What urea (mmol/L) to creatinine (µmol/L) ratio threshold corresponds to the approximately 35:1 BUN‑to‑creatinine ratio used to identify upper gastrointestinal bleeding?

BUN/Creatinine Ratio Threshold for Upper GI Bleeding

The urea (mmol/L) to creatinine (µmol/L) ratio threshold of approximately 0.10 corresponds to the 35:1 BUN-to-creatinine ratio used to identify upper gastrointestinal bleeding.

Unit Conversion and Threshold

To convert the commonly cited BUN/Cr ratio of approximately 35:1 (mg/dL : mg/dL) to SI units:

• BUN (mg/dL) to urea (mmol/L): Divide by 2.8
• Creatinine (mg/dL) to creatinine (µmol/L): Multiply by 88.4
• Resulting ratio: A BUN/Cr of 35 (mg/dL : mg/dL) = approximately 0.10 (mmol/L : µmol/L)

The most recent high-quality research identified a BUN/Cr ratio of 34.59 mg/g (equivalent to 34.59:1 in conventional units) as the optimal cut-off for distinguishing upper from lower GI bleeding, with an area under the curve of 0.831 1. This translates to approximately 0.10 in SI units (mmol/L : µmol/L).

Diagnostic Performance

A ratio ≥0.10 (or ≥35:1 in conventional units) strongly suggests upper GI bleeding with high specificity:

• Specificity: 90.16% for values ≥35 2
• Positive predictive value: 89.09% 2
• Sensitivity: Only 19.63%, meaning many upper GI bleeds will have ratios <35 2

The American College of Physicians recognizes this ratio as a diagnostic tool, with the 34.59 threshold demonstrating reliable performance in differentiating bleeding sources 3.

Clinical Application Algorithm

When evaluating a patient with GI bleeding:

1. Calculate the urea/creatinine ratio immediately upon presentation 3, 4

2. If ratio ≥0.10 (≥35:1): Upper GI bleeding is highly likely—proceed directly to upper endoscopy 5, 2

3. If ratio <0.10 (<35:1): The ratio cannot reliably exclude upper GI bleeding due to low sensitivity; clinical judgment and other factors must guide diagnostic approach 5, 2

4. Integrate with other clinical features: Hematemesis, melena, hemodynamic instability, history of peptic ulcer disease, portal hypertension, or antiplatelet use all increase likelihood of upper GI source regardless of ratio 4

Important Caveats

The ratio has critical limitations that must be recognized:

• Low sensitivity (19.63%) means 80% of upper GI bleeds will have ratios <35 2—never use a low ratio to rule out upper GI bleeding

• Dehydration from any cause elevates BUN disproportionately to creatinine, potentially causing false-positive results 4

• Pre-renal azotemia from volume depletion (the mechanism in GI bleeding) causes 40-50% of filtered urea to be reabsorbed in the proximal tubule while creatinine reabsorption remains minimal, explaining the disproportionate BUN rise 4

• Patients on RAAS inhibitors with diuretics are at increased risk for elevated BUN/Cr ratios due to excessive diuresis and impaired renal autoregulation, independent of GI bleeding 4

The ratio is most useful when ≥0.10 (≥35:1) to confirm upper GI bleeding with high probability, but values below this threshold provide no diagnostic value 5, 2.

Prognostic Significance

Beyond diagnosis, BUN elevation predicts worse outcomes in confirmed upper GI bleeding:

• BUN is a core component of the Blatchford score, which accurately identifies patients requiring clinical intervention 3, 4

• An increase in BUN at 24 hours compared to admission predicts 2.75-fold increased odds of death, rebleeding, or need for intervention 3, 4

• The Rockall score incorporates BUN alongside hemoglobin, pulse, blood pressure, syncope, melena, and comorbidities to predict rebleeding and mortality 3