Fecal occult blood test (guaiac and FIT)

Principle

• Guaiac test (gFOBT): heme has pseudoperoxidase activity; with hydrogen peroxide developer it oxidizes guaiac to a blue color.
• Fecal immunochemical test (FIT): antibodies detect human globin; it is specific for human blood and mainly detects lower GI bleeding, because globin from upper GI bleeding is digested.

Specimen

• gFOBT: samples from 3 consecutive bowel movements, two areas of each stool applied to the card windows.
• FIT: usually a single sample collected with the probe into buffer; no dietary restriction needed.

Reagents and equipment

• Guaiac cards with developer and on-card performance controls; FIT devices (qualitative cassettes or quantitative analyzers).

Procedure

• gFOBT: apply a thin smear to each window; allow to dry; in the lab add developer to the back of the card and read within the time stated (usually about 60 seconds); then develop the performance monitor areas.
• FIT: follow manufacturer instructions for sample load, buffer and read time or analyzer run.

Results

• gFOBT: any trace of blue within the read time is positive.
• FIT: result reported as positive/negative or as µg hemoglobin per gram of feces against a program-specific cutoff.
• A positive screening result needs follow-up colonoscopy.

Quality control and pitfalls

• Check on-card positive and negative monitors with every gFOBT card; run kit controls for FIT.
• gFOBT false positives: red meat, peroxidase-rich raw vegetables (e.g. turnip, horseradish, radish, broccoli), NSAIDs/aspirin, bleeding gums, menstrual blood.
• gFOBT false negatives: vitamin C above about 250 mg/day, old or dry samples, intermittent bleeding.
• Do not rehydrate guaiac slides; it raises false positives.
• FIT samples degrade with heat and delay; return promptly.

Clinical use

• Colorectal cancer screening (FIT is preferred in many programs) and investigation of occult GI bleeding.