Understanding Red Blood Cell Indices for Anemia Diagnosis
Red Cell Indices
Red cell indices, also known as absolute values, include Mean Cell Volume (MCV), Mean Cell Hemoglobin (MCH), and Mean Cell Hemoglobin Concentration (MCHC). These are derived from hemoglobin, packed cell volume (PCV or hematocrit), and red cell count values. Automated hematology analyzers measure these indices accurately. It is crucial to correlate red cell indices with other red cell parameters like hemoglobin, hematocrit, red cell count, and other indices to rule out technical errors.
Reference Ranges
- Mean Cell Volume (MCV): 80-100 fl
- Mean Cell Hemoglobin (MCH): 27-32 pg
- Mean Cell Hemoglobin Concentration (MCHC): 30-35 g/dl
Uses of Red Cell Indices
- Morphological Classification of Anemia: Based on red cell indices, anemia is classified into three main types: normocytic normochromic, microcytic hypochromic, and macrocytic normochromic anemia.
- Differentiation of Iron Deficiency Anemia from Thalassemia Trait: In iron deficiency anemia, MCV, MCH, and MCHC are typically low. In thalassemia trait, MCV and MCH are low, while MCHC is usually normal.
Methods for Calculation
There are two methods for calculating red cell indices: manual and automated.
Mean Cell Volume (MCV)
MCV measures the average size of red blood cells. Automated instruments measure it directly. With semi-automated instruments or manual methods, MCV is calculated by dividing PCV by the red cell count:
MCV = (PCV in %) / (Red cell count in million/cmm) x 10
MCV is expressed in femtoliters (fl) and corresponds to the red cell diameter on a blood smear. Normal MCV is 80-100 fl.
MCV is used to classify anemia into microcytic (MCV < 80 fl), macrocytic (MCV > 100 fl), and normocytic (MCV: 80-100 fl) types. MCV often correlates with red cell diameter; low MCV cells appear smaller, and high MCV cells appear larger.
Causes of Increased MCV
- Megaloblastic anemia
- Non-megaloblastic macrocytosis: Chronic alcoholism, liver disease, hypothyroidism, normal pregnancy, reticulocytosis
- Newborns
Causes of Low MCV
- Microcytic hypochromic anemia
MCV is normal in normocytic normochromic anemia (e.g., acute blood loss, hemolysis, aplastic anemia). In the presence of abnormal red cells like sickle cells or in dimorphic anemia (mixed normocytic and microcytic), MCV may be normal (as it’s an average) and unreliable for morphological classification.
The Mentzer index (MCV divided by red cell count) can help differentiate conditions: a ratio less than 13 suggests thalassemia, while a ratio greater than 13 suggests iron deficiency anemia.
Mean Cell Hemoglobin (MCH)
MCH represents the average amount of hemoglobin in a single red blood cell. It is calculated by dividing the hemoglobin value by the red cell count:
MCH = (Hemoglobin in grams per dL) / (Red cell count in millions per cmm) x 10
MCH is expressed in picograms (pg) and the reference range is 27-32 pg.
MCH is decreased in microcytic hypochromic anemia and increased in macrocytic anemia and in newborns. MCH should be interpreted alongside MCV due to their direct linear relationship: smaller red cells contain less hemoglobin, and larger cells contain more.
In some anemias, changes in MCV are accompanied by corresponding changes in intracellular hemoglobin, resulting in normochromic cells (normal MCHC). However, in other anemias, a significant decrease in MCH relative to MCV leads to hypochromic cells (low MCHC). Therefore, cells with low MCV and low MCH are not necessarily hypochromic, and cells with high MCV (macrocytes) and high MCH are often normochromic.
Mean Cell Hemoglobin Concentration (MCHC)
MCHC is calculated by dividing the hemoglobin value by the PCV and is expressed in grams/dL or grams/L. It indicates the concentration of hemoglobin within a given volume of packed red cells.