What laboratory tests should be ordered for a patient with a positive Hemoccult (guaiac fecal occult blood test)?

Laboratory Tests for Positive Hemoccult (Fecal Occult Blood Test)

A positive Hemoccult test requires complete colonic evaluation with colonoscopy as the definitive next step in the diagnostic workup. 1

Rationale for Colonoscopy

A positive guaiac fecal occult blood test (gFOBT) indicates the presence of blood in the stool, which may originate from colorectal cancer, advanced adenomas, or other gastrointestinal sources. Guidelines consistently recommend that:

• Any positive FOBT should be followed up with colonoscopy 1
• Repeating the stool test or follow-up with non-colonoscopy tests is inappropriate 1
• The sensitivity of a single gFOBT for detecting colorectal cancer ranges from 37.1% to 79.4%, depending on the test used 1

Laboratory Evaluation to Consider

In addition to colonoscopy, the following laboratory tests should be ordered:

1. Complete Blood Count (CBC)

• To assess for anemia, which may indicate chronic blood loss
• Evaluate red blood cell indices:
  • Microcytosis may reflect iron deficiency from chronic GI blood loss 1
  • Macrocytosis may be associated with vitamin B12 or folate deficiency 1
  • Abnormal white blood cell count or platelet count may reflect more generalized disturbances 1

2. Iron Studies

• Serum iron and total iron binding capacity (TIBC)
• Serum ferritin to assess iron stores
• Transferrin saturation (TSAT)
• These tests help determine if iron deficiency is present, which is common in GI bleeding 1

3. Additional Tests Based on Clinical Suspicion

• Liver function tests if metastatic disease is suspected
• Carcinoembryonic antigen (CEA) if colorectal cancer is confirmed
• Electrolytes, BUN, and creatinine to assess overall health status

Important Considerations

1. Upper vs. Lower GI Source: While gFOBT is often used to detect lower GI bleeding, it can also detect blood from upper GI sources. Research shows that small amounts of blood from the upper GI tract (5-20 ml) can be detected by guaiac-based tests 2.

2. Test Limitations:

   • False positives can occur with certain foods (red meat) or medications
   • False negatives can occur with vitamin C intake 1
   • Single-sample in-office FOBT during digital rectal examination is not recommended due to very low sensitivity 3

3. Risk of False Negatives: Patients with false-negative FOBT results who later develop colorectal cancer may have increased mortality compared to those whose tests convert to positive 4. This emphasizes the importance of proper follow-up and repeated testing.

4. Test Performance: Immunochemical FOBT (FIT) has shown superior sensitivity (81%) compared to unrehydrated gFOBT (13-25%) for colorectal cancer detection 5.

Clinical Pitfalls to Avoid

1. Do not repeat the stool test if the initial test is positive - proceed directly to colonoscopy 1
2. Do not attribute fecal occult blood solely to hemorrhoids without proper colonic evaluation 1
3. Do not rely on single-sample office-based FOBT during digital rectal examination 3
4. Remember that a positive stool guaiac test should not be attributed to hemorrhoids until the colon is adequately evaluated 1

By following this evidence-based approach, you can ensure appropriate evaluation of patients with positive Hemoccult tests, potentially leading to earlier detection of colorectal cancer and improved outcomes.