Is a BUN of 21 mg/dL Normal?
A BUN of 21 mg/dL is at the upper end of normal or mildly elevated, depending on the laboratory reference range used, but this value alone requires clinical context to determine its significance.
Normal Reference Range
- The typical normal range for BUN is approximately 7-20 mg/dL, though some laboratories use slightly different cutoffs 1
- A BUN of 21 mg/dL falls just above the conventional upper limit but would not be considered significantly elevated 1
- Reference ranges are not sex-specific for BUN 2
Clinical Interpretation Depends on Context
When BUN of 21 is Likely Benign
- In outpatients with no symptoms, stable chronic conditions, and normal creatinine, a BUN of 21 mg/dL typically does not warrant aggressive investigation 3
- This level is well below thresholds used to define clinically significant azotemia in most guidelines 2
When BUN of 21 Warrants Further Evaluation
In critically ill or hospitalized patients, even this modest elevation has prognostic significance:
- BUN ≥20 mg/dL is used as a minor criterion for ICU admission in pneumonia severity scoring (CURB-65) 1
- In heart failure patients, any progressive rise in BUN—even within the "normal" range—reflects congestion, fluid retention, and cardiac dysfunction and predicts worse outcomes 2, 1
- Among ICU patients, BUN >28 mg/dL is independently associated with adverse long-term mortality, but values in the 20s still carry increased risk 4
In patients with acute coronary syndromes, BUN of 21 mg/dL falls into a range associated with increased mortality risk independent of kidney function 5
Key Clinical Pearls
- Never interpret BUN in isolation—always evaluate alongside serum creatinine, estimated GFR, and clinical volume status 1, 3
- The BUN:creatinine ratio matters: A ratio >20:1 suggests prerenal azotemia (dehydration, heart failure, decreased renal perfusion) rather than intrinsic kidney disease 6
- Trending is more important than a single value: Progressive increases in BUN, even within the "normal" range, indicate clinical deterioration in heart failure and other conditions 2, 1
Common Causes of BUN Around 21 mg/dL
- Mild dehydration or volume depletion 3, 6
- Early or compensated heart failure 2, 1
- High protein intake (>100 g/day) 6
- Increased protein catabolism from infection, steroids, or hypercatabolic states 6
- Early kidney dysfunction not yet reflected in creatinine 7
- Advanced age with lower muscle mass (creatinine may appear falsely reassuring) 3, 6
Recommended Next Steps
If the patient is asymptomatic and outpatient:
- Confirm with serum creatinine and calculate estimated GFR 3
- Assess hydration status clinically 3
- No immediate intervention needed if creatinine is normal and patient is well-hydrated 3
If the patient has heart failure, is hospitalized, or critically ill:
- Evaluate volume status and optimize diuretic therapy if fluid overloaded 2, 3
- Monitor BUN serially as it predicts outcomes better than creatinine in these populations 2, 1
- Consider intravenous fluids if signs of dehydration are present 3
If BUN:creatinine ratio is elevated (>20:1):