Low MCV and Low MCH Indicates Smaller-Than-Normal and Pale RBCs
Low MCV and low MCH is the pair of values that indicates microcytic (smaller-than-normal) and hypochromic (pale) red blood cells. 1
Understanding Red Blood Cell Indices and Microcytic Hypochromic Anemia
Automated cell counters provide measurements of red blood cell characteristics that help identify various types of anemia:
Mean Corpuscular Volume (MCV) measures the average size of red blood cells in femtoliters (fL). A low MCV (<80 fL) indicates microcytosis or smaller-than-normal red blood cells 1
Mean Corpuscular Hemoglobin (MCH) measures the average hemoglobin content per red blood cell. A low MCH indicates hypochromia or pale red blood cells due to decreased hemoglobin content 1
When both MCV and MCH are low, this represents microcytic hypochromic anemia, most commonly caused by iron deficiency 1, 2
Common Causes of Microcytic Hypochromic Anemia
Microcytic hypochromic anemia (low MCV and low MCH) is typically associated with:
- Iron deficiency anemia - the most common cause 3, 2
- Thalassemia (a genetic disorder affecting hemoglobin production) 1, 4
- Anemia of chronic disease (in some cases) 1
- Sideroblastic anemia 1, 2
Distinguishing Features of Other Value Pairs
The other pairs mentioned in the question do not indicate microcytic hypochromic RBCs:
Low Hgb and elevated Hct - This combination is inconsistent with any common anemia pattern, as hemoglobin and hematocrit typically move in the same direction 1
Elevated Hct and elevated TIBC - Elevated TIBC (Total Iron Binding Capacity) can be seen in iron deficiency, but would not be accompanied by elevated hematocrit in anemic states 1
Elevated MCV and low MCH - This combination represents macrocytic (large cells) with low hemoglobin content, which is not consistent with the classic presentation of microcytic hypochromic anemia 1
Laboratory Differentiation of Microcytic Anemias
When microcytic hypochromic anemia (low MCV and low MCH) is identified, additional tests help determine the specific cause:
- Serum ferritin - the most specific test for iron deficiency in the absence of inflammation 1
- Transferrin saturation and TIBC - helpful when iron deficiency is suspected 1
- Red cell distribution width (RDW) - often elevated in iron deficiency anemia but may be normal in thalassemia 1, 5
- Hemoglobin electrophoresis - recommended when thalassemia is suspected 1, 4
Clinical Significance
Identifying microcytic hypochromic anemia through low MCV and low MCH values is clinically important because:
- It narrows the differential diagnosis to conditions affecting hemoglobin synthesis 1, 2
- It guides further diagnostic testing to determine the specific cause 1
- It helps prevent inappropriate treatment, such as unnecessary iron supplementation in cases of thalassemia 4
- Early identification and treatment of iron deficiency can prevent significant morbidity, especially in vulnerable populations like pregnant women 2