What blood test indicates a patient is swallowing red blood cells?

Blood Test to Detect Swallowed Red Blood Cells

The blood urea nitrogen (BUN) test, particularly an elevated BUN-to-creatinine ratio, is the primary blood test that indicates a patient is swallowing red blood cells from upper gastrointestinal bleeding. 1

Mechanism and Interpretation

When red blood cells are swallowed and digested in the gastrointestinal tract, the breakdown of hemoglobin releases protein that is absorbed in the small intestine, leading to:

• Elevated BUN levels while creatinine remains normal or stable, creating a disproportionately high BUN-to-creatinine ratio (typically >20:1 or >30:1) 1
• This elevation occurs because the digested blood protein is metabolized to urea, which is measured as BUN 1

Initial Laboratory Workup for Suspected Blood Swallowing

When evaluating a patient for potential swallowed blood from upper GI bleeding, order the following tests immediately:

• Complete blood count (CBC) to assess hemoglobin, hematocrit, and detect anemia 1, 2
• BUN and creatinine to calculate the BUN-to-creatinine ratio 1, 2
• Coagulation profile (PT/INR, PTT) 2
• Type and cross-match for potential blood transfusion 1, 2
• Serum electrolytes 1

Supporting Laboratory Findings

Additional blood test abnormalities that may accompany swallowed blood include:

• Anemia (low hemoglobin/hematocrit) indicating blood loss 1
• Elevated reticulocyte count (corrected for anemia) suggesting active blood loss or hemolysis 1, 3
• Iron deficiency markers in chronic cases: low serum iron, elevated TIBC, low ferritin 1

Stool Testing Considerations

While not a blood test, fecal occult blood testing can confirm GI bleeding but has significant limitations:

• Guaiac-based tests have high false-positive rates (72-76%) 4
• Modified guaiac tests (Hemoccult) have lower false-positive rates (12%) but reduced sensitivity 4
• Swallowed blood from epistaxis or oral sources will produce positive results 4, 5

Clinical Context and Pitfalls

Critical caveat: The BUN elevation from swallowed blood typically appears 6-24 hours after the bleeding episode, as digestion and absorption take time. 1

Common pitfall to avoid: Do not rely solely on initial hemoglobin/hematocrit values in acute bleeding, as hemodilution takes time and initial values may not reflect the true extent of blood loss. 1, 2 Serial measurements over several hours provide more accurate assessment. 1

Important distinction: An elevated BUN with normal creatinine suggests upper GI bleeding with swallowed blood, while elevated BUN with proportionally elevated creatinine suggests renal dysfunction or dehydration. 1

When Swallowed Blood is Suspected

If upper GI bleeding with swallowed blood is suspected based on elevated BUN-to-creatinine ratio:

• Perform nasogastric tube aspiration to confirm upper GI source (though this has limitations) 1
• Proceed with upper endoscopy within 24 hours after hemodynamic stabilization 1, 2
• Monitor for hemodynamic instability requiring immediate intervention 1, 2

The BUN test remains the most specific blood marker for detecting swallowed red blood cells, though it must be interpreted in the full clinical context with supporting laboratory and clinical findings. 1