From the Guidelines
An isolated elevated BUN/creatinine ratio of 37 with normal creatinine and GFR most likely indicates pre-renal azotemia, which means decreased blood flow to the kidneys rather than kidney damage itself. This condition can be caused by various factors, including dehydration, where the body reabsorbs more urea (measured as BUN) while creatinine clearance remains relatively normal 1. Other potential causes include:
- Gastrointestinal bleeding (where blood proteins in the gut get broken down to urea)
- High-protein diet
- Use of certain medications (like corticosteroids or tetracyclines)
- Catabolic states such as fever or infection If symptoms like excessive thirst, dark urine, dizziness, or fatigue are present, these support the dehydration theory 1. While this finding alone isn't usually concerning when other kidney function markers are normal, persistent elevation warrants follow-up testing in 1-3 months. The normal BUN/creatinine ratio is typically 10-20, so a ratio of 37 is elevated but not severely alarming when creatinine and GFR remain normal, suggesting the kidneys themselves are functioning properly 1. It's essential to note that clinicians should not use serum creatinine concentration as the sole means to assess the level of kidney function 1, and clinical laboratories should report an estimate of GFR using a prediction equation in addition to reporting the serum creatinine measurement 1. In the context of real-life clinical medicine, it's crucial to prioritize the patient's overall health and consider factors that may influence kidney function, such as age, muscle mass, and comorbid conditions 1. Therefore, increasing fluid intake to 2-3 liters of water daily, unless restricted, is a reasonable recommendation to help manage pre-renal azotemia and prevent further kidney damage.
From the Research
Isolated Elevated BUN/Cr Ratio
- An isolated elevated BUN/Cr ratio of 37 with normal creatinine and GFR may indicate pre-renal azotemia, but it can also be caused by other factors such as increased protein catabolism or an excessive protein load 2.
- A normal BUN:Cr ratio is typically considered to be 10-15:1, and a disproportionate rise in BUN:Cr (> 20:1) may imply pre-renal azotemia 2.
- However, in some cases, a severely disproportionate BUN:Cr ratio can be multifactorial and may not be indicative of uncomplicated renal hypoperfusion 2.
- Factors that can contribute to a disproportionate BUN:Cr ratio include:
- Hypovolemia
- Congestive heart failure
- Septic or hypovolemic shock
- High-dose steroids
- High protein intake
- Infection
- Gastrointestinal bleeding
- HIV 2
- It is worth noting that a fractional Na excretion of < 1% is consistent with pre-renal azotemia, but this was only present in a subset of patients with severely disproportionate BUN:Cr ratios 2.