What does a slightly elevated blood urea nitrogen (BUN) level with a high BUN/creatinine ratio, normal creatinine level, and normal estimated glomerular filtration rate (eGFR) indicate?

Interpretation of Elevated BUN with Normal Creatinine and eGFR

An elevated blood urea nitrogen (BUN) of 28 mg/dL with normal creatinine (0.88 mg/dL) and normal eGFR (71 mL/min/1.73m²), but high BUN/creatinine ratio (32) most likely indicates a pre-renal condition rather than intrinsic kidney disease, suggesting possible dehydration, high protein catabolism, or decreased renal perfusion. 1, 2

Understanding the Laboratory Values

• BUN of 28 mg/dL is above the normal range (7-25 mg/dL), indicating increased nitrogen waste products in the blood 2
• Creatinine of 0.88 mg/dL is within normal range (0.60-1.00 mg/dL), suggesting preserved glomerular filtration 2
• eGFR of 71 mL/min/1.73m² is normal (>60 mL/min/1.73m²), confirming adequate kidney function 2
• BUN/creatinine ratio of 32 is significantly elevated above normal (6-22), suggesting a disproportionate rise in BUN relative to creatinine 3

Clinical Significance

• A disproportionately elevated BUN/creatinine ratio (>20:1) often indicates pre-renal azotemia rather than intrinsic kidney disease 3
• Unlike intrinsic kidney disease where both BUN and creatinine rise in tandem, this pattern suggests factors affecting BUN independently of glomerular filtration 1, 3
• This laboratory pattern is commonly seen in conditions with decreased renal perfusion, increased protein catabolism, or excessive protein intake 3

Potential Causes to Consider

Decreased Renal Perfusion

• Volume depletion/dehydration (most common cause) 3
• Heart failure with reduced cardiac output 1, 4
• Hypotension or shock states 3

Increased Protein Catabolism

• Gastrointestinal bleeding (protein breakdown in the gut) 3
• Catabolic states (severe illness, infections, burns) 3
• Corticosteroid therapy 3

Other Causes

• High protein diet or parenteral nutrition 3
• Hyperthyroidism (can increase BUN/creatinine ratio through increased protein catabolism) 5
• Certain medications that affect tubular reabsorption of urea 2

Recommended Next Steps

1. Assess hydration status:

   • Check for clinical signs of dehydration (dry mucous membranes, decreased skin turgor) 2
   • Review fluid intake and output records 2

2. Evaluate cardiovascular function:

   • Assess for signs of heart failure (peripheral edema, jugular venous distension) 1
   • Check vital signs, particularly for hypotension or orthostatic changes 1

3. Review medication history:

   • Identify nephrotoxic medications, particularly NSAIDs 1, 2
   • Check for diuretic use that may contribute to pre-renal state 1

4. Consider additional laboratory testing:

   • Repeat BUN and creatinine to confirm persistence 2
   • Urinalysis to assess for concentrated urine (high specific gravity) 2
   • Consider thyroid function tests if hyperthyroidism is suspected 5

Management Considerations

• Hydration: If dehydration is suspected, appropriate fluid repletion should be initiated 2
• Medication adjustment: Consider temporarily discontinuing medications that may worsen kidney function, such as NSAIDs, ACE inhibitors, or ARBs in the setting of volume depletion 1
• Monitoring: Follow BUN, creatinine, and BUN/creatinine ratio to assess response to interventions 1, 2
• Underlying cause: Address the primary condition causing the elevated BUN/creatinine ratio 2

Common Pitfalls to Avoid

• Misinterpreting as kidney disease: Avoid assuming kidney dysfunction when creatinine and eGFR are normal 2
• Overlooking heart failure: In patients with heart failure, elevated BUN/creatinine ratio is an independent predictor of mortality, even with normal eGFR 4, 6
• Ignoring protein intake: High protein diets can increase BUN without affecting kidney function 3
• Neglecting age effects: BUN/creatinine ratio tends to increase with age due to age-related changes in muscle mass and renal function 5

When to Consider Nephrology Referral

• If the elevated BUN persists despite addressing obvious causes 2
• If there is subsequent development of elevated creatinine or decreased eGFR 2
• If there are other signs of kidney dysfunction (proteinuria, hematuria) 2