BUN 6 and Creatinine 0.73: Interpretation
These values are both below the normal reference ranges and suggest either excellent kidney function with low protein intake, low muscle mass, or a dilutional state from overhydration. 1
Normal Reference Ranges
- Normal BUN: 10-20 mg/dL 1
- Normal Creatinine: 0.6-1.2 mg/dL 1
- Normal BUN:Creatinine ratio: 10-15:1 1, 2
Your values yield a BUN:Creatinine ratio of approximately 8:1, which is below the normal range. 1
Clinical Significance of Low BUN
Low BUN (below 10 mg/dL) typically indicates:
- Decreased protein intake or malnutrition - BUN is produced in the liver as a degradation product of proteins, so reduced protein consumption directly lowers BUN production 3, 4
- Overhydration or excessive fluid intake - Dilutional effect from increased total body water 4
- Severe liver disease - Impaired hepatic urea synthesis, though this would typically present with other abnormal liver function tests 3
- Pregnancy - Increased glomerular filtration and plasma volume expansion 5
Clinical Significance of Normal-Low Creatinine
Creatinine of 0.73 mg/dL is within normal range but on the lower end, suggesting:
- Low muscle mass - Creatinine is produced from muscle metabolism, so individuals with reduced muscle mass (elderly, sarcopenic, chronically ill) produce less creatinine 6, 5
- Excellent kidney function - Normal to supranormal GFR with efficient creatinine clearance 6
The National Kidney Foundation emphasizes that serum creatinine alone does not provide sufficiently accurate assessment of kidney function and should be interpreted using GFR estimation equations (MDRD or CKD-EPI) that account for age, sex, race, and body size. 6
Key Clinical Considerations
When evaluating these values, assess for:
- Nutritional status - Check serum albumin, total lymphocyte count, and dietary protein intake history 2
- Hydration status - Evaluate for signs of fluid overload or excessive water intake 4
- Muscle mass - Consider age, body habitus, chronic illness, and mobility status 6
- Liver function - If concerned about hepatic dysfunction, obtain liver enzymes and synthetic function tests 6
Clinical Pitfalls to Avoid
Do not assume normal kidney function based solely on creatinine level. The MDRD Study equation shows tighter correlation with measured GFR than serum creatinine alone, as patients can maintain seemingly normal creatinine levels despite declining GFR. 6
BUN and creatinine are best interpreted together, comparing the two results rather than in isolation. 1, 5 The low BUN:creatinine ratio in this case suggests the BUN is disproportionately low relative to creatinine, pointing away from kidney dysfunction and toward nutritional or dilutional factors. 4, 1
In elderly patients or those with low muscle mass, creatinine may overestimate kidney function. Consider calculating estimated GFR using validated equations that adjust for these factors. 6