What is the significance of Blood Urea Nitrogen (BUN) in assessing kidney function compared to creatinine?

From the Guidelines

Creatinine is generally considered a more reliable indicator of kidney function than Blood Urea Nitrogen (BUN) because it's produced at a relatively constant rate and is almost exclusively filtered by the kidneys. BUN levels can be affected by many factors unrelated to kidney function, including protein intake, dehydration, gastrointestinal bleeding, certain medications, and liver function 1. The BUN-to-creatinine ratio is often more informative than BUN alone, with elevated ratios potentially indicating dehydration, gastrointestinal bleeding, or certain kidney conditions.

Key Points to Consider

• Normal BUN levels typically range from 7-20 mg/dL, though this can vary by laboratory.
• When evaluating kidney function, healthcare providers typically consider both BUN and creatinine together, along with other tests like estimated glomerular filtration rate (eGFR), to get a comprehensive picture of kidney health.
• The Modification of Diet in Renal Disease (MDRD) formula, which includes BUN, can be used to estimate GFR, but it has its own limitations and may not be accurate in all populations, such as the elderly or those with severe malnutrition or obesity 2.
• The MDRD formula has been validated in certain populations, including kidney transplant recipients and African-Americans with nephrosclerosis, but its generalizability to other populations is still a topic of debate 3.

Clinical Implications

• Healthcare providers should not rely solely on serum creatinine concentration to assess kidney function, as it can be affected by factors other than GFR, such as creatinine secretion and generation and extrarenal excretion 3.
• Clinical laboratories should report an estimate of GFR using a prediction equation, such as the MDRD or CKD-EPI formula, in addition to reporting the serum creatinine measurement 3.
• When monitoring renal function, the trend in creatinine (or eGFR over months) is often more important than the absolute value, and a patient-based monitoring regimen should be developed to minimize the risk of worsening renal function and acute kidney injury 1.

From the Research

Significance of Blood Urea Nitrogen (BUN) in Assessing Kidney Function

• BUN is a commonly used test to estimate renal function, along with plasma creatinine concentration and creatinine clearance 4.
• The serum creatinine level is less influenced by extra-renal factors than the serum urea nitrogen level, making it a more accurate test 5.
• BUN levels are often viewed in concert with creatinine levels, and their absolute levels as well as their relation to one another are observed 5.

Comparison with Creatinine

• Creatinine is considered a more accurate test for assessing kidney function, as it is less influenced by extra-renal factors 5.
• The BUN to creatinine ratio can provide valuable information in assessing renal function, with a ratio of 20 or greater indicating various physiological conditions 6.
• Measuring BUN and creatinine simultaneously can eliminate most external influences and improve the assessment of kidney function 7.

Clinical Applications

• BUN and creatinine levels are essential in the assessment of renal function in the Emergency Department, as they are relatively low-cost and easy to run 5.
• A high BUN to creatinine ratio can cause misestimation of chronic kidney disease (CKD) stage, and GFR estimates for patients with high BCR should be interpreted cautiously 6.
• BUN and creatinine levels can be used to diagnose kidney diseases, such as diabetic nephropathy, and their association with glomerular filtration rate (GFR) can provide valuable information 8.