Microcytic Anemia with Elevated RDW: Iron Deficiency Anemia
This CBC pattern (Hgb 10.7, MCV 76, elevated RDW 18.2) is most consistent with iron deficiency anemia, and you should immediately order serum ferritin as the single most powerful diagnostic test. 1
CBC Pattern Analysis
Your patient has:
- Microcytic anemia (MCV 76 fL, normal 80-100 fL)
- Hypochromic cells (MCH 24.0 pg, low)
- Elevated RDW (18.2%, indicating significant variation in red cell size)
This triad is characteristic of iron deficiency anemia 2. The elevated RDW is particularly important—it distinguishes iron deficiency from thalassemia trait, where RDW is typically normal and red blood cell count is elevated 1, 3.
Immediate Diagnostic Workup
First-Line Test: Serum Ferritin
Order serum ferritin immediately as it is the most powerful test for iron deficiency 1:
- Ferritin <12 μg/dL: Diagnostic of iron deficiency
- Ferritin 12-30 μg/dL: Consistent with iron deficiency in absence of inflammation
- Ferritin 30-100 μg/dL: May still indicate iron deficiency if inflammation/malignancy present 1, 4
- Ferritin >100 μg/dL: Iron deficiency almost certainly not present 1
Second-Line Tests (if ferritin equivocal)
If ferritin is 30-100 μg/dL, add:
- Transferrin saturation (<15-20% suggests iron deficiency) 2
- Total iron binding capacity (TIBC) (elevated in iron deficiency)
- Serum iron (low in iron deficiency)
Source Investigation Required
In adults, iron deficiency anemia is presumed to be caused by blood loss until proven otherwise 1. The most common source is gastrointestinal bleeding, and asymptomatic GI malignancy must be excluded 1.
Mandatory GI Evaluation
All patients with confirmed iron deficiency anemia require both upper and lower GI tract examination 1:
- Upper endoscopy with small bowel biopsies (2-3% have celiac disease) 1
- Colonoscopy or barium enema to exclude colonic malignancy 1
This applies even without GI symptoms, as symptoms rarely correlate with findings 1. Dual pathology (lesions in both upper and lower GI tract) occurs in approximately 10% of cases 1.
History Priorities
Document specifically:
- Menstrual history (if premenopausal woman—most common cause in this group) 1
- NSAID/aspirin use (common cause of occult GI bleeding) 1
- Dietary iron intake (though dietary deficiency alone should not preclude GI investigation) 1
- Family history of thalassemia or other hemoglobinopathies 1
- Visible blood loss (stool, urine)
Differential Diagnosis Considerations
While iron deficiency is most likely, consider:
- Thalassemia trait: Usually has normal or elevated RBC count, normal RDW, and MCV often <70 fL 1, 3
- Anemia of chronic disease: Can present with microcytosis but typically has ferritin >100 μg/dL, low TIBC, and transferrin saturation <20% 2, 4
- Combined deficiency: If folate/B12 deficiency coexists, elevated RDW may be the only clue 1
Critical Pitfall to Avoid
Do not assume dietary deficiency or menstrual blood loss as the sole cause without completing GI evaluation. Even with a positive dietary history or menstruation, full GI investigation is still required in adults to exclude malignancy 1. Mild anemia is no less indicative of serious disease than severe anemia 1.