Normal Plasma Ranges for Blood Urea Nitrogen and Creatinine
The normal plasma range for Blood Urea Nitrogen (BUN) is 7-20 mg/dL, and for serum creatinine is 0.7-1.3 mg/dL for adult men and 0.6-1.1 mg/dL for adult women. These ranges represent the standard reference intervals used in clinical practice for assessing kidney function.
Standard Reference Ranges
Blood Urea Nitrogen (BUN):
- Normal range: 7-20 mg/dL
- Elderly patients (70+ years) may have slightly higher normal ranges of 14-23 mg/dL 1
Serum Creatinine:
- Adult men: 0.7-1.3 mg/dL
- Adult women: 0.6-1.1 mg/dL
- Elderly men (70+ years): 0.9-1.3 mg/dL 1
- Elderly women (70+ years): 0.7-1.1 mg/dL 1
Clinical Significance
BUN and creatinine are key markers for assessing kidney function, particularly glomerular filtration rate (GFR). According to clinical guidelines, baseline renal function (BUN and creatinine) should be no worse than 1.5 times the upper range of normal values when evaluating patients for clinical trials 2.
BUN:Creatinine Ratio
- Normal ratio: 10-15:1
- Ratio >20:1 may suggest:
- Pre-renal azotemia
- Increased protein catabolism
- Excessive protein load
- Gastrointestinal bleeding
- High-dose steroid use 3
Factors Affecting BUN and Creatinine Levels
Factors that can elevate BUN:
- Decreased renal perfusion (dehydration, heart failure)
- High protein intake
- Gastrointestinal bleeding
- Catabolic states
- Medications (corticosteroids)
- Advanced age
Factors that can elevate Creatinine:
- Decreased kidney function
- Muscle injury (rhabdomyolysis)
- Certain medications
- High meat intake
Clinical Applications
Screening for Renal Insufficiency
- BUN <15 mg/dL provides strong evidence against renal insufficiency (sensitivity 99.7%) 4
- BUN <20 mg/dL has 98% sensitivity for ruling out significant renal insufficiency 4
Monitoring Kidney Function
- Serial measurements of BUN and creatinine are more valuable than single measurements
- Disproportionate elevation of BUN relative to creatinine (BUN:Cr >20:1) is associated with higher mortality in acute kidney injury 5
Special Considerations in Elderly Patients
Elderly patients typically have:
- Higher normal BUN values (14-23 mg/dL)
- Slightly higher normal creatinine values
- Positive correlation between BUN levels and age, particularly in men 1
- Lower muscle mass, which can result in lower creatinine production despite reduced kidney function
Interpreting Abnormal Values
When interpreting BUN and creatinine values that fall outside the normal range, consider:
- Rate of change over time
- Presence of acute illness
- Medication effects
- Hydration status
- Dietary protein intake
- Muscle mass (particularly important for creatinine interpretation)
Remember that BUN and creatinine should be interpreted together with other clinical findings and not in isolation. Estimated GFR calculations (such as MDRD or CKD-EPI equations) provide more accurate assessment of kidney function than either value alone.