Kidney Function Tests in Pediatrics
Kidney Function Tests in Pediatrics
Key Points
- Age-dependent normal values
- Essential for monitoring renal development
- Critical in managing chronic conditions
- Important for medication dosing
Common Indications
- Acute kidney injury evaluation
- Chronic kidney disease monitoring
- Urinary tract infections
- Medication monitoring (nephrotoxic drugs)
- Fluid and electrolyte disorders
- Hypertension workup
Basic Panel Components
- Serum creatinine
- Blood Urea Nitrogen (BUN)
- Estimated GFR
- Urinalysis
- Urine protein/creatinine ratio
Creatinine Studies
Physiological Background
Creatinine is a breakdown product of muscle metabolism, filtered by glomeruli with minimal tubular reabsorption or secretion.
Age-Specific Reference Ranges
- Newborn: 0.3-1.0 mg/dL
- Infant (1-12 months): 0.2-0.4 mg/dL
- Child (1-12 years): 0.3-0.7 mg/dL
- Adolescent: 0.5-1.0 mg/dL
Clinical Considerations
- Influencing Factors:
- Muscle mass
- Dietary protein intake
- Growth and development
- Medications
- Limitations:
- Late marker of kidney injury
- Affected by muscle mass
- Variable in malnutrition
- Changes lag behind GFR changes
Blood Urea Nitrogen (BUN)
Clinical Significance
BUN reflects protein metabolism and is influenced by both renal and non-renal factors.
Reference Ranges
- Newborn: 3-12 mg/dL
- Child: 7-20 mg/dL
- Adolescent: 7-18 mg/dL
Elevated BUN
- Pre-renal Causes:
- Dehydration
- Heart failure
- Shock
- Renal Causes:
- Acute kidney injury
- Chronic kidney disease
- Glomerulonephritis
- Post-renal Causes:
- Urinary tract obstruction
- Posterior urethral valves
Low BUN
- Causes:
- Malnutrition
- Low protein intake
- Liver disease
- Overhydration
Glomerular Filtration Rate (GFR)
Estimation Methods
- Schwartz Formula:
- eGFR = (k × Height) / Serum Creatinine
- k values vary by age and gender
- Heights in cm, creatinine in mg/dL
- CKiD Formula (more accurate):
- Uses height, BUN, creatinine, cystatin C
- Preferred for chronic kidney disease
Normal Values by Age
- Term newborn: 20-40 mL/min/1.73m²
- 2 weeks: 30-50 mL/min/1.73m²
- 1 year: 70-110 mL/min/1.73m²
- >2 years: 90-130 mL/min/1.73m²
CKD Staging in Children
- Stage 1: >90 mL/min/1.73m²
- Stage 2: 60-89 mL/min/1.73m²
- Stage 3: 30-59 mL/min/1.73m²
- Stage 4: 15-29 mL/min/1.73m²
- Stage 5: <15 mL/min/1.73m²
Urine Studies
Urinalysis
- Key Parameters:
- Specific gravity
- pH
- Protein
- Blood
- Leukocyte esterase
- Nitrites
Protein Studies
- Spot Urine Protein/Creatinine:
- Normal: <0.2 mg/mg
- Nephrotic range: >2.0 mg/mg
- 24-hour Collection:
- Normal: <4 mg/m²/hour
- Nephrotic: >40 mg/m²/hour
Other Urine Studies
- Fractional excretion of sodium (FENa)
- Urine osmolality
- Urine electrolytes
- Beta-2 microglobulin
Clinical Scenarios and Interpretation
Acute Kidney Injury
- Pre-renal:
- ↑ BUN/Cr ratio (>20:1)
- FENa <1%
- Concentrated urine
- Intrinsic Renal:
- Normal BUN/Cr ratio
- FENa >2%
- Abnormal urinalysis
Monitoring Points
- Trends more important than single values
- Consider body composition changes
- Account for medications
- Note fluid status
Common Pitfalls
- Using adult reference ranges
- Not adjusting for height/age
- Overlooking non-renal factors
- Missing pre-analytical errors
