Iron Supplementation and Fecal Occult Blood Testing Protocol
Direct Answer
You do NOT need to discontinue iron tablets before sending a stool sample for occult blood testing (OBT). Oral iron supplementation does not cause false-positive results on standard guaiac-based fecal occult blood tests (Hemoccult II or Hemoccult Sensa) or immunochemical tests 1, 2.
Evidence Supporting Continued Iron Use
Prospective randomized controlled trials demonstrate that therapeutic doses of ferrous sulfate (324 mg three times daily) and ferrous gluconate do not produce false-positive Hemoccult test results 1, 2.
In a double-blind crossover study of 78 healthy volunteers, zero positive results occurred after iron ingestion when tested by either Hemoccult II or Hemoccult Sensa methods 1.
Quantitative fecal blood assays (HemoQuant) confirmed that oral iron preparations in therapeutic doses do not increase gastrointestinal blood loss compared to controls 2.
Proper FOBT Collection Protocol
The patient should collect stool samples at home using the standard 3-day protocol, regardless of iron supplementation:
Collect 2 samples from each of 3 consecutive bowel movements (total of 6 samples) 3, 4.
Do NOT collect a single sample during digital rectal examination in the office - this method has unacceptably low sensitivity (only 4.9% for advanced neoplasia) 4, 5.
Dietary and Medication Restrictions
For guaiac-based tests (gFOBT), the patient should avoid:
Red meat, poultry, and fish for 3 days before and during testing 4, 5.
NSAIDs (ibuprofen, naproxen) for 7 days before testing 4, 5.
High-dose aspirin (more than one adult aspirin per day) for 7 days before testing 4, 5.
Vitamin C in excess of 250 mg for 3 days before testing 4.
Iron tablets do NOT need to be stopped 1, 2.
Important Clinical Caveat
If the FOBT is positive, any positive result on any of the 6 specimens requires colonoscopy - never repeat the FOBT 3, 4. The American Cancer Society emphasizes there is no justification for repeating FOBT in response to an initial positive finding 3.
Alternative Testing Option
Consider using a fecal immunochemical test (FIT) instead of guaiac-based FOBT, as FIT does not require dietary restrictions and has equal or better sensitivity and specificity 3, 5. FIT is more patient-friendly and eliminates concerns about dietary interference 3.
Context: When FOBT Should NOT Be Used
FOBT is inappropriate for diagnosing active gastrointestinal bleeding in hospitalized patients or symptomatic individuals 3, 6. FOBT is validated specifically for colorectal cancer screening in asymptomatic outpatients 6. For patients with suspected active bleeding, direct endoscopic visualization is the appropriate diagnostic approach 6.