Why BUN is Elevated in Upper Gastrointestinal Bleeding
BUN rises in UGIB because blood in the upper GI tract is digested and absorbed as protein, releasing urea nitrogen that is absorbed into the bloodstream before it can be excreted, creating an elevated BUN-to-creatinine ratio that distinguishes upper from lower GI bleeding. 1
Mechanism of BUN Elevation
- Blood contains approximately 16-20% protein by weight, and when it enters the upper GI tract, digestive enzymes break down hemoglobin and other blood proteins into amino acids 2
- These amino acids are absorbed in the small intestine, metabolized in the liver, and converted to urea, which raises the BUN level disproportionately to creatinine 3
- This process does not occur to the same degree in lower GI bleeding because blood passes through the colon more rapidly with less protein digestion and absorption 2, 3
Diagnostic Value of BUN/Creatinine Ratio
- The BUN/creatinine ratio is significantly higher in UGIB compared to LGIB, with mean values of 22.5 ± 11.5 versus 15.9 ± 8.2 (p = 0.0001) 2
- A BUN/creatinine ratio ≥36 strongly suggests an upper GI source, as no lower GI bleeders demonstrate ratios this high, while 38% of upper GI bleeders exceed this threshold 3
- The optimal cut-off value for differentiating upper from lower GI bleeding is 34.59 mg/g, with an area under the ROC curve of 0.831 4
Clinical Utility and Limitations
- Elevated BUN/creatinine ratio is particularly useful when patients present with bright red rectal bleeding or hematochezia without hematemesis, as 10-15% of patients with severe hematochezia actually have an upper GI source 1, 5
- Risk factors that increase suspicion for UGIB include past medical history of peptic ulcer disease, portal hypertension, elevated blood urea/creatinine ratio, and use of antiplatelet drugs 1
- However, the BUN/creatinine ratio has poor specificity (17%) for lower GI bleeding when using a cut-off of ≤33, meaning significant overlap exists and ratios <36 cannot reliably exclude upper GI sources 2
Prognostic Significance
- An increase in BUN at 24 hours after admission (compared to presentation BUN) independently predicts worse outcomes in acute nonvariceal UGIB, including increased risk of inpatient death (8% vs 1%, p = 0.004) and composite adverse outcomes (22% vs 9%, p = 0.014) 6
- Rising BUN at 24 hours likely reflects inadequate resuscitation and ongoing bleeding, with an adjusted odds ratio of 2.75 for adverse outcomes even after controlling for AIMS65 score 6
- Early dynamic elevation of BUN/creatinine ratio within 6-48 hours after admission shows an area under the ROC curve of 0.806 for predicting primary clinical outcomes, and when combined with Rockall scoring, improves to 0.909 4
Correlation with Severity
- BUN level correlates significantly with transfusion requirements rather than admission vital signs, making it a marker of bleeding severity 2
- BUN is a weak predictor of ICU admission when adjusted for systolic blood pressure (OR 1.03,95% CI 1.00-1.06, p = 0.08), but does not reliably predict high-risk endoscopic lesions 7