Can a lower gastrointestinal (GI) bleed cause elevated Blood Urea Nitrogen (BUN)?

From the Guidelines

Yes, a lower gastrointestinal (GI) bleed can cause elevated blood urea nitrogen (BUN) levels. When significant bleeding occurs in the lower GI tract, blood proteins are broken down and absorbed in the intestines, leading to increased protein load that is metabolized by the liver into urea. This process results in higher BUN levels in the bloodstream. Additionally, blood loss from a GI bleed can cause decreased kidney perfusion, which triggers the kidneys to increase water reabsorption, further concentrating the BUN. The elevation is often accompanied by a disproportionate increase in the BUN to creatinine ratio, typically greater than 20:1, which can help distinguish this cause from other reasons for elevated BUN.

Key Points to Consider

• Clinicians should be aware that the degree of BUN elevation often correlates with the severity of bleeding, and very high levels may indicate substantial blood loss requiring urgent intervention 1.
• Other signs that may accompany this finding include hemodynamic instability, hematochezia, melena, and a drop in hemoglobin levels.
• The British Society of Gastroenterology recommends that patients presenting with lower gastrointestinal bleeding (LGIB) are stratified as unstable or stable, and stable bleeds should then be categorized as major or minor, using a risk assessment tool such as the Oakland score 1.
• It is essential to note that the management of lower GI bleed should prioritize the patient's hemodynamic stability and the severity of bleeding, and that the elevation of BUN levels should be considered in the context of the overall clinical picture.

Management Considerations

• The guidelines from the British Society of Gastroenterology suggest that patients with a major bleed should be admitted to hospital for colonoscopy, and that CT angiography provides the fastest and least invasive means to localize the site of blood loss before planning endoscopic or radiological therapy 1.
• In cases of unstable gastrointestinal hemorrhage, anticoagulation should be reversed with prothrombin complex, and restrictive red blood cell (RBC) thresholds should be used, unless the patient has a history of cardiovascular disease 1.

From the Research

Lower GI Bleed and Elevated BUN

• The relationship between lower GI bleed and elevated blood urea nitrogen (BUN) is not directly addressed in the provided studies, but we can infer some information from the studies on the BUN/creatinine ratio in distinguishing upper from lower gastrointestinal bleeding.
• According to the study by 2, the mean BUN/creatinine ratio was significantly higher in upper GI bleeders (34.8) than in lower GI bleeders (17.8), suggesting that a higher BUN level may be more indicative of upper GI bleeding.
• However, the study by 3 found that a BUN/creatinine ratio of greater than 35 can predict upper GI bleeding with high probability, but values less than 35 are not diagnostic, implying that lower GI bleeding may not necessarily result in elevated BUN levels.
• Another study by 4 found that a BUN/creatinine ratio greater than 36 had a sensitivity of 90% and a specificity of 27% for predicting an upper GI source of bleeding, which suggests that lower GI bleeding may not typically result in significantly elevated BUN levels.

BUN/Creatinine Ratio in GI Bleeding

• The BUN/creatinine ratio has been studied as a potential marker for distinguishing upper from lower GI bleeding, with higher ratios suggesting upper GI bleeding 2, 3, 4.
• However, the studies suggest that while a high BUN/creatinine ratio may indicate upper GI bleeding, a low ratio does not necessarily rule out upper GI bleeding, and other diagnostic methods should be used in conjunction with this ratio.

Clinical Implications

• In clinical practice, the BUN/creatinine ratio may be used as one of several factors to help distinguish upper from lower GI bleeding, but it should not be relied upon as the sole diagnostic criterion 2, 3, 4.
• Further studies are needed to fully understand the relationship between lower GI bleeding and elevated BUN levels, as well as the clinical utility of the BUN/creatinine ratio in diagnosing GI bleeding.