What is the normal Blood Urea Nitrogen (BUN)-to-creatinine ratio in a typical adult patient?

What is the BUN/Creatinine Ratio?

The BUN/creatinine ratio is a calculated value comparing blood urea nitrogen (BUN) to serum creatinine levels, with a normal range of 10:1 to 20:1 (or 10-20 when expressed as a simple ratio), used to help distinguish between different causes of kidney dysfunction and assess volume status. 1

Normal Reference Values

• Normal BUN levels range from 10-20 mg/dL in adults 1
• Normal serum creatinine ranges from 0.6-1.2 mg/dL 1
• The normal BUN/creatinine ratio is 10:1 to 15:1, though values up to 20:1 are generally considered within normal limits 1, 2
• In elderly subjects aged 70 years or older, the normal BUN range is slightly higher at 14-23 mg/dL, with creatinine 0.9-1.3 mg/dL in males and 0.7-1.1 mg/dL in females 3

Clinical Interpretation

Elevated Ratio (>20:1)

• A BUN/creatinine ratio greater than 20:1 traditionally suggests prerenal azotemia (kidney hypoperfusion from dehydration, heart failure, or shock), though this interpretation has significant limitations 1, 2
• Common causes of elevated ratio include hypovolemia, congestive heart failure, septic or hypovolemic shock, high protein intake (>100 g/day), gastrointestinal bleeding, and high-dose corticosteroid use 2
• Markedly elevated ratios (BUN ≥100 mg/dL with creatinine ≤5 mg/dL) are frequently multifactorial, particularly in elderly ICU patients, and carry high mortality due to severe underlying illness 2

Low Ratio (10:1 or less)

• A ratio of 10:1 suggests intrinsic renal failure (acute or chronic kidney disease where both BUN and creatinine rise proportionally) 1
• Low ratios can also indicate severe muscle wasting or malnutrition, where inappropriately low creatinine (from decreased muscle mass) makes the ratio unreliable for assessing true renal function 4

Important Clinical Pitfalls

• The BUN/creatinine ratio has limited utility in distinguishing prerenal azotemia from acute tubular necrosis in hospitalized patients with AKI - approximately half of AKI patients have ratios >20:1, and those with high ratios actually have higher mortality than those with low ratios, contrary to traditional teaching 5
• Do not rely solely on the BUN/creatinine ratio to assess kidney function - it should always be interpreted alongside estimated GFR (using MDRD or CKD-EPI equations), urinalysis for proteinuria, and clinical context 4
• In elderly patients and those with decreased muscle mass, serum creatinine may be inappropriately low, masking significant renal dysfunction and making the ratio misleading 4, 3
• A BCR ≥20 causes misestimation of CKD stage when using standard GFR equations, requiring cautious interpretation 6
• Fractional sodium excretion <1% (the traditional marker of prerenal azotemia) is present in only a minority of patients with elevated BUN/creatinine ratios, further questioning the ratio's diagnostic value 2

When Both Values Are Abnormal

• Prerenal and intrinsic renal problems can coexist, requiring comprehensive evaluation beyond the ratio alone 1
• Multiple factors often contribute simultaneously, especially in critically ill, elderly, or malnourished patients 2