FIT Test and Iron Therapy
Iron therapy does not interfere with FIT testing and should not be discontinued before performing the test. 1
Key Evidence on Iron Therapy and FIT
Unlike the older guaiac-based fecal occult blood tests (gFOBT), FIT is not affected by oral iron supplementation:
No dietary or medication adjustments are needed when using FIT, including continuation of iron supplements, anticoagulants, or antiplatelet agents. 1
Prospective studies examining FIT test characteristics in patients using various medications found no negative impact on test performance from iron supplementation. 1
A randomized, double-blind study of 78 healthy volunteers demonstrated that oral iron supplementation (ferrous sulfate or ferrous gluconate) does not cause false-positive results on fecal occult blood testing. 2
To simplify testing and enhance patient adherence, patients should be explicitly instructed that they do not need to adjust diet or medications to complete FIT testing. 1
Important Distinction: Iron Deficiency Anemia vs. Iron Therapy
While iron therapy does not affect FIT results, there is a critical distinction regarding iron deficiency anemia itself:
FIT should not be used in patients with iron deficiency anemia as a screening tool, as it has reduced sensitivity (58%) and specificity (84%) in this population. 1
The 2023 APAGE-APSDE guidelines explicitly state: "We do not recommend the use of FIT in patients with iron deficiency anaemia." 1
Patients with iron deficiency anemia require direct colonoscopy evaluation rather than FIT screening, as they are at higher risk for colorectal pathology. 1, 3
Clinical Algorithm
For patients on iron therapy:
- Continue iron supplementation without interruption
- Proceed with FIT testing as scheduled
- No dietary restrictions needed 1
For patients with iron deficiency anemia:
- Bypass FIT testing entirely
- Proceed directly to colonoscopy for diagnostic evaluation 1
- FIT-positive results in iron deficiency anemia patients predict bleeding lesions (OR 9.5) and cancer (OR 4.0), but the test's reduced sensitivity makes it unreliable for screening in this population 3
Common Pitfall to Avoid
Do not confuse iron therapy (which does not affect FIT) with iron deficiency anemia (which is a contraindication to using FIT for screening). The former refers to medication use, while the latter refers to the underlying clinical condition requiring direct endoscopic evaluation. 1