Microcytic Anemia with High RDW: Iron Deficiency Until Proven Otherwise
This presentation—low hemoglobin, low hematocrit, low MCV, high MCH (likely a lab artifact), and high RDW—is most consistent with iron deficiency anemia, which is the most common cause of microcytic anemia and requires immediate confirmation with serum ferritin and iron studies. 1, 2
Understanding the Lab Pattern
The combination of low MCV with high RDW is particularly characteristic of iron deficiency 2. The high RDW reflects the heterogeneous population of red blood cells—some older normocytic cells mixed with newer microcytic cells as iron stores become depleted. The reported "high MCH" is likely spurious or a lab error, as MCH typically decreases in parallel with MCV in iron deficiency; verify this value with the laboratory.
Immediate Diagnostic Work-Up
First-Line Testing (Order Now)
Serum ferritin is the single most powerful test for iron deficiency 2:
- Ferritin <12-15 μg/L = diagnostic of iron deficiency (no further testing needed)
- Ferritin <30 μg/L without inflammation = iron deficiency 3
- Ferritin 30-100 μg/L with inflammation = likely mixed picture (iron deficiency + anemia of chronic disease) 3
- Ferritin >100 μg/L = iron deficiency almost certainly excluded 2
Confirmatory Iron Studies
- Transferrin saturation <15-16% supports iron deficiency 1, 3
- Total iron binding capacity (TIBC): elevated in iron deficiency
- Serum iron: low in iron deficiency
- Peripheral blood smear: look for microcytic hypochromic pencil cells 3
Critical Caveat
If the patient has concurrent inflammation, malignancy, or liver disease, ferritin may be falsely elevated. In these cases, ferritin up to 100 μg/L may still indicate iron deficiency 2, 3.
Differential Diagnosis to Exclude
While iron deficiency is most likely, you must rule out:
Thalassemia trait: Usually has elevated or normal RBC count (not just low hemoglobin), family history, specific ethnic background. Confirm with hemoglobin electrophoresis if suspected 1, 2
Anemia of chronic disease: Ferritin >100 μg/L, transferrin saturation <20%, may have microcytosis 1, 2
Combined deficiency: High RDW can indicate coexisting folate/B12 deficiency masking macrocytosis 2
Sideroblastic anemia: Rare, requires bone marrow examination 1
Source Investigation (Essential for Adults)
For Adult Men and Postmenopausal Women
Blood loss from the GI tract is the most common cause 2. You must investigate for:
- Upper GI endoscopy with small bowel biopsies (2-3% have celiac disease) 2
- Colonoscopy or barium enema to exclude colorectal cancer 2
- Both studies are required even if one is positive (10% have dual pathology) 2
Common GI causes to consider 2:
- Colorectal cancer/polyps
- Gastric cancer
- NSAID use (document and stop if possible)
- Celiac disease (test with small bowel biopsy during endoscopy)
- Angiodysplasia
- Inflammatory bowel disease
For Premenopausal Women
- Menstrual blood loss is the most common cause 2
- GI investigation still warranted if menstrual losses don't fully explain severity or if other symptoms present
Additional History Points
- Dietary intake: vegetarian/vegan diet, poor nutrition
- Medications: NSAIDs, aspirin, anticoagulants 2
- Family history: thalassemia, hereditary hemorrhagic telangiectasia, bleeding disorders 2
- H. pylori infection: can impair iron absorption 4
Treatment Approach
Oral Iron (First-Line)
- Start with ferrous sulfate, ferrous fumarate, or ferrous gluconate (least expensive, equally effective) 4
- Dosing: Take on empty stomach for best absorption; if not tolerated, take with meals 4
- Add vitamin C 500 mg to enhance absorption 4
- Trial for 3 weeks: Response confirms diagnosis 2
- Intermittent dosing (alternate days) may be as effective with fewer side effects 5
Intravenous Iron (Second-Line)
Consider if:
- Oral iron not tolerated
- Malabsorption (celiac disease, inflammatory bowel disease)
- No response to oral iron after adequate trial
- Severe anemia requiring rapid correction 4, 5
Treat Underlying Cause
- Celiac disease: Strict gluten-free diet essential for iron absorption 4
- H. pylori: Eradicate infection 4
- Stop NSAIDs whenever possible 2
- Endoscopic treatment of bleeding lesions (angiodysplasia, polyps) 4
Common Pitfalls to Avoid
- Don't assume dietary deficiency alone—always investigate for GI pathology in adults 2
- Don't skip ferritin testing—it's the most powerful diagnostic test 2
- Don't ignore mild anemia—severity doesn't correlate with likelihood of serious disease 2
- Don't stop investigation after finding one lesion—10% have dual pathology 2
- Don't accept "high MCH" at face value—verify with lab, as this is inconsistent with microcytic anemia