What are the chances of a positive Fecal Immunochemical Test (FIT) result being due to colorectal cancer in an average-risk individual over 50 years old with a family history of colorectal cancer?

Positive Predictive Value of FIT for Colorectal Cancer

If you have a positive FIT test, your chance of actually having colorectal cancer is approximately 2-12%, meaning that 88-98% of positive FIT results are NOT due to colorectal cancer. 1

Understanding What a Positive FIT Means

The positive predictive value (PPV) for colorectal cancer specifically is quite low, though the PPV for any significant neoplasia (cancer plus advanced adenomas) is considerably higher:

• PPV for colorectal cancer alone: 1.2-2.4% 2
• PPV for advanced neoplasia (cancer + advanced adenomas): 18.9-24.4% 2
• This means approximately 75-80% of positive FIT results lead to colonoscopies that find no advanced neoplasia at all 2

How This Relates to FIT's Specificity

Specificity tells you the opposite story - it measures how often FIT correctly identifies people WITHOUT disease:

• FIT specificity ranges from 91-96% 3, 4
• This means 4-9% of people without colorectal cancer will have a false-positive FIT result 3
• In absolute numbers: among 100 people without advanced neoplasia, 4-9 will still test positive 3

The Mathematical Relationship

The low PPV despite high specificity occurs because colorectal cancer is relatively uncommon even in screening populations:

• Sensitivity for cancer: 73-88% (FIT catches most cancers) 3, 4
• Specificity: 91-96% (FIT correctly identifies most people without disease) 3, 4
• But prevalence of cancer in average-risk screening populations is only about 0.5-1% 2

When disease prevalence is low, even a highly specific test will generate many false positives relative to true positives 1.

What Causes Positive FIT Results Besides Cancer

The majority of positive FIT results are due to:

1. Advanced adenomas (large polyps ≥1cm): 60-70% of all advanced neoplasia detected 1
2. Non-advanced adenomas and other benign findings: 20-30% 2
3. No identifiable source (false positives): 39-44% 2
4. Upper GI or other proximal cancers: <1% (0.6-0.7% cumulative 3-year incidence) 5

Risk Factors That Increase False-Positive Rates

• NSAID use increases false-positive risk by 16% (RR 1.16) 6
• Female sex has higher false-positive rates (males have RR 0.84 for false positives) 6

Clinical Implications for Your Patient

When counseling a patient with a positive FIT:

• Emphasize that colonoscopy is mandatory - this is a strong recommendation with moderate-quality evidence 1
• Explain that most positive results (75-80%) will not show advanced disease, but colonoscopy is still essential because we cannot distinguish true from false positives without it 2
• Do NOT repeat the FIT - this is inappropriate and delays necessary evaluation 7
• Do NOT pursue upper GI evaluation if colonoscopy is negative and there are no iron-deficiency anemia or upper GI symptoms 1

Special Consideration for Family History

Your patient has a family history of colorectal cancer, which:

• Increases their baseline risk of having advanced neoplasia 1
• Increases their risk of false-negative results (RR 1.61) if the FIT were negative 6
• May slightly improve the PPV of their positive result compared to someone without family history, though specific data on this interaction is limited 1

Common Pitfalls to Avoid

• Never delay colonoscopy to repeat stool testing - the positive FIT has already triggered the need for definitive evaluation 7
• Do not order upper endoscopy reflexively after negative colonoscopy unless there are specific indications like iron-deficiency anemia 1
• Do not falsely reassure patients that a positive FIT "probably means nothing" - while cancer is unlikely, advanced adenomas requiring removal are found in about 20-25% of cases 2
• Ensure colonoscopy occurs within 60-270 days - delays beyond 270 days significantly increase risk of late-stage cancer (OR 1.48) 8