What does a positive Fecal Immunochemical Test (FIT) result indicate?

What Does a Positive Fecal Immunochemical Test (FIT) Mean?

A positive FIT result indicates the detection of human hemoglobin in stool, which signals a significantly elevated risk of colorectal cancer or advanced neoplasia (precancerous polyps), requiring immediate follow-up colonoscopy rather than repeat stool testing. 1

Clinical Significance and Predictive Value

A positive FIT carries substantial diagnostic weight:

• The positive predictive value for colorectal cancer ranges from 2.9% to 7.8%, meaning approximately 3-8 out of every 100 people with a positive FIT will have cancer 1
• The positive predictive value for advanced neoplasia (cancer plus advanced adenomas) ranges from 33.9% to 54%, indicating that roughly one-third to one-half of positive FIT results will reveal significant colorectal pathology requiring intervention 1
• FIT demonstrates approximately 80% sensitivity for detecting colorectal cancer with a single application, making it highly effective at identifying existing cancers 1

What FIT Actually Detects

FIT is an immunochemical test that specifically identifies human hemoglobin in stool specimens 2. Unlike older guaiac-based tests, FIT:

• Detects only human blood, not dietary sources or animal blood 1
• Requires no dietary restrictions or medication adjustments before testing, including continuation of iron supplements, anticoagulants, or antiplatelet agents 3
• Identifies bleeding from colorectal sources, though positive results can occasionally indicate bleeding from upper gastrointestinal, esophageal, stomach, or small intestine cancers 4, 5

Mandatory Next Step: Colonoscopy

Given the high positive predictive value for significant neoplasia, colonoscopy is the required follow-up test when FIT is positive—not repeat FIT testing. 1

Timing Requirements

• Colonoscopy should be scheduled within 3 months of the positive result, with optimal timing being as soon as possible 6
• The maximum acceptable delay is 6 months; delays beyond 10 months are associated with significantly increased risk of colorectal cancer (OR 1.48 at 10-12 months, OR 2.25 beyond 12 months) and advanced-stage disease (OR 1.97 at 10-12 months, OR 3.22 beyond 12 months) 6, 7
• Healthcare systems should aim for ≥95% of follow-up colonoscopies performed within 6 months, with at least 80% offered an appointment within 3 months 6

Why Colonoscopy Is Essential

Colonoscopy is the optimal follow-up because it directly evaluates the entire colorectal mucosa and simultaneously allows removal of detected neoplasia 1, 6. This dual diagnostic and therapeutic capability makes it superior to other imaging modalities for positive FIT follow-up.

Important Clinical Distinctions

When FIT Should NOT Be Used

FIT is a screening tool for asymptomatic, average-risk individuals—not a diagnostic test for symptomatic patients 8:

• Patients with visible blood in stool should proceed directly to colonoscopy, not FIT testing, as visible bleeding is an alarm symptom requiring immediate evaluation 8
• Patients with iron deficiency anemia should bypass FIT entirely and proceed directly to colonoscopy, as FIT has reduced sensitivity (58%) and specificity (84%) in this population 3

Appropriate FIT Use

FIT is recommended for:

• Average-risk, asymptomatic individuals aged 50 years and older as part of routine colorectal cancer screening 8
• Annual or biennial screening programs to maximize cancer detection and prevention effectiveness 1

Common Pitfalls to Avoid

• Never repeat FIT testing after a positive result—this delays necessary diagnostic evaluation and potentially misses significant lesions 1
• Do not delay colonoscopy to perform additional testing in patients with positive FIT 8
• Do not use FIT as a diagnostic tool in symptomatic patients, as it has not been validated for this purpose 8
• Recognize that a negative colonoscopy after positive FIT generally does not require upper gastrointestinal evaluation unless iron-deficiency anemia or upper GI symptoms are present 1

Programmatic Considerations

Positivity rates decrease with successive screening rounds (from 8-10% in first round to 4-6% in subsequent rounds) as prevalent cancers and advanced adenomas are detected and removed 1. However, the positive predictive value for advanced neoplasia remains high (30-40%) throughout multiple screening rounds 1, maintaining the clinical significance of positive results over time.