What causes a slightly low Mean Corpuscular Hemoglobin (MCH) with a normal Red Cell Distribution Width (RDW) and a normal Complete Blood Count (CBC)?

Evaluation of Low MCH (26.5 pg) with Elevated RDW (18%) and Otherwise Normal CBC

This pattern most likely represents early or evolving iron deficiency anemia, and you should immediately obtain iron studies (serum ferritin and transferrin saturation) to confirm the diagnosis and guide treatment. 1, 2

Understanding the Laboratory Pattern

The combination of low MCH with elevated RDW is highly suggestive of iron deficiency, even when other CBC parameters remain normal:

• MCH (26.5 pg) is below the normal range (typically 27-33 pg), indicating that individual red blood cells contain less hemoglobin than normal, which is characteristic of iron deficiency 2, 3
• MCH is actually more reliable than MCHC for detecting iron deficiency because it is less dependent on storage conditions and laboratory equipment 2
• Elevated RDW (18%) indicates increased variation in red blood cell size, which is one of the most sensitive early markers for nutritional deficiencies 4, 3
• RDW has 72% sensitivity for detecting iron deficiency in patients with anemia, making it a valuable screening parameter 4

Why This Pattern Occurs

This represents early-stage iron deficiency where:

• The bone marrow is producing red blood cells with progressively less hemoglobin content (low MCH) 5, 3
• Both normal-sized older cells and smaller, hypochromic newer cells coexist in circulation, creating the high RDW 5, 3
• Hemoglobin and hematocrit may still appear "normal" because the anemia is just beginning to develop 1, 6
• Low MCH with elevated RDW specifically suggests iron-deficient erythropoiesis, even before frank anemia develops 5, 2

Recommended Diagnostic Workup

Obtain the following tests immediately:

• Serum ferritin: <30 μg/L confirms iron deficiency in the absence of inflammation 1, 2
• Transferrin saturation: <15-20% supports iron deficiency and is less affected by inflammation than ferritin 1, 2
• C-reactive protein (CRP): To assess for inflammation that could falsely elevate ferritin 1, 2
• Reticulocyte count: To evaluate bone marrow response 1, 2

Alternative Diagnoses to Consider

If iron studies are normal, consider:

• Mixed nutritional deficiencies (iron plus folate or B12): RDW elevation with normal MCV can occur when multiple deficiencies counterbalance each other 1, 4
• Thalassemia trait: Obtain hemoglobin electrophoresis, particularly in patients of Mediterranean, African, or Southeast Asian descent 1, 2
• Anemia of chronic disease: Check for underlying inflammatory conditions, though this typically shows normal or elevated ferritin 1, 2

Clinical Pitfalls to Avoid

• Do not dismiss this as "normal" simply because hemoglobin is within reference range - this pattern indicates evolving iron deficiency that will progress to overt anemia if untreated 5, 1
• Do not rely on MCV alone - MCV may remain normal in early iron deficiency or when mixed deficiencies are present 1, 3
• Ferritin must be interpreted in clinical context - it can be falsely elevated by inflammation, chronic disease, malignancy, or liver disease 2
• If iron deficiency is confirmed, investigate the underlying cause: gastrointestinal bleeding in men and postmenopausal women, menstrual losses in premenopausal women 2

Treatment Approach Once Diagnosis Confirmed

If iron deficiency is confirmed:

• First-line treatment is oral iron supplementation (intermittent dosing is as effective as daily dosing with fewer side effects) 6
• Reserve intravenous iron for patients who cannot tolerate, cannot absorb, or do not respond to oral iron 2, 6
• Recheck CBC and iron studies in 4-8 weeks to confirm response to treatment 2