From the Guidelines
An elevated BUN of 20 mg/dL with normal creatinine levels typically indicates pre-renal causes, such as dehydration, high protein intake, gastrointestinal bleeding, or increased catabolism, rather than intrinsic kidney damage. This pattern suggests that the kidneys are functioning properly, but there is an issue with blood flow or urea reabsorption. When BUN rises disproportionately to creatinine, it often reflects reduced blood flow to the kidneys rather than kidney dysfunction itself 1.
Possible Causes
- Dehydration: increasing fluid intake to 2-3 liters daily is recommended
- High protein intake: moderating protein consumption may help normalize BUN levels
- Gastrointestinal bleeding: medical evaluation is necessary
- Increased catabolism: addressing the underlying cause of increased catabolism is essential
Clinical Significance
The elevated BUN occurs because urea is reabsorbed in the kidney tubules during states of decreased renal perfusion, while creatinine is not reabsorbed, explaining why creatinine remains normal while BUN increases 1. This pattern is generally less concerning than when both values rise together, which would suggest actual kidney impairment. According to the most recent study 1, AKI is common, affecting up to 20% of hospital inpatients, and has a significant impact on patient morbidity and mortality.
Monitoring and Management
Monitoring renal function is crucial, and the trend in creatinine (or eGFR over months) is more important than the absolute value 1. A patient-based monitoring regimen should be developed, considering both medication and individual risk factors, and suggesting a monitoring interval based on a patient’s combined risk, facilitating early intervention to reduce risk of renal deterioration, hospital admission, and mortality.
From the Research
Elevated BUN without Elevation in Creatinine
- An elevated BUN of 20 without elevation in creatinine may indicate pre-renal azotemia, which can be caused by various factors such as hypovolemia, congestive heart failure, or septic shock 2.
- The normal BUN:Cr ratio is 10-15:1, and a disproportionate rise in BUN:Cr (> 20:1) may imply pre-renal azotemia or increased protein catabolism 2.
- Other possible causes of elevated BUN without elevation in creatinine include:
- It is essential to note that severely disproportionate BUN:Cr is frequently multifactorial and may not be indicative of uncomplicated renal hypoperfusion 2.
- Fractional Na excretion can help differentiate between pre-renal and intrinsic renal causes, with a value < 1% consistent with pre-renal azotemia 2.