Testing Stools for Iron
You should NOT test stools for iron content directly—instead, use fecal occult blood testing (FOBT) to detect gastrointestinal bleeding as the cause of iron deficiency. The purpose is to identify blood loss, not to measure iron itself in the stool.
Primary Testing Method
A stool guaiac test for occult blood is the recommended approach when investigating iron deficiency to detect gastrointestinal bleeding 1. This test detects the presence of blood in stool, which indicates potential GI blood loss causing iron depletion.
Available Testing Options
- Guaiac-based FOBT (gFOBT): Traditional chemical test that detects peroxidase activity of hemoglobin 1
- Fecal Immunochemical Test (FIT): More specific test that detects human hemoglobin directly 1
Critical Limitations and Caveats
When NOT to Use FOBT
FIT should NOT be used in patients with iron deficiency anemia for diagnostic purposes 1. The sensitivity is only 58% (95% CI 0.53-0.63) with specificity of 84% (95% CI 0.75-0.89), meaning 42% of patients with identifiable causes of iron deficiency anemia will have false-negative results 2.
FOBT should be avoided during acute diarrhea episodes, as sensitivities range from only 38% to 87% with variable specificity 1.
Effect of Oral Iron Supplementation
There is contradictory evidence regarding whether oral iron causes false-positive results:
- Older data (1982): Ferrous sulfate and ferrous gluconate caused 50-65% false-positive Hemoccult reactions and 25-65% false-positive Hematest reactions 3, 4
- More recent data (1990): A controlled study found oral iron supplementation (ferrous sulfate or ferrous gluconate) did NOT cause false-positive results with Hemoccult II or Hemoccult Sensa methods 5
In clinical practice, the more recent controlled study suggests oral iron does not interfere with modern Hemoccult testing 5.
Proper Clinical Approach to Iron Deficiency
Instead of Relying on Stool Testing
Bidirectional endoscopy (gastroscopy and colonoscopy) should be performed as first-line investigation in adults with newly diagnosed iron deficiency anemia 6. This is far superior to FOBT for identifying bleeding sources:
- Gastroscopy reveals a cause in 30-50% of patients 6
- Colonoscopy should be performed even if upper endoscopy finds a lesion, as dual pathology occurs in 10-15% of patients 6
- FOBT has poor sensitivity (54%) for non-colorectal cancer lesions that commonly cause iron deficiency 2
Complementary Testing
- Coeliac disease screening should be performed serologically (tissue transglutaminase antibody with IgA level), as it's found in 3-5% of iron deficiency anemia cases 6
- Urinalysis should be performed to exclude urinary tract bleeding 6
- H. pylori testing should be considered after negative bidirectional endoscopy 1
Key Clinical Pitfall
Do not use a positive or negative FOBT result to guide decisions about whether to pursue endoscopic evaluation in iron deficiency anemia 2. The test performance is inadequate for this purpose, and endoscopy should be pursued based on clinical criteria, not FOBT results 1.