What should be done for a patient with elevated red blood cell count, low mean corpuscular volume (MCV), low mean corpuscular hemoglobin (MCH), low mean corpuscular hemoglobin concentration (MCHC), and elevated red cell distribution width (RDW)?

Elevated RBC with Low MCV, MCH, MCHC and High RDW: Diagnostic Approach

Check serum ferritin immediately to confirm iron deficiency, and if ferritin is normal or elevated (>30 μg/L) with appropriate ethnic background, obtain hemoglobin electrophoresis to rule out thalassemia trait. This pattern of microcytic hypochromic red cells with elevated RDW strongly suggests either iron deficiency or thalassemia trait, and distinguishing between them is critical for appropriate management 1.

Understanding the Blood Picture

Your laboratory findings show:

• Elevated RBC count (polycythemia/erythrocytosis)
• Low MCV, MCH, MCHC (microcytic, hypochromic red cells)
• Elevated RDW (increased variation in red cell size)

This combination creates a "microcytic polycythemia" pattern that narrows the differential diagnosis significantly.

Primary Differential Diagnosis

1. Iron Deficiency (Most Common)

The low MCV, MCH, and MCHC with elevated RDW is the classic pattern for iron deficiency 1. However, the elevated RBC count is somewhat atypical, as iron deficiency usually causes anemia (low RBC count).

Diagnostic approach:

• Serum ferritin is the single most useful test 1

  • <15 μg/L: highly specific for iron deficiency (99% specificity)
  • <30 μg/L: generally indicates low iron stores
  • <45 μg/L: optimal cutoff balancing sensitivity/specificity (92% specificity)

  • 150 μg/L: essentially rules out iron deficiency even with inflammation 1

• Additional iron studies if ferritin equivocal:

  • Transferrin saturation <16% supports iron deficiency 2
  • Total iron-binding capacity (raised in iron deficiency)

Important caveat: Ferritin is an acute phase reactant and can be falsely normal in the presence of inflammation or chronic disease. If clinical suspicion is high despite normal ferritin (30-150 μg/L), consider transferrin saturation or a therapeutic trial of iron 1.

2. Thalassemia Trait (Beta or Alpha)

Thalassemia trait classically presents with:

• Elevated RBC count (often >5.0 × 10¹²/L)
• Disproportionately low MCV (often <70 fL, out of proportion to degree of anemia)
• Normal or mildly elevated RDW (though your case has elevated RDW)
• Normal iron studies 1, 3

Key distinguishing feature: The RDW/RBC ratio has 100% specificity for differentiating thalassemia from iron deficiency, with 91.18% sensitivity 3. In thalassemia, this ratio is typically lower.

Diagnostic approach:

• Hemoglobin electrophoresis is recommended when microcytosis exists with normal iron studies, particularly in patients of Mediterranean, Middle Eastern, African, or Southeast Asian descent 1

3. Polycythemia Vera with Concurrent Iron Deficiency

This represents true polycythemia (elevated red cell mass) that has become iron deficient, often from chronic phlebotomy or occult bleeding 4.

Diagnostic features:

• History of polycythemia vera diagnosis
• JAK2 mutation positive
• Iron deficiency develops secondarily
• After iron repletion, RBC count remains elevated but MCV normalizes 4

4. Secondary Polycythemia with Iron Deficiency

Conditions causing chronic hypoxia (COPD, sleep apnea, high altitude, cyanotic heart disease) or erythropoietin-secreting tumors can cause elevated RBC counts. Concurrent iron deficiency creates the microcytic picture 4.

Recommended Diagnostic Algorithm

Step 1: Obtain serum ferritin

• If <30 μg/L → Diagnose iron deficiency, investigate source (GI blood loss most common in adults)
• If 30-45 μg/L with inflammation → Consider additional iron studies or therapeutic trial
• If >45 μg/L → Proceed to Step 2

Step 2: If iron studies normal, obtain hemoglobin electrophoresis

• Particularly important if patient has appropriate ethnic background
• Elevated HbA2 (>3.5%) confirms beta-thalassemia trait
• Normal electrophoresis with microcytosis suggests alpha-thalassemia trait (requires genetic testing for confirmation)

Step 3: If both iron studies and hemoglobin electrophoresis normal

• Evaluate for secondary causes of polycythemia:
  • Pulse oximetry/arterial blood gas (hypoxia)
  • Sleep study if indicated
  • Erythropoietin level
  • Abdominal imaging if concern for EPO-secreting tumor
• Consider JAK2 mutation testing for polycythemia vera

Critical Management Points

If iron deficiency is confirmed:

• Investigate the source - In adults, assume GI blood loss until proven otherwise 1
• Consider GI investigation at any level of iron deficiency, especially with more severe degrees 1
• Men with Hb <110 g/L or non-menstruating women with Hb <100 g/L warrant fast-track referral for suspected GI malignancy 1
• Therapeutic trial: A hemoglobin rise ≥10 g/L within 2 weeks of iron therapy strongly confirms iron deficiency even if iron studies were equivocal 1

Common pitfall: Don't assume elevated RBC count rules out iron deficiency. While unusual, iron deficiency can coexist with conditions causing erythrocytosis 4.

Quality of evidence note: The British Society of Gastroenterology 2021 guidelines 1 provide the most current, high-quality recommendations for iron deficiency evaluation. The RDW/RBC formula for thalassemia discrimination has excellent performance characteristics in the 2015 study 3, though it was validated in Thai adults and may perform differently in other populations.