Red blood cell (RBC) indices are calculated parameters that describe the physical characteristics of erythrocytes. They are derived from the measured CBC values of RBC count, hemoglobin, and hematocrit, and are essential for classifying anemias and identifying underlying erythropoietic abnormalities.
Mean Corpuscular Volume (MCV)
MCV measures the average volume of individual red blood cells, expressed in femtoliters (fL). It is calculated as (Hematocrit × 1000) ÷ RBC count. The normal range is approximately 80–100 fL. MCV is the primary index for morphologic classification of anemias: values below 80 fL define microcytic anemias (iron deficiency, thalassemia, anemia of chronic disease), values within the normal range define normocytic anemias (acute blood loss, hemolysis, anemia of chronic disease, bone marrow failure), and values above 100 fL define macrocytic anemias (vitamin B12 deficiency, folate deficiency, liver disease, alcohol use, myelodysplastic syndromes, reticulocytosis).
Mean Corpuscular Hemoglobin (MCH)
MCH quantifies the average mass of hemoglobin per red blood cell, expressed in picograms (pg). It is calculated as Hemoglobin × 10 ÷ RBC count. The normal range is 27–33 pg. MCH generally parallels MCV in most conditions: microcytic anemias show low MCH (hypochromia), macrocytic anemias show high MCH, and normocytic anemias show normal MCH. Isolated reduction of MCH without low MCV can occur in early iron deficiency and some hemoglobinopathies.
Mean Corpuscular Hemoglobin Concentration (MCHC)
MCHC reflects the average concentration of hemoglobin per unit volume of packed red blood cells, expressed in g/dL. It is calculated as Hemoglobin × 100 ÷ Hematocrit. The normal range is 33–36 g/dL. MCHC is the most stable of the indices and is less affected by cell size. Low MCHC indicates hypochromia, most commonly seen in iron deficiency anemia and thalassemia. MCHC above 36 g/dL (hyperchromia) is rare and suggests spherocytosis, hereditary spherocytosis, or autoimmune hemolytic anemia. Very high MCHC values may also result from hemoglobin C disease or cold agglutinin interference.
Red Cell Distribution Width (RDW)
RDW measures the degree of anisocytosis (variation in RBC size) and is reported as the coefficient of variation (CV%) or standard deviation (SD) of the RBC volume distribution curve. The normal range is 11.5–14.5%. An elevated RDW indicates increased heterogeneity of RBC sizes and is a sensitive marker for early anemia before MCV changes. RDW is particularly useful for distinguishing iron deficiency anemia (high RDW) from thalassemia trait (normal RDW) — a classic laboratory distinction. High RDW with normal MCV may indicate early nutritional deficiency, while high RDW with high MCV suggests megaloblastic anemia.
Reticulocyte Hemoglobin Equivalent (RET-He)
Modern analyzers provide the reticulocyte hemoglobin equivalent (RET-He or CHr), which measures hemoglobin content in reticulocytes. This parameter reflects the most recent iron availability for erythropoiesis (3–4 days prior) and is the earliest indicator of iron-deficient erythropoiesis. It is useful for diagnosing iron deficiency in the presence of inflammation, monitoring response to iron therapy, and assessing functional iron deficiency in patients receiving erythropoiesis-stimulating agents.
Clinical Interpretation of the Indices
The combined pattern of MCV, MCH, MCHC, and RDW guides the differential diagnosis. Low MCV with low MCHC and high RDW suggests iron deficiency anemia. Low MCV with low MCHC and normal RDW suggests thalassemia trait. High MCV with elevated RDW suggests megaloblastic anemia due to B12 or folate deficiency. Normal MCV with high RDW may indicate early nutritional deficiency or mixed deficiency. The peripheral blood smear provides visual confirmation. Integration with reticulocyte count, iron studies (ferritin, TIBC, transferrin saturation), and hemoglobin electrophoresis is often required for definitive diagnosis.
