Normal Ranges for BUN and Creatinine
The normal range for Blood Urea Nitrogen (BUN) is 10-20 mg/dL, and for serum creatinine is 0.6-1.2 mg/dL, with a normal BUN:creatinine ratio of 10-15:1.
Normal Reference Ranges
Blood Urea Nitrogen (BUN)
- Normal range: 10-20 mg/dL (3.6-7.1 mmol/L) 1
- Values may vary slightly between laboratories
- Interpretation should consider clinical context
Serum Creatinine
- Normal range: 0.6-1.2 mg/dL (53-106 μmol/L) 1
- Values are influenced by:
- Age
- Sex
- Muscle mass
- Hydration status
- Medication use
BUN:Creatinine Ratio
- Normal ratio: 10-15:1 2
- Ratio >20:1 may suggest:
- Pre-renal azotemia
- Increased protein catabolism
- Excessive protein intake
- Gastrointestinal bleeding
- Volume depletion
Clinical Significance
BUN Elevation
- May reflect:
- Decreased renal perfusion
- Renal dysfunction
- Increased protein catabolism
- Heart failure
- Dehydration
- Gastrointestinal bleeding
Creatinine Elevation
- More specific for changes in glomerular filtration rate (GFR) 3
- Better indicator of actual kidney function than BUN alone
- Less affected by non-renal factors compared to BUN
BUN:Creatinine Ratio Clinical Applications
- Elevated ratio (>20:1) traditionally suggested pre-renal azotemia
- However, recent research questions this traditional interpretation 4
- Higher than normal BUN:creatinine ratio is associated with:
Special Considerations
Gender-Affirming Hormone Therapy
- Creatinine levels are influenced by gender-affirming hormone therapy 3
- For patients on gender-affirming hormones for ≥12 months:
- Interpret using reference range for affirmed gender
- For patients who stopped hormone therapy for ≥3 months:
- Interpret using reference range for sex assigned at birth
Elderly Patients
- Elderly patients may have disproportionately elevated BUN:creatinine ratios due to:
- Lower muscle mass
- Decreased renal perfusion
- Multiple comorbidities 2
Heart Failure
- BUN is a better predictor of outcome than creatinine or estimated GFR in acute heart failure 3
- Elevated BUN in heart failure may reflect:
- Congestion and fluid retention
- Cardiac dysfunction
- Renal dysfunction
- Neurohormonal activation
Common Pitfalls in Interpretation
Relying on BUN or creatinine alone: Always interpret these values together for a more complete picture of renal function 1
Ignoring clinical context: BUN and creatinine should be interpreted in the context of the patient's clinical status, medications, and comorbidities
Failing to consider non-renal factors: BUN can be affected by protein intake, gastrointestinal bleeding, catabolic states, and medications
Overlooking the prognostic value: Abnormal BUN:creatinine ratio has independent prognostic value beyond the individual components 5, 6
Not considering gender-affirming hormone therapy: This can significantly affect creatinine levels and should be taken into account when interpreting results 3
Remember that while these laboratory values are important indicators of kidney function, they should always be interpreted within the broader clinical context and in conjunction with other relevant laboratory and clinical findings.