From the Guidelines
The BUN (blood urea nitrogen) of 39 mg/dL and creatinine of 1.85 mg/dL yields a BUN:creatinine ratio of approximately 21:1, which suggests prerenal azotemia as the likely etiology. This ratio is elevated above the normal range of 10-15:1, indicating that urea is being disproportionately reabsorbed compared to creatinine. Prerenal azotemia typically occurs due to decreased renal perfusion, which can result from volume depletion (dehydration, bleeding, excessive diuresis), decreased cardiac output (heart failure, shock), or renal artery stenosis. The management should focus on identifying and treating the underlying cause of decreased renal perfusion, typically through volume repletion with intravenous fluids if dehydration is present. Other interventions may include adjusting medications that affect renal perfusion (such as ACE inhibitors, ARBs, or diuretics), improving cardiac output if heart failure is present, or addressing other causes of hypoperfusion, as suggested by the European Society of Cardiology guidelines 1, 2, 3. It's essential to monitor kidney function closely with serial BUN and creatinine measurements to ensure improvement with treatment, and to consider the potential impact of medications such as ACE inhibitors and ARBs on renal function, as outlined in the guidelines 1, 3. If the elevated ratio persists despite appropriate interventions, further evaluation for intrinsic renal disease may be necessary. In patients with heart failure, it is crucial to monitor renal function and adjust medications accordingly, as worsening renal function and hyperkalaemia can occur, and the guidelines provide recommendations for managing these complications 1, 3. Overall, the management of prerenal azotemia should prioritize the identification and treatment of the underlying cause, as well as close monitoring of kidney function and adjustment of medications as needed.
From the Research
Etiology of Elevated BUN to Creatinine Ratio
The BUN to Creatinine ratio of 21.1, given a BUN of 39 and a Cr of 1.85, is elevated. Key points to consider:
- The normal BUN: Cr ratio is 10-15: 1 4
- Disproportionate rises in BUN: Cr (> 20: 1) often imply pre-renal azotemia but may be caused by increased protein catabolism or an excessive protein load 4
- A high BUN/creatinine ratio is a strong prognostic indicator in patients with acute decompensated heart failure (ADHF) 5, 6, 7
Possible Causes
Possible causes of the elevated BUN to Creatinine ratio include:
- Pre-renal azotemia 4
- Increased protein catabolism 4
- Excessive protein load 4
- Heart failure 5, 6, 7
- Dehydration or hypovolemia 4
- Congestive heart failure 4
- Septic or hypovolemic shock 4
- High-dose steroids 4
- Low serum albumin 4
- High protein intake 4
- Infection 4
Clinical Implications
The clinical implications of an elevated BUN to Creatinine ratio include:
- Higher all-cause mortality in patients with ADHF 5
- Higher post-discharge all-cause mortality in patients with ADHF 5
- Identification of a high-risk but potentially reversible form of renal dysfunction in patients with decompensated heart failure 6
- Association with worse prognosis independently from both creatinine and BUN in patients with AHF 7