What is the most likely cause and appropriate workup and treatment for a patient with low hematocrit, low mean corpuscular volume, low mean corpuscular hemoglobin, and elevated red cell distribution width (RDW‑CV)?

Microcytic Anemia with Elevated RDW: Iron Deficiency Until Proven Otherwise

The combination of low hematocrit, low MCV, low MCH, and elevated RDW-CV is highly characteristic of iron deficiency anemia and should prompt immediate iron studies and investigation for the underlying cause of iron loss. 1, 2

Most Likely Diagnosis

Iron deficiency anemia (IDA) is the most common cause of microcytic anemia (MCV <80 fL), and the elevated RDW-CV is a key distinguishing feature that develops early in iron deficiency, even before frank anemia appears. 1, 2

• The elevated RDW reflects increased variation in red blood cell size (anisocytosis), which occurs as iron-deficient cells are produced alongside older normal cells 2
• RDW-CV >15% has 77% sensitivity for iron deficiency anemia, though specificity is only 54-91% depending on the population 3, 4
• The combination of low MCV and elevated RDW is more specific for iron deficiency than either parameter alone 2, 5

Important Differential Considerations

While iron deficiency is most likely, other causes of microcytic anemia must be considered 1:

• Thalassemia: typically presents with elevated red blood cell count, normal or only mildly elevated RDW, and microcytosis out of proportion to the degree of anemia 2
• Anemia of chronic disease: can cause microcytosis but usually has normal or only mildly elevated RDW 1, 2
• Combined deficiency: elevated RDW with normal or high MCV suggests concurrent folate or B12 deficiency 2

Diagnostic Workup

Initial Iron Studies (Essential)

Serum ferritin is the single most powerful test for iron deficiency and should be ordered immediately 2:

• Ferritin <12-15 μg/L is diagnostic of iron deficiency 2
• Ferritin <30 μg/L indicates iron deficiency in patients without inflammation 6, 7
• In the presence of inflammation, malignancy, or liver disease, ferritin up to 100 μg/L may still represent iron deficiency 2, 6

Additional confirmatory tests 1:

• Transferrin saturation <15-16% supports iron deficiency 1
• Total iron binding capacity (TIBC) is typically elevated 1
• Serum iron is low but less reliable than ferritin 2

Clinical History (Critical Details)

Focus on identifying the source of iron loss 2:

Gastrointestinal blood loss (most common in adult men and postmenopausal women):

• NSAID use, aspirin, anticoagulants 2
• Symptoms of peptic ulcer, gastritis, or inflammatory bowel disease 2
• Change in bowel habits, blood in stool, or black tarry stools 2

Menstrual blood loss (most common in premenopausal women):

• Heavy or prolonged menstrual periods 2

Malabsorption:

• Symptoms suggesting celiac disease (diarrhea, weight loss, bloating) 2
• History of gastrectomy or gastric bypass 2

Dietary insufficiency:

• Vegetarian/vegan diet, poor nutritional intake 2

Family history:

• Thalassemia, hereditary hemorrhagic telangiectasia, bleeding disorders 2

Gastrointestinal Evaluation

For adult men and postmenopausal women with confirmed iron deficiency anemia, bidirectional endoscopy is mandatory to exclude gastrointestinal malignancy 2:

• Upper endoscopy with small bowel biopsies to evaluate for gastric cancer, peptic ulcer disease, angiodysplasia, and celiac disease (2-3% of IDA cases) 2
• Colonoscopy or barium enema to exclude colorectal cancer and polyps, even if upper endoscopy reveals a lesion, as dual pathology occurs in ~10% of cases 2
• Testing for Helicobacter pylori should be performed if IDA persists or recurs after negative endoscopy, as eradication can reverse anemia 8

For premenopausal women, gastrointestinal evaluation is indicated if:

• Menstrual blood loss does not adequately explain the degree of anemia 2
• Gastrointestinal symptoms are present 2
• Anemia fails to respond to iron supplementation 2

Treatment Approach

Iron Replacement Therapy

Oral iron supplementation is first-line treatment 7, 8:

• Ferrous sulfate 200 mg twice daily (or equivalent elemental iron from other ferrous salts) 8
• Lower doses may be equally effective with better tolerability and should be considered if side effects occur 8
• Continue for 3 months after correction to replenish iron stores 8
• Taking iron on an empty stomach improves absorption; if not tolerated, take with meals or meat protein 7
• Vitamin C 250-500 mg with iron may enhance absorption 8

Intravenous iron is indicated when 7, 8:

• Oral iron is not tolerated despite trying multiple formulations 7, 8
• Malabsorption is present (celiac disease, inflammatory bowel disease, gastric bypass) 7, 8
• Severe anemia requires rapid correction 7
• No response to adequate oral iron trial (3 weeks) 2, 8

Available IV formulations include iron sucrose, ferric carboxymaltose, and iron dextran, with varying dosing schedules and safety profiles 8

Treatment of Underlying Cause

Addressing the source of iron loss is essential to prevent recurrence 2:

• Discontinue NSAIDs and aspirin when possible 2
• Treat identified gastrointestinal lesions (polyp removal, H. pylori eradication, management of inflammatory bowel disease) 2, 8
• Strict gluten-free diet for celiac disease to improve iron absorption 7, 9
• Gynecologic evaluation and management for menorrhagia 2

Common Pitfalls to Avoid

• Do not assume dietary insufficiency is the sole cause without completing gastrointestinal evaluation in at-risk populations, as occult malignancy may be present 2
• Do not rely on ferritin alone in inflammatory conditions; use transferrin saturation or other markers to confirm iron deficiency when ferritin is 30-100 μg/L 2, 6
• Do not stop investigation after finding one lesion on upper endoscopy; proceed with colonoscopy as dual pathology is common 2
• Do not use fecal occult blood testing as it is insensitive and non-specific for iron deficiency anemia 8
• Do not assume thalassemia without laboratory confirmation, especially in microcytic anemia with elevated red blood cell count 2

Expected Response and Follow-up

• Reticulocyte count should increase within 7-10 days of starting iron therapy 1
• Hemoglobin should rise by approximately 1 g/dL every 2-3 weeks with adequate iron replacement 8
• If no response after 3 weeks of oral iron, consider non-compliance, ongoing blood loss, malabsorption, incorrect diagnosis, or need for IV iron 2, 8
• Recheck complete blood count and iron studies after completing treatment to confirm normalization 8