What is MCV (Mean Corpuscular Volume)?
MCV is a red blood cell index that measures the average volume of individual red blood cells in femtoliters (fL), serving as a critical parameter for classifying anemia into microcytic (MCV <80 fL), normocytic (MCV 80-100 fL), or macrocytic (MCV >100 fL) categories. 1
Clinical Significance in Anemia Classification
MCV provides an initial framework for investigating anemia etiology, though it has important limitations:
Microcytic Anemia (MCV <80 fL)
- Most commonly indicates iron deficiency anemia, which is the predominant cause of microcytic anemia 1, 2
- Other causes include thalassemia trait (which typically shows normal or only slightly elevated RDW, distinguishing it from iron deficiency) 3
- Rare causes include sideroblastic anemia and certain chronic diseases 2
Normocytic Anemia (MCV 80-100 fL)
- May indicate acute hemorrhage, hemolysis, or anemia of chronic inflammation 1
- Can also represent mixed deficiencies where microcytic and macrocytic processes coexist, masking each other 3
Macrocytic Anemia (MCV >100 fL)
- Suggests vitamin B12 or folate deficiency, particularly when MCV exceeds 120 fL (megaloblastic macrocytosis) 4, 1
- Medication effects from hydroxyurea, azathioprine, and 6-mercaptopurine can cause macrocytosis resembling pernicious anemia 4
Critical Limitations of MCV-Guided Diagnosis
MCV should not be used as the sole parameter for ruling out specific anemia etiologies, as it has poor sensitivity and specificity:
- In primary care patients with newly diagnosed anemia, 16% of microcytic patients had etiologies inconsistent with expected causes (like vitamin B12 deficiency), and 90% of macrocytic patients had unexpected etiologies 5
- For vitamin B12 deficiency specifically, MCV sensitivity ranges from only 17-30% in screened populations, meaning up to 84% of B12-deficient cases can be missed if relying on MCV alone 6
- Normal MCV values do not exclude iron deficiency, as 85% of anemic patients have MCV within the normal range 5
Recommended Diagnostic Approach
Rather than using MCV as a gatekeeper, obtain a comprehensive initial panel regardless of MCV value:
Essential Initial Tests 4, 1
- Complete blood count with all red cell indices (MCV, MCH, MCHC)
- Red cell distribution width (RDW) - elevated RDW suggests iron deficiency even before MCV changes 3
- Reticulocyte count - distinguishes impaired production from increased destruction 1
- Serum ferritin and transferrin saturation - ferritin <30 μg/L confirms iron deficiency without inflammation 1, 3
- Vitamin B12 and folate levels - essential when MCV >100 fL 4, 1
- C-reactive protein - helps interpret ferritin in inflammatory states 4
Interpretation Nuances
MCH (mean corpuscular hemoglobin) is more reliable than MCV for detecting iron deficiency because it's less dependent on storage conditions and laboratory equipment 1. The combination of low MCH with elevated RDW has 72% sensitivity for iron deficiency, even before frank anemia develops 1.
Ferritin interpretation requires clinical context: it can be falsely elevated with inflammation, chronic disease, malignancy, or liver disease, potentially masking iron deficiency 1, 3. In inflammatory states, ferritin up to 100 μg/L may still indicate iron deficiency 3.
Common Pitfalls to Avoid
- Never exclude vitamin B12 deficiency based on normal MCV alone - substantial neurological damage can occur without macrocytosis 6
- Don't assume microcytosis always means iron deficiency - check iron studies to distinguish from thalassemia trait, which has different management 3
- Recognize that normal MCV with elevated RDW suggests early or mixed deficiencies requiring full workup 1, 3
- In inflammatory bowel disease patients on thiopurines, macrocytosis is expected and doesn't automatically warrant extensive investigation if otherwise stable 4