What Low MCV and MCH Mean
Low MCV (Mean Corpuscular Volume) and low MCH (Mean Corpuscular Hemoglobin) indicate microcytic, hypochromic red blood cells, most commonly caused by iron deficiency anemia, though thalassemia and chronic disease must also be considered. 1, 2
Primary Diagnostic Significance
- Low MCV (<80 fL in adults) indicates microcytosis, meaning red blood cells are smaller than normal 3
- Low MCH indicates hypochromia, meaning red blood cells contain less hemoglobin than normal 1
- MCH is actually a more reliable marker than MCV because it is less dependent on storage conditions and the counting machine used, and it decreases in both absolute and functional iron deficiency 1
- MCH may be more sensitive for detecting iron deficiency than MCV 1
Most Common Causes to Consider
Iron Deficiency Anemia (Most Common)
- Iron deficiency is the most common cause of microcytic anemia and typically presents with low MCV, low MCH, AND elevated RDW (>14.0%) 2, 3
- Serum ferritin <15 μg/L is highly specific (99% specificity) for iron deficiency 1
- In the presence of inflammation, ferritin may be falsely normal; consider iron deficiency even with ferritin up to 45 μg/L 1, 2
- Ferritin >100-150 μg/L essentially rules out iron deficiency even with inflammation 1, 2
Thalassemia Minor (Second Most Common)
- Thalassemia typically presents with low MCV, low MCH, BUT normal RDW (≤14.0%), making this distinction clinically critical 2, 4
- Microcytosis is often more pronounced relative to the degree of anemia compared to iron deficiency 2
- Serum ferritin and iron studies are normal or elevated in thalassemia, unlike in iron deficiency 2
- MCV is typically reduced out of proportion to the level of anemia in thalassemia 1
Other Causes
- Anemia of chronic inflammation or infection can cause microcytosis, especially when prolonged 2
- Lead poisoning can result in microcytic red blood cells 2
- Sideroblastic anemia is a less common inherited cause 3
Practical Diagnostic Algorithm
Step 1: Check RDW to differentiate the two main causes
- Low MCV + Low MCH + High RDW (>14.0%) → strongly suggests iron deficiency anemia 2, 4
- Low MCV + Low MCH + Normal RDW (≤14.0%) → strongly suggests thalassemia minor 2, 4
Step 2: Confirm with iron studies
- Measure serum ferritin as the single most useful marker 1, 2
- Ferritin <15 μg/L definitively indicates iron deficiency 1, 2
- Ferritin <45 μg/L warrants consideration of iron deficiency, especially with chronic inflammation 1, 2
- Consider transferrin saturation if false-normal ferritin is suspected 1
Step 3: If iron studies are normal with low MCV/MCH
- Hemoglobin electrophoresis is recommended to diagnose thalassemia, particularly in patients with appropriate ethnic background 1
Critical Management Implications
For Iron Deficiency
- Identify and address the underlying cause, particularly gastrointestinal blood loss in adult men and post-menopausal women 2
- A good response to iron therapy (Hb rise ≥10 g/L within 2 weeks) is highly suggestive of absolute iron deficiency, even if iron studies are equivocal 1
For Thalassemia
- Avoid unnecessary iron supplementation, which is ineffective in thalassemia and may cause dangerous iron overload 2, 4
- Genetic counseling may be appropriate, but specific treatment is usually not required for thalassemia trait 2
Common Pitfalls to Avoid
- Don't assume all microcytic anemias are iron deficiency—the RDW is your key differentiator 4
- Both microcytosis and hypochromia lose sensitivity for iron deficiency in the presence of chronic disease, thalassemia, or vitamin B12/folate deficiency 1
- Thalassemia and iron deficiency can coexist, complicating the diagnostic picture 2, 4
- Normal MCV and MCH values do not exclude iron deficiency, as these are only moderately accurate diagnostic tests 5
- When microcytosis and macrocytosis coexist (combined deficiency states), they may neutralize each other and result in normal MCV despite underlying abnormalities 2
- Avoid empiric iron therapy without proper diagnosis, as it's ineffective in thalassemia and may cause harm 4