Assessment of GI Blood Loss
To assess GI blood loss, order a complete blood count (CBC) to check for anemia and a fecal occult blood test (FOBT), with stool Hemoccult and CBC recommended as the initial screening tests. 1
Initial Laboratory Evaluation
The screening workup for suspected GI blood loss should include:
- Complete blood count (CBC) to identify iron deficiency anemia, which is the most common manifestation of chronic GI bleeding 1
- Fecal occult blood test (FOBT) using either guaiac-based tests (Hemoccult II or Hemoccult II SENSA) or immunochemical tests 1
- Serum ferritin concentration, which is the most powerful test for iron deficiency 1
Understanding FOBT Limitations
While FOBT is recommended for screening, it has important limitations:
- Fecal occult blood testing alone is insensitive and non-specific for determining the bleeding source and should not be used as the sole diagnostic test 2
- Guaiac-based tests can detect upper GI bleeding, not just lower GI sources - studies show Hemoccult II SENSA detected blood in 64% of subjects ingesting small amounts of blood (10-20 mL), while immunochemical tests failed to detect any upper GI blood 3
- A positive FOBT does not localize the bleeding site - it simply indicates blood is present somewhere in the GI tract and requires complete evaluation 4
Additional Screening Tests
Based on clinical presentation, consider:
- Sedimentation rate (particularly in younger patients) 1
- Serum chemistries and albumin 1
- Stool for ova and parasites if diarrhea-predominant symptoms or endemic infection risk 1
- Celiac serology (tissue transglutaminase antibody) as 2-3% of patients with iron deficiency anemia have celiac disease 1
- Urine testing for blood as approximately 1% of patients with iron deficiency anemia have renal tract malignancy 1
Common Pitfalls to Avoid
- Do not rely on single-sample FOBT collected during digital rectal examination, which has poor sensitivity and may yield false positives 2
- Do not assume a positive guaiac test indicates only lower GI bleeding - upper GI sources can produce positive results 3
- Do not accept FOBT results as definitive - they require endoscopic confirmation regardless of result 2
When FOBT is Positive
A positive FOBT mandates further investigation:
- Colonoscopy is the preferred initial diagnostic procedure as it allows direct visualization and therapeutic intervention 2
- Upper endoscopy (EGD) should follow even if colonoscopy identifies a lesion, as dual pathology occurs in 10-15% of patients 1
- Patients with occult GI blood loss and no anemia may not require evaluation beyond colonoscopy unless upper tract symptoms are present 1
- Patients with occult GI blood loss and iron deficiency anemia need comprehensive evaluation including both upper and lower endoscopy 1