What is occult stool?

Understanding Occult Stool

Occult stool refers to the presence of blood in fecal matter that is not visible to the naked eye and can only be detected through specific laboratory testing. This microscopic bleeding is typically identified through fecal occult blood tests (FOBTs) designed to detect small amounts of blood that may indicate gastrointestinal pathology 1.

Types of Fecal Occult Blood Tests

There are two primary categories of fecal occult blood tests:

1. Guaiac-based FOBT (gFOBT):

   • Detects blood through the pseudoperoxidase activity of heme or hemoglobin 1
   • Can detect as little as 2-5 mg of hemoglobin per gram of stool 2
   • Requires dietary restrictions before testing (avoid red meat, certain vegetables, vitamin C) 1
   • May produce false positives from non-human blood sources or certain foods 1
   • May detect bleeding from both upper and lower GI tract 3

2. Fecal Immunochemical Test (FIT):

   • Reacts specifically to human globin 1
   • More specific for lower GI bleeding as globin degrades in the upper GI tract 2, 3
   • No dietary restrictions required 2
   • Higher specificity than gFOBT 2

Clinical Significance

Occult blood in stool is clinically significant for several reasons:

• Cancer screening: Primary use is for colorectal cancer screening in average-risk adults aged 50+ 1
• Early detection: Can detect bleeding from colorectal cancer or large polyps (>1-2 cm) before they cause visible bleeding 1
• Mortality reduction: Screening programs using FOBT have demonstrated 15-33% reduction in colorectal cancer mortality 1

Proper Testing Protocol

For accurate results:

• Collect 2 samples from each of 3 consecutive bowel movements 1
• Follow dietary restrictions for guaiac-based tests (avoid red meat, poultry, fish, vitamin C, NSAIDs) 1
• Do not perform single-sample FOBT during digital rectal examination (very low sensitivity) 2
• A positive FOBT requires follow-up with colonoscopy to examine the entire colon 1

Causes of Occult Bleeding

Blood in stool may originate from various sources:

• Lower GI tract: Colorectal cancer, adenomatous polyps (>1-2 cm), inflammatory bowel disease
• Upper GI tract: Peptic ulcer disease, gastritis, esophagitis
• Small intestine: Angiodysplasia, Crohn's disease, tumors 1

Common Pitfalls

1. False positives can occur from:

   • Dietary factors with gFOBT (red meat, certain vegetables) 1
   • Iron supplements 4
   • Non-human blood sources 1

2. False negatives can occur from:

   • Vitamin C intake with guaiac tests 1
   • Intermittent bleeding from lesions 1
   • Improper sample collection 1

3. Improper follow-up:

   • Repeating FOBT after a positive result instead of proceeding to colonoscopy 5
   • Using toilet bowl tests instead of proper collection methods 2

Clinical Application

When occult blood is detected:

1. For screening purposes: A positive FOBT requires diagnostic workup with colonoscopy to examine the entire colon 1

2. For obscure GI bleeding: When the source is not identified by standard endoscopy, additional evaluation with capsule endoscopy may be needed 1

3. For symptomatic patients: FOBT has shown 100% sensitivity and 86.3% specificity for identifying colorectal cancer in symptomatic patients without overt rectal bleeding 6

Remember that a single FOBT has limited sensitivity (30-50%), but this improves with repeated annual testing 2. The presence of occult blood is a nonspecific finding that requires further investigation to determine the underlying cause.