From the Guidelines
A BUN to creatinine ratio of 27 suggests pre-renal acute kidney injury rather than intrinsic kidney damage. This elevated ratio (normal is typically 10-20) indicates that blood urea nitrogen is disproportionately increased compared to creatinine, which commonly occurs in conditions that reduce kidney perfusion such as dehydration, heart failure, or excessive diuretic use 1. The kidneys respond to decreased blood flow by increasing urea reabsorption while creatinine clearance remains relatively less affected, creating this imbalance. This distinction is clinically important because pre-renal causes are often reversible with appropriate fluid resuscitation or treatment of the underlying condition. However, this ratio should not be interpreted in isolation; other clinical parameters including absolute BUN and creatinine values, urinalysis findings, and the patient's volume status should be considered for a complete assessment.
Some key points to consider in the evaluation of acute kidney injury (AKI) include:
- The definition of AKI, which involves an increase in serum creatinine by 0.3 mg/dL within 48 hours or an increase in serum creatinine to 1.5 times baseline (within prior 7 days) or urine volume < 0.5 mL/kg/hr for 6 hours 1
- The classification of AKI into three stages based on the KDIGO guidelines, which considers changes in serum creatinine and urine output 1
- The importance of identifying the underlying cause of AKI, whether it be pre-renal, renal, or post-renal, to guide appropriate treatment 1
If this elevated ratio is found, prompt evaluation of hydration status and cardiac function is warranted, with consideration of intravenous fluid administration if dehydration is confirmed and not contraindicated. It is also essential to consider the broader clinical context, including the presence of conditions that can be modified with renal replacement therapy (RRT), and trends of laboratory tests, rather than single BUN and creatinine thresholds alone, when making decisions about RRT 1.
From the Research
Blood Urea Nitrogen to Creatinine Ratio
- The Blood Urea Nitrogen (BUN) to creatinine ratio is used to help differentiate between prerenal acute kidney injury (PR AKI) and intrinsic AKI (I AKI) 2.
- A ratio of 27 is higher than the traditional threshold of 20, which is sometimes used to distinguish pre-renal azotemia (PRA) from acute tubular necrosis (ATN) 3.
- However, studies have shown that the BUN to creatinine ratio is not a reliable parameter for distinguishing between PR AKI and I AKI 2, and that a high ratio (>20) is associated with higher hospital mortality 3.
Acute Kidney Injury
- Acute kidney injury (AKI) is a common and devastating medical condition, and its definition and diagnosis are still evolving 4, 5.
- The RIFLE criteria and the Acute Kidney Injury Network definition are two commonly used classification systems for AKI 4, 5.
- AKI can be caused by a variety of factors, including prerenal azotemia, acute tubular necrosis, and hepatorenal syndrome 6.
Clinical Implications
- A BUN to creatinine ratio of 27 may indicate a higher risk of hospital mortality, as shown in a study where patients with a high BUN to creatinine ratio (>20) had a higher odds ratio of hospital mortality compared to those with a low ratio (<20) 3.
- The diagnosis and treatment of AKI should be based on a comprehensive evaluation of the patient's clinical condition, including serum creatinine levels, urine output, and other relevant factors 4, 6, 5.