Why is Blood Urea Nitrogen (BUN) elevated in cases of acute bleeding?

From the Guidelines

Blood Urea Nitrogen (BUN) becomes elevated in acute bleeding primarily due to increased protein catabolism and reduced renal perfusion. When significant bleeding occurs, the body breaks down red blood cells and other proteins from the hemorrhaged blood, leading to higher protein metabolism. This process generates more urea as a byproduct, which directly increases BUN levels. Additionally, acute blood loss causes hypotension and decreased blood volume, reducing blood flow to the kidneys. With diminished renal perfusion, the kidneys cannot filter blood efficiently, resulting in decreased urea excretion and further BUN elevation. The gastrointestinal tract may also absorb more blood proteins during internal bleeding, contributing to higher urea production.

Key factors to consider in the management of acute bleeding include:

• Volume resuscitation via adequate fluid therapy and packed red blood cell (PRBC) transfusion to restore and maintain hemodynamic stability 1
• Restrictive PRBC transfusion, initiating at a hemoglobin threshold of 7 g/dL and maintaining at 7–9 g/dL, to decrease bleeding-related mortality and serious adverse events 1
• Avoiding excessive fluid therapy/PRBC transfusion to prevent increased portal pressure and aggravation of bleeding from varices 1

This elevation typically occurs within hours of significant bleeding and can be an important diagnostic clue, especially when the BUN-to-creatinine ratio exceeds the normal 10:1 to 20:1 range. Clinicians should consider acute bleeding as a potential cause when encountering unexplained BUN elevation, particularly when accompanied by falling hemoglobin levels or other signs of blood loss.

From the Research

Blood Urea Nitrogen (BUN) Elevation in Acute Bleeding

• BUN elevation in cases of acute bleeding can be attributed to several factors, including:
  • Decreased blood volume and renal perfusion, leading to increased reabsorption of urea by the kidneys 2
  • Release of urea from broken-down blood cells and tissues 3
  • Increased catabolism and stress response, resulting in elevated urea production 4
• The relationship between acute bleeding and BUN elevation is complex and influenced by various factors, such as:
  • Severity and source of bleeding 3
  • Patient's underlying health conditions and comorbidities 4
  • Timeliness and effectiveness of resuscitation and treatment 5, 2
• Studies have shown that fluid resuscitation strategies, such as those guided by pulse-indicated continuous cardiac output (PiCCO) monitoring, can help improve renal function and reduce BUN levels in patients with septic shock-induced acute kidney injury 5
• The use of blood products and tranexamic acid in the management of critical bleeding may also impact BUN levels, although the exact mechanisms and effects are not fully understood 6