What tests can be ordered to check for blood in stool?

Tests to Check for Blood in Stool

Order a fecal immunochemical test (FIT) or high-sensitivity guaiac-based fecal occult blood test (gFOBT) with proper 3-day home collection from consecutive bowel movements. 1

Primary Testing Options

Two main categories of stool blood tests are available for detecting occult blood:

Fecal Immunochemical Test (FIT)

• FIT is the preferred option because it requires no dietary restrictions, has fewer false positives, and must be processed in a clinical laboratory 2
• Reacts specifically to human globin, eliminating dietary interference 1
• Requires annual testing with collection of specimens from consecutive bowel movements 1

Guaiac-Based Fecal Occult Blood Test (gFOBT)

• Detects blood through pseudoperoxidase activity of heme or hemoglobin 1
• The only stool blood test with proven mortality reduction (15-33%) from randomized controlled trials 1
• Can be processed in physician's office or clinical laboratory 1

Critical Testing Requirements

Proper Collection Protocol

• Must collect 2 samples from each of 3 consecutive bowel movements at home 1
• Single-sample office testing after digital rectal exam has unacceptably poor sensitivity of only 4.9% for advanced disease and should never be used 2, 3
• Collection of all 3 samples is essential because test sensitivity improves with each additional stool sample 1

Pre-Test Restrictions for gFOBT (Not Required for FIT)

• Avoid NSAIDs and aspirin (>1 adult dose/day) for 7 days before testing 2, 3
• Avoid vitamin C (>250 mg), red meat, poultry, fish, and some raw vegetables for 3 days before testing 1, 2
• These restrictions minimize false-positive results from dietary pseudoperoxidase activity and medication-induced bleeding 2

Common Pitfalls to Avoid

What NOT to Do

• Never perform single-sample FOBT during office digital rectal examination - this method has only 4.9% sensitivity and leads to false reassurance 2, 3
• Never rehydrate gFOBT slides, as this substantially increases false-positive rates 2
• Never repeat FOBT if the initial test is positive - proceed directly to colonoscopy 2, 3

False Positive Causes

• Recent red meat consumption strongly interferes with gFOBT results 2
• Iron supplements can interfere with test interpretation 2
• Digital rectal examination immediately before sample collection can cause trauma-related bleeding 2

Follow-Up Based on Results

If Test is Positive

• Any positive result requires colonoscopy within 60 days to examine the entire colon 1, 3
• Never repeat the stool test - colonoscopy is mandatory 2, 3
• Flexible sigmoidoscopy alone is inadequate as it misses proximal lesions 3

If Test is Negative

• Requires annual repetition for screening purposes 2
• A negative test does not rule out significant pathology in symptomatic patients 3, 4
• For patients over age 50, colonoscopy may still be recommended based on screening guidelines regardless of FOBT result 3

Test Performance Characteristics

• Sensitivity of properly performed 3-day gFOBT ranges from 37.1% (non-rehydrated Hemoccult II) to 64.3% (Hemoccult SENSA) for cancer 2
• Specificity is 97.7-98.1% for low-sensitivity tests but lower (86.7-87.5%) for high-sensitivity tests 2
• Program sensitivity with repeated annual testing can detect up to 92% of cancers over time 2
• FIT generally has fewer false positives than gFOBT due to lack of dietary interference 2

Important Clinical Context

• These tests primarily detect cancer rather than small adenomatous polyps, which do not tend to bleed 1
• Bleeding from cancers or large polyps may be intermittent, which is why multiple consecutive samples are essential 1
• Guaiac-based tests can detect upper GI bleeding, while immunochemical tests (FIT) are specific to lower GI sources 5