How to Collect a Stool Specimen for Fecal Immunochemical Test (FIT)
Collect a single stool sample from one spontaneously passed bowel movement at home using the collection device provided in the FIT kit, without any dietary or medication restrictions, and return it to the laboratory promptly. 1
No Dietary or Medication Restrictions Required
Unlike older guaiac-based tests, FIT does not require any dietary modifications before or during sample collection. 1
- Continue eating normally – red meat, poultry, fish, and vegetables do not interfere with FIT results because the test specifically detects human hemoglobin globin rather than peroxidase activity from food. 1
- Continue all medications – aspirin, NSAIDs, warfarin, and clopidogrel do not need to be stopped, as high-quality prospective studies show these medications do not negatively impact FIT test characteristics. 1
- Patients should be explicitly instructed that no dietary or medication adjustments are necessary, which simplifies testing and enhances adherence. 1
Proper Sample Collection Method
Use only spontaneously passed stool specimens collected at home, not samples obtained during a digital rectal examination in the office. 1
- Collect the sample from a single bowel movement using the collection device provided in your FIT kit. 1
- Follow the specific instructions included with your particular FIT kit, as collection devices vary by manufacturer. 1
- Some FIT kits require inserting a sampling probe into the stool multiple times and checking visually that the probe is completely filled with stool, though violations of these instructions generally have only small effects on test performance. 2
Critical Pitfall to Avoid: No Office-Based DRE Samples
Do not use stool obtained during a digital rectal examination in the clinical office, as this approach shows significantly different test performance compared to spontaneously passed stool. 1
- DRE-based samples have a higher positivity rate (5.4% vs 3.5%) but significantly lower positive predictive value for both adenomas and cancer. 1
- The Task Force explicitly recommends against using in-office DRE samples for FIT screening. 1
Timing and Return of Sample
Return the specimen to the laboratory as soon as possible after collection to minimize degradation of hemoglobin. 3
- FIT specimens stored at room temperature show reduced hemoglobin concentrations over time, with specimens near the clinical cutoff particularly susceptible to false negatives. 3
- While manufacturer stability claims may state 15 days at room temperature, specimens should ideally be returned within 3-7 days to reduce the risk of false negative results. 3
- Refrigeration improves specimen stability but does not completely prevent hemoglobin degradation. 3
Single Sample vs. Multiple Samples
Most current FIT protocols use a single-sample approach collected from one bowel movement. 1
- A two-sample FIT (from multiple bowel movements) shows higher detection rates for advanced neoplasia compared to single-sample FIT (4.1% vs 3.1%), but the single-sample approach is more commonly recommended to enhance patient adherence. 1
- The Task Force suggests a one-sample annual FIT screening approach as an acceptable balance between detection and adherence. 1
What Happens After Collection
If your FIT result is positive, you will need a colonoscopy to examine your entire colon – never simply repeat the FIT test. 1