What initial laboratory tests are recommended for a patient presenting with a gastrointestinal (GI) bleed?

Initial Laboratory Testing for Gastrointestinal Bleeding

For any patient presenting with suspected GI bleeding, immediately obtain a complete blood count (CBC), basic metabolic panel including blood urea nitrogen (BUN) and creatinine, coagulation studies (PT/INR and aPTT), and type and crossmatch for blood products. 1, 2

Essential First-Line Laboratory Panel

The following tests must be ordered at initial presentation:

• Complete Blood Count (CBC): Obtain hemoglobin and hematocrit to quantify blood loss severity and determine transfusion needs 3, 1, 2

• Basic Metabolic Panel: Include BUN and creatinine to assess volume status, renal function, and help localize bleeding source 3, 1, 2

• Coagulation Studies: Order PT/INR and aPTT to identify coagulopathy that may contribute to bleeding or complicate management 3, 1, 2

• Type and Crossmatch: Essential at initial assessment for patients with signs of severe bleeding or hemodynamic instability to ensure blood products are immediately available 3, 1

• Liver Function Tests: Include albumin and transaminases, particularly important for patients with suspected liver disease or variceal bleeding 3

BUN/Creatinine Ratio for Source Localization

• A BUN/Cr ratio ≥36 strongly suggests upper GI bleeding with 90% specificity, though sensitivity is only 20% 4, 5

• Values <36 do not help localize the bleeding source and should not be used to rule out upper GI bleeding 4, 5

• The elevated ratio in upper GI bleeding results from blood protein digestion and absorption in the small intestine 4

Risk Stratification Laboratory Parameters

Use hemoglobin values for risk stratification scoring systems 1, 6:

• Hemoglobin <70 g/L: highest risk (22 points in Oakland score) 1
• Hemoglobin 70-89 g/L: 17 points 1
• Hemoglobin 90-109 g/L: 13 points 1
• Hemoglobin 110-129 g/L: 8 points 1
• Hemoglobin 130-159 g/L: 4 points 1
• Hemoglobin ≥160 g/L: 0 points 1

Additional laboratory markers predicting poor outcomes include elevated BUN, elevated creatinine, elevated serum aminotransferases, low serum bicarbonate, low arterial pH, elevated lactic acid, and marked leukocytosis 3

Context-Specific Additional Testing

For suspected lower GI bleeding specifically:

• Iron studies to evaluate chronic blood loss 1
• ABO blood group determination 1
• Thyroid function tests (thyroid disorders affect von Willebrand factor levels) 1

For women of childbearing age:

• Pregnancy test with patient consent if any possibility of pregnancy 3, 1

Critical Pitfalls to Avoid

• Do not rely on initial hemoglobin alone: Hemoglobin may not reflect acute blood loss for several hours until hemodilution occurs 2

• Do not assume hemorrhoidal bleeding from occult blood: Complete colonic evaluation by colonoscopy is mandatory when fecal occult blood testing is positive, even if hemorrhoids are present, as hemorrhoidal bleeding typically presents as bright red blood, not occult blood 1

• Do not overlook coagulopathy: Always check coagulation studies before any intervention, as anticoagulation significantly impacts management 3, 2

• Do not delay type and crossmatch: Order immediately at presentation for any patient with hemodynamic instability (systolic BP <100 mmHg, heart rate >100 bpm) or visible severe bleeding 3, 1

• Do not use BUN/Cr ratio <36 to exclude upper GI bleeding: The low sensitivity (20%) means most upper GI bleeds will have ratios <36 4, 5