Differential Diagnosis for Microcytic Anemia with Rectal Bleeding in a Young Male Immigrant
In a 22-year-old male immigrant presenting with rectal bleeding and microcytic anemia (low MCV), the differential diagnosis must prioritize gastrointestinal blood loss as the primary cause, with iron deficiency anemia being most likely, but thalassemia trait and parasitic infections (particularly hookworm given immigration history) must also be considered.
Primary Diagnostic Considerations
Iron Deficiency Anemia (Most Likely)
- 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.
- Serum ferritin <15 μg/dL is diagnostic of iron deficiency, though values <30 μg/dL indicate low body iron stores 2.
- The combination of low MCV with elevated red cell distribution width (RDW >14.0%) strongly suggests iron deficiency rather than thalassemia 2.
- 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 2.
Specific GI Causes to Consider in This Patient
Common bleeding sources 1:
- Colorectal cancer or polyps (despite young age, must be excluded given rectal bleeding)
- Inflammatory bowel disease (Crohn's disease or ulcerative colitis)
- Angiodysplasia
- NSAID-induced lesions (though no NSAID use mentioned, nicotine use noted)
Non-bleeding causes that present with iron deficiency 1, 3:
- Celiac disease occurs in 2-3% of patients with iron deficiency anemia and should be screened for serologically or via small bowel biopsy 1.
- Helicobacter pylori gastritis (found in 18% of IDA cases without GI symptoms) 3
- Atrophic gastritis (found in 27% of IDA cases without GI symptoms) 3
- Hookworm infection is specifically listed as an uncommon but important cause in the differential, particularly relevant given immigration history 1.
Thalassemia Trait (Critical Alternative)
- Haemoglobinopathies frequently cause microcytosis in certain ethnic groups and must not be presumed without laboratory confirmation 1.
- In thalassemia, the red cell count is usually elevated and MCV is reduced out of proportion to the degree of anemia 1.
- Low MCV with RDW ≤14.0% suggests thalassemia minor rather than iron deficiency 2.
- Hemoglobin electrophoresis is warranted if iron studies are normal despite microcytosis, particularly given the patient's immigration background 2.
Anemia of Chronic Disease
- Can present with microcytosis and low MCH/MCHC, though less common than iron deficiency 1.
- Ferritin may be falsely elevated (>15 μg/dL) in the presence of inflammation, malignancy, or hepatic disease, though ferritin >100 μg/dL essentially excludes iron deficiency 1.
Critical Diagnostic Workup Required
Initial Laboratory Assessment
- Serum ferritin is the single most powerful test for iron deficiency and should be obtained first 1, 2.
- Complete iron studies including transferrin saturation (<16-20% indicates insufficient circulating iron) 2.
- Complete blood count with RDW to differentiate iron deficiency (elevated RDW) from thalassemia (normal RDW) 2.
- Celiac disease screening (tissue transglutaminase antibodies or endomysial antibodies) is mandatory 1.
- Hemoglobin electrophoresis if iron studies are normal or if ethnic background suggests thalassemia risk 2.
Gastrointestinal Evaluation
- 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 1.
- Small bowel biopsies must be obtained during upper endoscopy to screen for celiac disease, as 2-3% of IDA patients have this diagnosis 1.
- Colonoscopy should not be deferred despite the patient's young age because dual pathology (lesions in both upper and lower GI tract) occurs in 10-15% of patients 1.
- Upper GI endoscopy reveals a cause in 30-50% of IDA patients and should include gastric and duodenal biopsies 1.
Additional Considerations for Immigrant Population
- Stool examination for ova and parasites to exclude hookworm infection, which is specifically listed as a cause of iron deficiency in the guidelines 1.
- Urinalysis to exclude urinary tract blood loss 1.
Important Clinical Pitfalls
Do Not Assume Dietary Deficiency Alone
- Even if dietary history suggests iron deficiency, a full GI investigation is still required because borderline deficient diets are common but should not be presumed as the sole cause 1.
Do Not Accept Superficial Lesions as Definitive
- Esophagitis, erosions, aphthous ulceration, and peptic ulcer should not be accepted as the cause of iron deficiency at initial evaluation until lower GI tract examination is complete, as dual pathology is common 1.
Ferritin Interpretation Requires Context
- Ferritin is an acute phase reactant and can be falsely normal or elevated during inflammation, infection, or malignancy despite true iron deficiency 2.
- However, ferritin >150 μg/L essentially excludes absolute iron deficiency even with concurrent inflammation 2.
Response to Iron Therapy as Diagnostic Tool
- 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 1.
- This can serve as a confirmatory test when other results are unclear 1.