What are the normal ranges of electrolytes, blood urea nitrogen (BUN), creatinine, and C-reactive protein (CRP) in pediatric patients?

Normal Ranges of Electrolytes, BUN, Creatinine, and CRP in Pediatric Patients

Normal laboratory values in children vary significantly by age, and local laboratory reference ranges should always be consulted, as standardization remains incomplete across different assay methods and populations. 1

Key Principle: Age-Dependent Variation

Laboratory values in pediatric patients change with growth and development, requiring more frequent screening than adults and age-specific interpretation 1. The following ranges represent general guidelines, but individual laboratory reference ranges must be verified as methodologies differ 1.

Electrolytes

Sodium (Na+)

• Infants (1-5 years): Mean ~138 mmol/L, with tighter distribution than older children 2
• School-age to adolescents: Mean ~140 mmol/L (approaching adult values) 2
• General pediatric range: 133-146 mmol/L, though this may be too wide for clinical precision 2
• Note: Hyponatremia is the most common electrolyte abnormality in children <4 years presenting to emergency departments (46.3% of cases) 3

Potassium (K+)

• Varies inversely with age: Higher in younger children, decreasing toward adult values 4, 2
• Monitoring critical: Hypokalemia from diuretics can cause fatal arrhythmias; hyperkalemia may complicate ACE inhibitor therapy 5
• Clinical significance: Hypokalemia is the most common electrolyte disorder in children >4 years old (54.2% in 4-11 years, 32.2% in 12-18 years) 3

Chloride (Cl-)

• DKA/HHS deficits: 3-5 mEq/kg in typical presentations 1
• Age-specific normal ranges vary by laboratory 1

Bicarbonate

• Starvation ketosis: Usually not lower than 18 mEq/L 1
• DKA diagnostic threshold: <15 mEq/L 1

Blood Urea Nitrogen (BUN)

General Pediatric Range

• Standard reference: 8.7-18.0 mg/dL (2.5th-97.5th percentiles) 6
• Adult comparison: 10-20 mg/100 mL 7
• Age relationship: BUN shows negative correlation with gestational age in newborns 4

Clinical Interpretation

• BUN/Creatinine ratio >25:1: Suggests extrarenal problem (pre-renal azotemia) 7
• BUN/Creatinine ratio ~10:1: Indicates intrinsic renal failure 7
• Pediatric-specific: Children have higher BUN/Creatinine ratios (10-34) than adults, likely due to higher protein intake 6
• Critical pitfall: Elevated BUN with normal creatinine does not always indicate kidney dysfunction—consider pre-renal causes first 5, 7

Creatinine

Age-Specific Values

• Pediatric range: 0.4-1.1 mg/L (2.5th-97.5th percentiles) 6
• Adult comparison: 0.6-1.2 mg/100 mL 7
• Age relationship: Creatinine increases with age and shows negative correlation with gestational age in newborns 4, 6

Estimation of GFR in Children

The revised "Bedside Schwartz" equation should be used for children aged 1-16 years (not valid for neonates) 1:

GFR (mL/min/1.73 m²) = 0.413 × [height (cm) / sCr (mg/dL)]

Or: GFR = 36.5 × [height (cm) / sCr (μmol/L)] 1

Critical Considerations

• Methodology matters: Enzymatic assays (isotope-dilution mass spectrometry-calibrated) should be used for pediatric populations, as Jaffe-based methods lack specificity at low creatinine values 1
• The original Schwartz equation is no longer valid and should not be used 1
• Muscle mass effect: Creatinine is affected by muscle mass, age, and gender 1

Alternative: Cystatin C

• Normal adult values: 0.51-0.98 mg/L (reached by 1 year of age) 1
• Predictive value: Cystatin C level of 1.06 mg/L predicted GFR <80 mL/min/1.73 m² with 91% sensitivity and 81% specificity in pediatric cohort 1
• Advantages: Not affected by muscle mass, age, or gender; almost completely filtered by glomerulus 1

C-Reactive Protein (CRP)

Pediatric Values in Acute Illness

• <10 mg/L: Found in 55% of influenza cases 1
• <20 mg/L: Found in 72% of influenza cases 1
• >80 mg/L: Found in only 5% of influenza cases 1
• Clinical utility: CRP is unhelpful in distinguishing influenza from bacterial infections in children 1

Special Populations and Clinical Contexts

Diabetic Ketoacidosis (DKA) in Pediatrics

• Potassium replacement: 20-40 mEq/L (2/3 KCl or potassium-acetate and 1/3 KPO₄) once renal function assured 1
• Sodium monitoring: Correct for hyperglycemia (add 1.6 mEq to sodium for each 100 mg/dL glucose >100 mg/dL) 1
• Fluid management: Initial isotonic saline 10-20 mL/kg/h, not exceeding 50 mL/kg over first 4 hours to avoid cerebral edema 1

Renal Assessment in Liver Disease

• Schwartz Formula or Cystatin C recommended over serum creatinine alone for children with chronic liver disease 1
• High-risk populations: Tyrosinemia, primary hyperoxaluria, congenital hepatic fibrosis, methylmalonic acidemia 1

Emergency Department Presentations

• Overall prevalence of electrolyte abnormalities: 0.14% of pediatric ED visits 3
• Hospital admission rate: 86.4% of children with electrolyte abnormalities require admission 3
• PICU admission: 32.4% of children with electrolyte abnormalities 3
• Associated conditions: Gastrointestinal, renal, and endocrine diseases most common 3

Monitoring Recommendations

Serial Monitoring

• Follow BUN and creatinine serially until normalization in cases of elevation 5
• Monitor serum potassium closely during diuretic or ACE inhibitor/ARB therapy 5
• Assess fluid status by measuring changes in body weight for short-term changes 5

When to Refer to Pediatric Nephrology

Referral warranted for 1:

• Persistent significant proteinuria (≥1+ on dipstick or protein-to-creatinine ratio ≥0.2 for 3 specimens)
• Persistent microscopic hematuria or gross hematuria without UTI
• Edema or hypertension
• Recurrent urinary tract infections
• Electrolyte abnormalities or persistent metabolic acidosis
• Elevated BUN or creatinine levels

Critical Clinical Pitfalls

Do not assume elevated BUN always indicates kidney dysfunction when creatinine is normal—pre-renal azotemia is common 5, 7. Do not stop ACE inhibitors in heart failure patients simply because of elevated BUN, as neurohormonal antagonism benefits persist 5. Do not use serum creatinine alone to assess renal function in children—use either Cystatin C or the revised Schwartz Formula 1.