Diagnostic Approach to Microcytic Anemia with Rectal Bleeding

Primary Diagnostic Considerations

Gastrointestinal blood loss is the leading cause of iron deficiency in adult men, and the presence of rectal bleeding makes this the top consideration 1, 2.
The American Gastroenterological Association suggests that serum ferritin <15 μg/dL is diagnostic of iron deficiency, though values <30 μg/dL indicate low body iron stores 3.
The combination of low MCV with elevated red cell distribution width (RDW >14.0%) strongly suggests iron deficiency rather than thalassemia 3.
The European Hematology Association recommends that MCH (mean corpuscular hemoglobin) is actually more reliable than MCV for detecting iron deficiency because it decreases in both absolute and functional iron deficiency and is less dependent on storage conditions 3.

The American College of Gastroenterology states that colorectal cancer or polyps, despite young age, must be excluded given rectal bleeding 4.
Inflammatory bowel disease (Crohn's disease or ulcerative colitis) is a possible cause of iron deficiency anemia 5.
Angiodysplasia is a potential cause of gastrointestinal bleeding 4.
NSAID-induced lesions can cause iron deficiency anemia, though no NSAID use is mentioned 5.

The British Society of Gastroenterology recommends that serum ferritin is the single most powerful test for iron deficiency and should be obtained first 5, 2, 3.
Complete iron studies, including transferrin saturation, should be performed to assess iron deficiency 3.
A complete blood count with RDW can help differentiate iron deficiency from thalassemia 3.
Celiac disease screening is mandatory, as 2-3% of patients with iron deficiency anemia have this diagnosis 6.

Bidirectional endoscopy (both upper endoscopy and colonoscopy) is the standard approach for evaluating iron deficiency anemia in men, even at age 22 with visible rectal bleeding 6.
Small bowel biopsies must be obtained during upper endoscopy to screen for celiac disease 5, 6.
Colonoscopy should not be deferred despite the patient's young age, as dual pathology occurs in 10-15% of patients 4, 5.

Stool examination for ova and parasites should be performed to exclude hookworm infection, which is a cause of iron deficiency 1, 2.
Urinalysis can help exclude urinary tract blood loss 6.
A good response to iron therapy (hemoglobin rise ≥10 g/L within 2 weeks) is highly suggestive of absolute iron deficiency, even if iron study results are equivocal 6, 2.