Is a slightly low mean corpuscular volume and a slightly high red‑cell distribution width possible in idiopathic erythrocytosis?

Slightly Low MCV and Slightly High RDW in Idiopathic Erythrocytosis

A slightly low MCV with elevated RDW is not characteristic of idiopathic erythrocytosis and instead strongly suggests iron deficiency anemia, which must be ruled out first. 1

Understanding the Red Cell Indices Pattern

The combination you describe—low MCV with high RDW—creates a distinctive hematologic signature that points toward specific diagnoses:

• Low MCV with elevated RDW (>14-16%) is pathognomonic for iron deficiency anemia, not erythrocytosis. 1, 2
• This pattern reflects heterogeneous red cell populations as iron stores become progressively depleted, creating mixed populations of microcytic cells alongside residual normocytic cells. 1
• In contrast, idiopathic erythrocytosis typically presents with normal or elevated MCV and normal RDW, as the primary defect involves increased red cell mass without abnormal hemoglobin synthesis. 3

Diagnostic Algorithm for This Presentation

When you encounter low MCV with high RDW in a patient you suspect has erythrocytosis, follow this sequence:

Step 1: Confirm Iron Deficiency

• Measure serum ferritin immediately—this is the single most powerful test for iron deficiency. 4, 1
• Ferritin <30 μg/L confirms iron deficiency in the absence of inflammation. 4, 2
• Ferritin <100 μg/L may still indicate iron deficiency if concurrent inflammation is present. 4, 1
• Add transferrin saturation: <16-20% confirms insufficient circulating iron for erythropoiesis. 4, 2

Step 2: Distinguish from Thalassemia Trait

• RDW >16% strongly favors iron deficiency over thalassemia trait, which typically shows normal or only slightly elevated RDW (≤14%). 1, 2, 5
• Thalassemia trait presents with MCV disproportionately low relative to the degree of anemia, but RDW remains normal in most cases. 4, 5
• If iron studies are normal despite microcytosis, order hemoglobin electrophoresis to exclude thalassemia. 4, 2

Step 3: Reassess the Erythrocytosis Diagnosis

• True idiopathic erythrocytosis involves increased red cell mass with normal oxygen-sensing pathways and normal EPO levels. 3
• The presence of microcytosis (low MCV) with anisocytosis (high RDW) contradicts this diagnosis and suggests you may be dealing with:
  • Concurrent iron deficiency masking an underlying erythrocytosis (rare but possible)
  • Misdiagnosis of erythrocytosis when the primary problem is actually a different hematologic disorder

Critical Clinical Pitfalls

Do Not Assume All Microcytic Anemia is Simple Iron Deficiency

• Anemia of chronic disease can present with low MCV and elevated RDW in 32% of cases, though less commonly than iron deficiency (90%). 5
• Ferritin >100 μg/L with transferrin saturation <20% indicates anemia of chronic disease, not iron deficiency. 6, 4

Do Not Overlook Combined Deficiencies

• Iron deficiency can coexist with B12 or folate deficiency, presenting with elevated RDW even when MCV appears normal due to the opposing effects on red cell size. 4, 1
• This creates a "mixed picture" where microcytosis from iron deficiency is partially masked by macrocytosis from B12/folate deficiency. 6

Do Not Delay Investigation of Iron Loss

• In adult men and post-menopausal women, confirmed iron deficiency mandates complete gastrointestinal evaluation regardless of symptom presence. 1, 2
• Upper endoscopy with small bowel biopsies detects celiac disease in 2-3% of iron deficiency cases. 1, 2
• Colonoscopy excludes colonic cancer, polyps, and angiodysplasia. 1, 2

When Iron Deficiency and Erythrocytosis Coexist

In the rare scenario where true erythrocytosis exists alongside iron deficiency:

• Treat the iron deficiency first with oral ferrous sulfate 200 mg three times daily for at least 3 months after hemoglobin normalizes. 2
• Expect hemoglobin to rise ≥10 g/L within 2 weeks if iron deficiency is contributing. 4, 2
• Reassess red cell mass after iron repletion to determine if true erythrocytosis persists. 3
• If erythrocytosis persists after iron correction, measure EPO levels to guide further workup: low EPO suggests primary causes, while normal/high EPO requires evaluation of the oxygen-sensing pathway. 3

Bottom Line

The pattern of slightly low MCV with elevated RDW is inconsistent with uncomplicated idiopathic erythrocytosis and demands immediate evaluation for iron deficiency as the primary explanation. 1, 2 Only after excluding and treating iron deficiency should you revisit the diagnosis of erythrocytosis.