urinary tract infection 1
• Elevated BUN or creatinine levels 1
• Electrolyte abnormalities or persistent metabolic acidosis 1

Common Pitfalls to Avoid

1. Relying solely on serum creatinine without eGFR calculation

   • Creatinine alone is inadequate due to age-dependent normal values 1, 2

2. Using adult reference ranges

   • Children have different normal values that change with age 2

3. Failing to confirm abnormal findings

   • Single abnormal urinalysis should be confirmed with repeat testing 3

4. Overlooking non-renal causes of abnormal values

   • Medications, diet, hydration status, and muscle mass can affect results 1

5. Inadequate sample collection

   • First morning void samples are preferred for protein assessment 1
   • Improper collection technique can lead to contamination

By following this comprehensive approach to laboratory evaluation, clinicians can effectively assess kidney function in 9-year-old children, allowing for early detection and management of kidney disease.