In idiopathic erythrocytosis, what are the typical red‑cell distribution width (RDW) and mean corpuscular volume (MCV) values?

RDW and MCV in Idiopathic Erythrocytosis

In idiopathic erythrocytosis, both RDW and MCV are typically normal, reflecting a homogeneous population of red blood cells without underlying nutritional deficiencies, hemolysis, or ineffective erythropoiesis.

Expected Laboratory Findings

Red Cell Distribution Width (RDW)

• Normal RDW (≤14.0%) is the expected finding in idiopathic erythrocytosis, indicating uniform red cell size without significant anisocytosis 1
• A normal RDW suggests absence of iron deficiency, vitamin B12/folate deficiency, or active hemolysis—conditions that would elevate RDW 1
• The homogeneous red cell population reflects stable, regulated erythrocytosis rather than pathologic bone marrow processes 1

Mean Corpuscular Volume (MCV)

• MCV remains within normal range (80-99 fL) in idiopathic erythrocytosis 1
• Normal MCV combined with normal RDW effectively excludes early nutritional deficiencies that could complicate the clinical picture 1
• The absence of macrocytosis distinguishes idiopathic erythrocytosis from conditions like myelodysplastic syndrome, where 54.7% of patients present with macrocytic anemia 2

Clinical Reasoning and Differential Diagnosis

Why These Values Matter

• The combination of normal MCV and normal RDW is diagnostically significant because it helps exclude secondary causes of erythrocytosis 1, 3
• Elevated RDW (>14.0%) would suggest concurrent iron deficiency, hemolysis, or mixed nutritional deficiencies requiring investigation 1, 4
• Macrocytosis would prompt evaluation for vitamin B12/folate deficiency, medication effects, or myelodysplastic processes 5

Distinguishing from Other Conditions

• Unlike iron deficiency anemia (low MCV + high RDW), idiopathic erythrocytosis shows normal values for both parameters 1, 4
• Unlike thalassemia trait (low MCV + normal RDW), idiopathic erythrocytosis maintains normal MCV 1, 6
• Unlike myelodysplastic syndrome where 84.7% have elevated RDW, idiopathic erythrocytosis maintains normal RDW 2

Common Pitfalls to Avoid

Laboratory Interpretation Caveats

• RDW values can vary between different laboratory instruments, so always interpret according to your specific laboratory's reference range 1
• If RDW or MCV are abnormal in a patient with suspected idiopathic erythrocytosis, investigate for concurrent conditions rather than dismissing the diagnosis 1
• A minimum workup should include reticulocyte count, serum ferritin, transferrin saturation, and CRP if any indices are abnormal 1

When to Investigate Further

• Any elevation in RDW warrants investigation for iron deficiency (serum ferritin <30 μg/L without inflammation, <100 μg/L with inflammation) 1
• Macrocytosis requires checking vitamin B12 and folate levels, particularly when MCV exceeds 100 fL 5
• Even with normal indices, if clinical suspicion exists for nutritional deficiency, additional testing is warranted 1